Heredity And Evolution Class 10 Science Exam Questions

Please refer to Heredity And Evolution Class 10 Science Exam Questions provided below. These questions and answers for Class 10 Science have been designed based on the past trend of questions and important topics in your class 10 Science books. You should go through all Class 10 Science Important Questions provided by our teachers which will help you to get more marks in upcoming exams.

Class 10 Science Exam Questions Heredity And Evolution

Class 10 Science students should read and understand the important questions and answers provided below for Heredity And Evolution which will help them to understand all important and difficult topics.

Very Short Answer:

Question. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plant bearing white flowers. What will be the result in F1 progeny ?
Answer : All the progeny of F1 generation will have violet flowers because violet colour is dominant over the recessive white colour.

Question. Why is variation important for a species ?
Answer : Variation is necessary for the survival of species as variation makes species more adapted to survive with the changing environmental conditions. The variant species are more adapted to changing environment. Therefore, they can survive better and reproduce to pass the genes to the offsprings.

Question. What is DNA copying ? State its importance.
Answer : Creating a new copy of DNA in a reproducing cell is known as DNA copying. Since DNA is the carrier of the blue print of the genetic characters, its copying is essential to pass on this blue print to the offsprings.

Question. Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually ? Why or why not ?
Answer : No, geographical isolation will not be a major factor in the speciation of a organism that reproduces asexually because offsprings are produced from a single parent and there is no involvement of any other organisms.

Question. If the weight of a tiger is reduced because of starvation, the progeny tigers will not have low weight. Give reason.
Answer : Decrease in weight of a tiger due to starvation will not cause a change in the DNA of germ cells as this is an example of acquired trait. Hence the progeny tigers will not have low weight.

Question. How does the creation of variations in species promote survival ? 
Answer : Due to errors in DNA copying some variations in a species occur. Species with useful variations get adapt to the changing environment and they have better chances of survival and produce their offsprings.

Question. Give reasons why acquired characters are not inherited ?
Answer : Acquired characters are developed by an individual during its life time. They are not present in the DNA of germ cells. They only involve changes in the nonreproductive tissues and hence these characters are not inherited.

Question. All the variations in a species do not have equal changes of survival. Why ?
Answer : Those variations which may not prove beneficial they would ultimately be eliminated so all the variations in a species do not have equal changes of survival.

Question. Why do all the gametes formed in human females have an X-chromosome ?
Answer : The sex chromosome in human female is homomorphic i.e., they contain same chromosome XX. During meiosis process at the time of gamete formation all egg cell will get one copy of X-chromosome, hence all the gametes formed in human females have an X-chromosome.

Question. Mendel took tall pea plants and short pea plants and produced F1 progeny through cross-fertilisation. What did Mendel observe in the F1 progeny ?
Answer : Mendel observed that all pea plants were tall in F1 generation.

Question. What factors could lead to the rise of a new species ?
Answer : The factors that could lead to the rise of a new species are genetic variations, genetic drift, reproductive isolation and natural selection.

Question: How does the creation of variations in a species promote survival?
Answer: Variations increases the adaptability of an organism to its changing environmental conditions.

Question: How do Mendel’s experiments show that traits are inherited independently?
Answer: When Mendel crossed pure bred tall pea plants with pure bred short pea plants, he found that only tall plants were produced in F₁ generation. Mendel, further crossed the tall pea plants obtained in F₁ generation with dwarf plants and obtained the ratio of Tall: Short plant as 3 : 1 in F₂ generation. This experiment proved that traits are inherited independently so other intermediate traits or new traits were formed.

Question: How is the sex of the child determined in human beings?
Answer: In case of human beings female sex have a pair of XX chromosomes (sex) and male sex have a pair of XY sex chromosome. When the crossing of male and female gametes takes place then the sex of the child is determined as follows:

This shows that the ratio of male: female sex of the child is same i.e., (1 : 1), 50% possibility of each is seen here.

Question: Why are traits acquired during the life-time of an individual not inherited?
Answer: The traits can be inherited from one generation to the other only if there is a variation/change in DNA. The traits acquired during the life-time of an individual may not bring change in the genes of DNA.

Question: What factors could lead to the rise of a new species?
Answer: The factors which can lead to the rise of a new species are gene flow, genetic drift, reproductive isolation and natural selection.

Question: Will geographical isolation be a major factor in the speciation of an organism that reproduces asexually? Why or why not?
Answer: No, because the asexually reproducing organisms does not depend on other organisms for their reproduction.

Question: Can the wing of a butterfly and the wing of a bat be considered homologous organ? Why or why not?
Answer: The wing of butterfly and the wing of a bat cannot be considered homologous organs because both have different structures but same function. They have different basic structural design and developmental origin. They are analogous organs.

Question: Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species?
Answer: Because irrespective differences in characters they have capacity of interbreeding.
Interbreeding is an important criteria to categorize them as one species.

Question: Give the respective scientific terms used for studying 
(a) the mechanism by which variations are  created and inherited and
(b) the development of new type of organisms from the existing ones.
Answer: (a) Heredity is the mechanism by which variations are created and inherited.
(b) Evolution is the process in which development of new type of organisms takes place from the existing ones. 

Question: Why do mice whose tails were surgically removed just afer birth for generations, continue to produce mice with tails?
Answer: A group of mice are normally bred and all their progeny possess tail. Now, if the tails of these mice are surgically removed in each generation, tail-less mice will not be produced.This is so because removal of tail during the life cycle of mice is an acquired character which is not transferred to future generations.The removal of the tail cannot change the genes of the germ cells of the mice.

Question: What is speciation?  
Answer: The process by which new species develop from the existing species is known as speciation.

Question: Write the contribution of Charles Darwin in the Field of ‘evolution’.
Answer: The contribution of Charles Darwin in the field of evolution is that only the organism which can adapt to the changing environmental conditions can survive and he also stated that the natural selection is a process which plays an important role in evolution of plants and animalsby selecting the organisms with traits favourable to the environment.

Short Answer: 1

Question: A man with blood group A marries a woman with blood group O and their daughter hasblood group O. Is this information enough to tell you which of the traits – blood group A or O is dominant? Why?
Answer: In humans, genes for blood groups A and B are dominant over genes for blood group O but codominant to each other. Hence, genotype of man with blood group A will be either IAIA (homozygous dominant) or IAIO (heterozygous dominant). Genotype of woman with blood group O will be IOIO. The children of this couple will have the following probabilities for their blood groups :

Thus, gene for blood group A is dominant over gene for blood group O. 

Question: Give one example each of characters that are inherited and the ones that are acquired in humans. Mention the difierence between the inherited and the acquired characters.
Answer: The difierence between inherited and acquired characters is that the inherited character of an organism is caused by a change in its genes (or DNA) on the other hand acquired character of an organism is not inherited but develops in response to the environment. Eye colour is an example of inherited character whereas swimming is an example of acquired character in humans.

Question: What are fossils? What do they tell about the process of evolution?
Answer: Fossils are the remains or impressions of the dead animals and plants that lived in the remote past.Te fossils provide evidence for evolution.
For example, a fossil bird called Archaeopteryx have characters of both reptiles and birds. It had feathers, fused bones and beak which are exclusive characters of birds and had teeth in the jaws, claws on fingers, a long tail, etc like reptiles. Thus, Archaeopteryx is a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles through the process of continuous evolution. 

Question: Write a difference between inherited traits and acquired traits giving one example of each.
Answer: A trait (or characteristic) of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait. For example, if a group of mice are normally bred, all their progeny will have tails. Now, if the tails of these mice are cut by surgery in each generation, tail-less mice will not be produced. This is so because removal of tail is an acquired character.
A trait (or characteristic) of an organism which is caused by a change in its genes (or DNA) is called an inherited trait . Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism. For example, skin colour in human beings

Short Answer: II

Question: (a) Classify the following as homologous or analogous pairs :
(i) Broccoli and Cabbage
(ii) Ginger and Raddish
(iii) Forelimbs of birds and lizard
(iv) Wings of a bat and Wings of a bird.
(b) State the main feature that categorises a given pair of organs as homologous or analogous.
Answer : (a) (i) Broccoli and cabbage are homologous pairs.
(ii) Ginger and raddish are analogous pairs.
(iii) Four limbs of birds and lizard are homologous pairs.
(iv) Wings of bat and wings of a bird are analogous pairs. 
(b) Homologous organs are defined as the organs of different animals that are having a similar structure but differ in their functions. Example of homologous organs are the forelimbs of frog, man and the flippers of the whale. The structure of these animals are the same but the functions are different. The flippers are used for swimming, the forelimbs of frogs are used for jumping and propping, and the forelimbs of man are used for writing, eating, holding things, etc. Having a similar structure shows that all these animals have a common ancestor.
Analogous organs are defined as the organs of different animals that are having different structure but perform the same functions. Example of analogous organ are the wings of insects and the wings of the birds. The structure of wings of a bird has bones covered by flesh, skin and feathers whereas the wings of insects are extension of integument. But in both the cases the wings are used for flying. These organisms would have to adapt to the same ecological niche as other organisms. Hence, they may have evolved similar (analogous) organs.

Question: A green stemmed rose plant denoted by GG and a brown stemmed rose plant denoted by gg are allowed to undergo a cross with each other.
(a) List your observations regarding
(i) Colour of stem in their F1 progeny
(ii) Percentage of brown stemmed plants in F2 progeny if F1 plants are self pollinated.
(iii) Ratio of GG and Gg in the F2 progeny.
(b) Based on the findings of this cross, what conclusion can be drawn ?
Answer : (a) (i) Colour of stems in F1 progeny will be all green.
(ii) The percentage of brown stem will be 25% i.e., one-fourth of the total number of progeny.
(iii) GG: Gg is 1: 2
(b) Based on the findings of this cross this can be concluded that green stemmed plants are dominant whereas brown stemmed plants are recessive.

Question: List two differences between acquired traits and inherited traits by giving an example of each.
Answer :

Question: How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism ?
Answer : Some traits that are inherited may not express themselves. Such hidden traits are known as recessive  traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, tall plant was crossed with a dwarf plant which produced all tall plants in F1 progeny.
However, when these F₁ tall plants were crossed with each other, both tall and dwarf pea plants were obtained in F₂ generation. Reappearance of the dwarf pea plants in F₂ generation proves that the dwarf trait was inherited but not expressed in F₁ generation.

So, dwarfness traits of plants (pea plant) were not lost but are suppressed in the F₁ generation and thus reappeares in F₂ generation.

Question: ‘‘Evolution and classification of organisms are interlinked’’. Give reasons to justify this statement.
Answer : Classification refers to the grouping and naming of organisms based on the similarities and difference in their characters. Classification is done on the basis of ancestral characteristics and derived characteristics. So as we move from simple life forms to the complex organisms, we are actually tracing the path of evolution.
All life forms on earth have progressively changed during evolution. So, the hierarchy develops during classification which throws the light on the evolutionary relationship among organisms. Common ancestors can also be predicted with the help of classification.

Question: With the help of two suitable examples, explain why certain experiences and traits earned by people during their lifetime are not passed on to their next generations. When can such traits be passed on ?
Answer : The experiences and qualifications that a person earns during their lifetime are examples of acquired traits. 
These traits are not inherited, they do not affect the genetic makeup. They are developed due to direct affect of the environment. Thus they are not passed to the next generation. Piercing of ears or large sized muscles of the wrestler are not carried by the next generation. Traits can only be passed on to the next generation only when they have some direct effect on the genes.
For example, mutation is caused in the germ cells of a person due to exposure to some harmful radiations,
it is likely that the mutation will be passed on to the subsequent generations.

Question: Explain with the help of an example each, how the following provide evidences in favour of evolution :
(a) Homologous organs
(b) Analogous organs
(c) Fossils 
Answer : (a) Homologous organs : These organs are similar in form but perform different functions in different organisms. These organs provide strong evidences in favour of evolution. For example : the bone structure observed in the forelimbs of birds and bats, flippers of dolphins and arms of human beings are similar and have same pentadactyl plan but they perform different functions.
(b) Analogous organs : These organs have different origin and different basic structure but perform same functions. For example, wings of birds and wings of bat have different structures but perform same function of flying. Thus, these organs provide evidences for evolution that they are different in origin but evolved to perform same function to survive in hostile environmental conditions.
(c) Fossils : Fossils are the preserved remains or traces of animals, plants and other organisms from the remote past. For example, Archaeopteryx is a connecting link between birds and reptiles and it suggests that the present animals have evolved from the existing ones through the process of continuous evolution.

Question: Mention the total number of chromosomes along with the sex chromosomes that are present in a human female and a human male. Explain how in sexually producing organisms the number of chromosomes in the progeny remains the same as that of the parents.
Answer : The total number of chromosomes present in both human male and female is 46. Out of these, two chromosomes are the sex chromosomes. In human males, the two sex chromosomes present are X and Y, while in human female, both sex chromosomes are X. During sexual reproduction, the new individual is formed by the fusion of gametes from both the parents. These gametes are haploid in nature i.e., they contain only one set of the chromosomes. They are formed by the meiosis half. When these haploid gametes fuse during fertilisation the nuclei of these two gametes fuse and the chromosome number is then restored to normal. Hence, the progeny formed has the same number of chromosomes as that of the parents.

Question: ‘‘Natural selection and speciation leads to evolution’’. Justify this statement.
Answer : Natural selection is the phenomenon by which the nature selects those species which possess survival advantage over the other species. According to theory of natural selection, there is ‘‘Struggle for existence’’ within the species of a population for the environmental resources and this struggle leads to survival of certain organisms and elimination of the less competent species. Thus, in this competition, some organisms might undergo genetic changes which help them in their survival. Thus the better adapted organism would, survive and pass on their traits to next generations, gradually leading to evolution. Speciation is a process of formation of new species from the existing one due to reproductive isolation of a part of its population. This reproduction and isolation can occur due to geographical isolation of a part of population with time, the genetic drift will accumulate different variations in each of the geographically separated, sub-population, ultimately, all the individuals of these two groups will isolate reproductively thus, leading to formation and
evolution of new species. Thus, we can say that both natural selection and speciation lead to the evolution of species on earth.

Question: If a trait A exists in 10% of a population of an asexually reproducing species and trait B exists in 60% of the same population, which trait is likely to have arisen earlier ?
Answer : Trait B would have arisen earlier than trait A because as species are asexually reproducing, there would be very minor differences generated due to small inaccuracies in DNA copying which results in variations in trait A. But as trait B occurs in more number in the population as compared to trait A so trait B would have arisen earlier than trait A.

Question: How do Mendel’s experiments show that traits may be dominant or recessive ?
Answer : When Mendel crossed pure tall (TT) pea plants with pure dwarf (tt) pea plants, in F₁ generation he found that all pea plants were tall (Tt). There were no dwarf plants produced in F₁ generation. When he self-pollinated these F₁ plants, in F₂ generation he obtained tall and dwarf plants in the ratio 3 : 1. Thus as three-fourths of the plant in F₂ generation are tall and one-fourth is dwarf so tall is a dominant trait whereas dwarf is a recessive trait [which expressed itself only in homozygous condition]. So, he concluded that for a particular trait [here in this example height of the plant] it may be dominant or recessive.

Question: Why are human beings who look so different from each other in terms of size, colour and looks said to belong to the same species ?
Answer : Human beings belong to a same species called Homosapiens. Based on the study of fossils, molecular hylogeny involving DNA sequence though human beings, who look so different from each other in terms of size, colour and looks belong to a single species. The study of fossils suggests that they have originated in Africa. Gradually, their number increases and they spread throughout the whole continent, some also migrate to other parts of the world. The geographical and climatic variations in different regions lead to variations in their genes as a result their colour, looks, size also varies.

Question: What evidence do we have for the origin of life from inanimate matter ?
Answer : The experiment of Stanley L. Miller and Harold C. Urey in 1953 provided the evidence that life originate from inanimate matter. They produced a mixture of methane, ammonia and hydrogen in the ratio of 2 : 2 : 1 in an apparatus along with water vapour and the temperature was maintained just below 100°C. Electric sparks were passed through this mixture to stimulate lightening. These mixtures of gases were believed to prevail in primitive atmosphere. By the end of a week 15% of carbon from methane had been converted to simple organic compounds like amino acids, sugars, purines, pyrimidines. These simple molecules interact with each other to form complex molecules like proteins, polysaccharides, nucleic acids etc., which are the main components of living matter.

Question: Mention few characteristics of genes.
Answer : The characteristics of genes are :
(a) It can maintain constancy from generation to generation.
(b) It may undergo mutation.
(c) The specific sequence of nucleotides on the gene determines the protein it codes for.
(d) Genes control a specific character.
(e) Genes are unit of inheritance.

Question: Differentiate between dominant and recessive trait ?
Answer :

Question: The gene for red hair is recessive to the gene for black hair. What will be the hair colour of a child if he inherits a gene for red colour from his mother and a gene of black hair from his father ? Express with the help of flowchart.
Answer : The colour of hair of the child will be black as black is dominant over red.

Question: Distinguish between homologous organs and analogous organs. In which category would you place wings of a bird and wings of a bat ? Justify your answer giving a suitable reason.
Answer :

Wings of bird and bat are examples of analogous organs as they have different basic structures but perform similar function i.e., flying. The wings of bird are formed of limb bones which are covered with flesh, skin and feathers whereas in wings of bats skin folds stretched mainly between elongated fingers.

Question. What are chromosomes ? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.
Answer : Long thread like structures made of DNA are called chromosomes. In sexually reproducing organisms gametes are formed which have only half the number of chromosomes or DNA as compared to nonreproductive body cells. The gametes fuse to form a zygote resulting in re-establishment of the number of chromosomes in the progeny.

Question: If we cross-bred tall (dominant) pea plant with purebred dwarf (recessive) pea plant, we will get plants of F₁ generation. If we now self-cross the pea plant of F₁ generation, we obtain pea plants of F₂ generation.
(a) What do the plants of F₁ generation look like ?
(b) State the ratio of tall plants to dwarf plants in F₂ generation.
(c) State the type of plants not found in F₁ generation but appeared in F₂ generation. Write the reason for the same.
Answer : (a) Tall
(b) 3 : 1
(c) Dwarf plants are not found in F₁ generation but appeared in F₂ generation because being a recessive trait, dwarfness can only be expressed in the absence of dominant trait or in its pure form. 

Question. List any three factors and mention how they could lead to the rise of a new species.
Answer : 1. Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution,
the change in the heritable traits characteristic of a population over generations.
2. Genetic drift is the variation in the relative frequency of different genotypes in a small population, owing to the change disappearance of particular genes as individuals die or do not reproduce.
3. A gene mutation is a permanent alteration in the DNA sequence that makes up a gene, such that the sequence differs from what is found in most people. Mutations range in size; they can affect anywhere from a single DNA building block (base pair) to a large segment of chromosome that includes multiple genes.

Question. Give an example of the characteristics being used to determine how close two species are in evolutionary terms.
Answer : Study of homologous organs as forelimbs of mammals, birds, reptiles and amphibian; show that though they perform different functions but they have similar basic or internal structure; this is because they have evolved from common ancestor and help us in determining the closeness between two species in evolutionary terms.

Question. How do variations arise in organisms ? ‘‘Variation is useful for the survival of species.’’ Justify this statement with the help of an example.
Answer : Variations may arise due to small inaccuracies in DNA copying during reproduction; as a result of sexual reproduction where genetic materials from two different organisms combine to form a new organism.
During adverse conditions some variations may give survival advantage to the organism e.g., a population of bacteria living in temperate waters. If the temperature of water increases suddenly then most of the bacteria would die but few variant bacteria resistant to heat would survive and grow further.

Question. Explain Mendel’s experiment with peas on inheritance of characters considering only one visible contrasting character.
Answer : Mendel conducted breeding experiments with garden peas :
(a) He studied (pure ) plants of a tall or short varieties.
(b) He crossed them and obtained F1 progeny.
(c) He found that F1 progeny was all tall plants.
(d) He selfed the (hybrid) plants of F1 progeny.
(e) He found that in F2 progeny there were tall as well as short plants.
(f) The three quarter plants were tall and one quarter was short.

Question: How did Mendel interpret his result to show that traits may be dominant or recessive?
Answer: Mendel crossed the pea plant for two contrasting characters under consideration.
The trait that expressed itself in F₁ generation was dominant and the one not expressed in  F₁ generation was recessive. He later selfed the plants of F₁ generation and recovered, both parental traits in a definite proportion in F₂ generation.

Question: With the help of an example justify the following statement: “A trait may be inherited, but may not be expressed.”19. List two difierences in tabular form between dominant trait and recessive traits. What percentage/proportion of the plants in the F₂ generation/progeny were round, in Mendel’s cross between round and wrinkled pea plants?
Answer: A trait may be inherited but may not be expressed, this could be explained by the given example. When a tall pea plant was crossed with a dwarf pea plant, then F₁ generation plants were all tall. When F₁ plants were selfed, then F₂ generation plants were both tall and dwarf. This shows that the F₁ plants had inherited both the parental traits but did not express dwarfness or recessive trait in the presence of the trait for tallness or dominant trait. This could be explained by the given cross : 

Question: In a monohybrid cross between tall pea plants (TT) and short pea plants (tt) a scientist obtained only tall pea plants (Tt) in the F₁ generation. However, on selfing the F₁ generation pea plants, he obtained both tall and short plants in F₂ generation. On the basis of above observations with other angiosperms also, can the scientist arrive at a law? If yes, explain the law. If not, give justification for your answer.
Answer: In the situation discussed in the question the scientist can arrive at two different laws, i.e., law of dominance and law of segregation (or law of purity of gametes). is can be explained with the help of following crosses :

In F₁ hybrid two dissimilar alleles are present for one character, i.e., height T is for tallness and t is for dwarfness, out of which only one allele called dominant allele expresses itself and the one which remains unexpressed is called recessive allele. This is called “law of dominance”.
Also the two dissimilar alleles that remain together in a heterozygous individual do not get mixed up and keep their distinct identity. Hence, at the time of gamete formation they separate so that each gamete receives only one allele and is always pure which enables reappearance of recessive trait in F₂ progenies when the two recessive alleles come together. This is called “law of purity of gametes.”

Question: Explain Mendel’s experiment with peas on inheritance of characters considering only one visible contrasting character.
Answer: Mendel crossed a pure tall pea plant with  pure dwarf pea plant. All the plants obtained in F₁generation were tall. When Mendel selfed plantsfrom F₁ generation then he obtained both tall anddwarf plants in F₂ generation in the ratio of 3 : 1This can be illustrated as follows :

This explains that for each pair of contrasting characters there are two alleles. The trait which is expressed in F₁ is dominant trait and is controlled by dominant allele and the trait which remains unexpressed in F₁1 is the recessive trait and is controlled by recessive gene. When both the contrasting alleles are present together in F₁ individuals, no mixing of alleles occurs and they again segregate at the time of gamete formation therefore, when the recessive alleles come together they result in reappearance of recessive trait in F₂ generation.

Question: How do Mendel’s experiment show that traits are inherited independently? 
Answer: In a dihybrid cross given by Mendel, it was observed that when two pairs of traits or characters were considered, each trait expressed independent of the other. us, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits. This could be explained clearly from the given cross:

Question:“It is a matter of chance whether a couple will have a male or a female child.” Justify this statement by drawing a flow chart.
Answer: Sex is determined at the time of fertilisation when male and female gametes fuse. Male produces two types of gametes, i.e., having X or Y chromosome and female produces same type of gametes containing X chromosomes. The sex of the child is determined at the time of fertilisation when male and female gametes fuse to form zygote. 
If a sperm (male gamete) carrying X chromosome fertilises an egg or ovum (female gamete) carrying X chromosome, then the offspring will be a girl (female). is is because the offspring will have XX combination of sex chromosomes.
If a sperm (male gamete) carrying Y chromosome fertilises an egg or ovum (female gamete) which has X chromosome, then the offspring will be a boy (male). This is because the offspring will have XY combination of sex chromosomes.
Therefore, there are 50% chance of a male child and 50% chance of a female child.

Question: A cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
(a) Write the phenotype of F₁ progeny. Give reason for your answer.
(b) Write the different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed.
Answer: (a) The given cross was made between pure breeding pea plants, one with round and green seeds and the other with wrinkled and yellow seeds.
Yellow seed colour and round seed shape is dominant over green seed colour and wrinkled seed shape. In F₁ generation, dominant traits express itself, whereas recessive traits get suppressed.

Therefore, the phenotype of F₁ progeny is round and yellow.
(b) The different types of F₂ progeny obtained along with their ratio when F₁ progeny was selfed could be illustrated by the given cross:

Question: “It is possible that a trait is inherited but may not be expressed.” Give a suitable example to justify this statement. 
Answer: A trait may be inherited but may not be expressed, this could be explained by the given example. When a tall pea plant was crossed with a dwarf pea plant, then F₁ generation plants were all tall. When F₁ plants were selfed, then F₂ generation plants were both tall and dwarf. This shows that the F₁ plants had inherited both the parental traits
but did not express dwarfness or recessive trait in the presence of the trait for tallness or dominant trait. This could be explained by the given cross :

Question: List in tabular form, two distingushing features between the acquired traits and the inherited traits with one example of each.
Answer:Differences between the inherited traits and acquired traits are as follows :

Question: “An individual cannot pass on to its progeny the experiences of its life-time”. Justify the statement with the help of an example and
also give reason for the same. 
Answer: If we breed a group of mice, all the progeny of mice will have tails just like their parents. Now, if we remove the tails surgically and again breed them,we still get new mice with tails. This is because cutting the tails of mice does not change the genes of their reproductive cells (or gametes). And since the acquired trait of ‘cut tails’ does not bring about a change in the genes of mice, this trait cannot be passed on to their next generations. From this we conclude that the experiences acquired by an individual during his lifetime (called acquired traits) cannot be passed on to its progeny, and hence cannot lead to evolution because they are not caused by the change in genes.

Question: Tabulate two distinguishing features between acquired traits and inherited traits with oneexample of each. 
Answer: Differences between the inherited traits and acquired traits are as follows :

Question: Describe any three ways in which individuals with a particular trait may increase in population.
Answer: Three ways in which individuals with a particular trait may increase in population are:
(i) Genetic drift : It is the random change in gene frequency occurring by chance fiuctuations.
In this phenomenon, an event may increase the frequency of a particular trait having little adaptive value and survival advantage.
(ii) Natural selection : It is the phenomenonwherein nature selects traits favourable to the species in the environment. Thus, a particular trait selected by the nature increases in number.
(iii) Geographical isolation : Interbreeding populations are geographically isolated by barriers such as mountain ranges, rivers and sea.
This geographical isolation leads to reproductive isolation and thus there is no gene flow between separated groups of population and therefore, population with particular character increases.

Question: What is speciation? List four factors responsible for speciation.
Answer: The process by which new species develop from the existing species is known as speciation. The four factors responsible for speciation are:
(i) Geographical isolation of a population leads to reproductive isolation and hence speciation. (ii) Genetic drift leads of formation of new species
by causing drastic changes in the frequencies of particular gene by chance alone.. 
(iii) Natural selection wherein nature in the wild selects traits favourable to the species in the given environment.
(iv) Random mating resulting in genetic variations in genotype frequencies within the population. 

Question: What is speciation? Explain in brief the role of natural selection and genetic drift in this process. 
Answer: The process by which new species develop from the existing species is known as speciation. Both natural selection and genetic drift play a very important role in speciation. By natural selection traits favourable to the species in the given environment are selected by the nature. Example, beetles develop colur variation during reproduction. Consequently, in the progeny, one beetle develops green body colour instead of normal red colour. This beetle can inherit this variation in colour on to its progeny so that all its offspring are green in colour. Crows cannot easily locate green-coloured beetles on the green leaves of bushes and continue to eat red beetles which are easily located on green leaves. As a result, in population of green beetles increases in each generation.
Genetic drift is the elimination of the genes of certain traits from the small population when a section of population dies of natural calamity or migrates to other region. It alters the gene frequency of the remaining population.
Suppose in sexually reproducing red beetle population, a colour variation arises wherein one beetle develops blue body colour instead of red.
This beetle can also pass this colour variation to its progeny so that all its offsprings are blue coloured beetles. As the population of beetles expand, initially there would be few blue coloured beetles among the majority of red coloured beetles. At this point, an elephant comes by and stamps on the bushes where the beetles live. Consequently, most of the beetles get killed. By chance, most of the survived beetles are of blue colour. This population again slowly multiply and will contain mostly blue coloured beetles over a period of time. Survival of more blue beetles in the population changed the coloured characteristic from normal red to blue over a period of time. In small population, accidents can change the frequency of some genes in a population, even if it does not give any survival advantage to the possessors.

Question: Explain the following:
(a) Speciation
(b) Natural selection. 
Answer: (a) Speciation : Origin of new species from existing one due to reproductive isolation of a part of its population is called speciation.
Suppose a large population of beetle living in an area gets split into two sub populations due to geographical barriers like river, mountain, sea, etc. Then members of these sub-populations will not be able to interbreed. Hence, there will be no gene flow between the members of these populations. After few generations, genetic driftwill accumulate different variations in each of the two geographically separated sub-populations.
Natural selection may also operate simultaneously in a different way in these eographically isolated sub-populations. This makes geographically isolated sub-populations to become more and more different from each other and ultimately reproductive isolation occurs between individuals of these groups and they transform into new species.
(b) Natural selection is the phenomenon wherein nature, in the wild, selects traits avourable to the species in its environment.

Question: List three main factors responsible for the rise of a new species giving a brief description about each.
Answer: Speciation is the process by which new species develop from the existing species.
The three important factors responsible for the speciation are :
(i) Geographical isolation of a population caused by various types of barriers such as mountain ranges, rivers and sea. Geographical isolation is the major factor in the speciation of sexually reproducing animals because it causes reproductive isolation and interrupts the flow of genes between their isolated populations through the gametes.
(ii) Genetic driftis the elimination of the genes of certain traits from the small population when a section of population dies of natural calamity or migrates to other region. It alters the gene frequency of the remaining population.
(iii) Variations introduced by random mating and hybridisation. Random fusion of gametes
adds new alleles and it result in formation of new species.

Question: List three factors that provide evidences in favour of evolution in organisms and state the role of each in brief. 
Answer: Some of the important sources which provide evidences for evolution are homologous organs, analogous organs and fossils.
(a) Homologous organs perform different functions in different species but have similar basic structure and similar embryonic origin. For example forelimbs of a frog, lizard, bird and human being show similarity in basic structure. However, these organs perform entirely different functions but the basic similarity in the forelimbs of these different vertebrates indicates common ancestry.
This means that these vertebrates have modified according to the special needs of the subsequent
generations during the course of evolution.
(b) Analogous organs are different in fundamental structure and embryonic origin but have similar appearance and perform similar functions. For example, the wings of an insect and a bird have different structures but they perform the same function of flying. The presence of these organs indicates that they are not derived from common ancestors but they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing nvironment. this provides a mechanism for evolution.

Question: “Evolution and classification of organisms are interlinked”. Give reasons to justify this statement.
Answer: Evolution is the process by which newer types of organisms are developed from the pre-existing ones through modifications. Classification is the arrangement of organisms into a series of groups based on physiological, biological, anatomical or other relationships. All systems of classification are hierarchial. The more closely two species are related, the more recently they have a common ancestor. Classification of organisms necessarily involves organising them in different groups, based on the similarities and difierences of characteristics. It helps in the recognisation of the basic arrangement of a hierarchical structure among diverse species. It facilitates studies or research of wide variety associated with organisms effortlessly. us, classification of species is infact a refiection of their evolutionary relationship or we can say that evolution and classification are interlinked. 

Question: What are fossils? How do they help in the study of evolution? 
Answer: Fossils are the remains or impressions of the dead animals and plants that lived in the remote past. e fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx have characters of both reptiles and birds. It had feathers, fused bones and beak which are exclusive characters of birds and had teeth in the jaws, claws on fingers, a long tail, etc like reptiles. Thus, Archaeopteryx is a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles through the process of continuous evolution.

Question:  (a) Give the evidence that the birds have evolved from reptiles.
(b) Insects, octopus, Planaria and vertebrates possess eyes. Can we group these animals together on the basis of eyes that they possess? Justify your answer giving reason. 
Answer: (a) Planaria, insects, octopus and vertebrates cannot be grouped together on the basis of eye.
Eyes of insects, octopus, Planaria and vertebrates are analogous organs which have developed over generation as an adaptation for similar function. They represent convergent evolution where distantly related groups develop similar
functional structure as an adaptation for same function.
(b) Fossil bird Archaeopteryx had features like feathers, fused bones, beak which are the characteristic features of birds. It also had some features of reptiles, like, teeth in jaws, claws on free fingers, a long tail, etc. Thus, it represents a
connecting link between reptiles and birds. This example provides a clue that birds have evolved from reptiles.

Question: Distinguish between homologous organs and analogous organs. In which category would you place wings of a bird and wings of a bat? Justify your answer giving a suitable reason.
Answer:  Those organs which have the same basic structure and similar embryonic origin but different functions are called as homologous organs. These organs follow same basic plant of organisation during development, but in adults got modified to perform different function as an adaptation to different environments. Those organs which have different basic structure and embryonic origin but perform similar functions are called analogous organs. The wings of bird and wings of bat are analogous organs because the basic structure or design of the wings of bird and bat are different but they perform the similar
function of flying.

Question: Explain how evolutionary relationship can be traced by the study of homologous organs.
Answer:  Homologous organs provide morphological and anatomical evidences of evolution. There are number of organs in different groups of animals or plants which have similar basic design but are used for different purposes. These are termed as homologous organs. For example, the forelimbs of frog, lizard, bird and human beings show similarity in basic structure. The basic similarity of forelimbs of these different vertebrates indicates that all these have evolved from a common ancestor who had five digited or pentadactyle limbs which became modified according to the special needs of subsequent generations during the course of evolution. Hence, homologous organs depict divergent evolution or adaptive radiation.

Question: How are fossils formed? Describe, in brief, two methods of determining the age of fossils.
Answer:  When organisms like plants or animals die, their bodies get decomposed by the action of microorganisms in the presence of oxygen, moisture, etc. Sometimes the conditions in the environment are such (like absence of oxygen, moisture, etc.), which do not let the body of the organism to get decompose completely. It is the body (or body part) of an organism which we get as fossil on digging the earth. In many cases the so parts of the organisms get decomposed and we get skeleton of hard parts (like teeth, bones, etc.) as fossil. Even the so parts of the plants and animals (which usually decompose quickly) are some times preserved as fossils in the form of their impressions inside the rocks. For example, if a dead leaf gets caught in mud, it will not decompose quickly. The mud around the leaf will set around it as a mould, gradually harden to form a rock and retain the impression of the leaf. This forms a leaf fossil which can be dug out from the earth a after a long time period. 
The age of fossils can be determined in two ways as given below :
(i) By the relative method : When we dig into the earth, we find fossils at different depths. The fossils which we find in layers closer to the surface of the earth are more recent and those fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of earth are the oldest ones.
(ii) Carbon dating method : When a living object dies and forms fossil, its carbon-14 radioactivity goes on decreasing gradually. By this method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.

Question: What are fossils? Describe the importance of fossils in deciding evolutionary relationship between organisms.
Answer:  Fossils are the remains or impressions of the dead animals and plants that lived in the remote past. Fossils occur in sedimentary rocks, peat, amber, asphalt, lava and snow. These provide direct evidence of evolution. Fossils play a very important role in establishing evolutionary relationship between organisms. The age of fossils can be approximately assumed through various techniques which gives an idea of the time period in the geological time scale when a particular organism existed. Some of the morphological features of organisms are well preserved which give a clue about their ancestors and descendants.
 Fossils are the remains or impressions of the dead animals and plants that lived in the remote past. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx have characters of both reptiles and birds. It had
feathers, fused bones and beak which are exclusive characters of birds and had teeth in the jaws, claws on fingers, a long tail, etc like reptiles. Thus,Archaeopteryx is a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles through the process of continuous evolution.

Question: Define the term “evolution”. Evolution should not be equated with progress.” Give reason to justify this statement.
Answer:  Evolution is the sequence of gradual changes which take place in the primitive organisms over millions of years in which new species are produced.
Evolution should not be equated with progress because there is no real progress in the concept of evolution. Evolution is just the production of diversity of life forms and shaping of this diversity by the environmental selection. The only progress in evolution appears to be that more and more complex body designs of organisms have emerged over the ages. This will become clear from the following examples. When a new species is formed, it is not necessary that the old species will disappear or get eliminated from earth. It will all depend on the environment. Also it is not as if the newly formed species are in any way better than the older ones. It is simply that genetic drift and natural selection processes have combined to form a population having different body design which cannot interbreed with the older population.
It is a common belief that chimpanzees are the ancestors of human beings. It is, however, not true that human beings have evolved from chimpanzees. Actually, both chimpanzees and human beings had a common ancestor long time ago. The two offsprings of that ancestor evolved in their own separated ways to form the modern day chimpanzees and human beings.

Question: Explain analogous organs and homologous organs. Identify the analogous and homologous organs amongst the following: Wings of an insect, wings of a bat, forelimbs of frog, forelimbs of a human.
Answer: The organs which are quite different in fundamental structure and embryonic origin but perform same function and may superficially look alike are called analogous organs The organs which perform different functions in different species but have similar basic structure and similar embryonic origin are called homologous organs.
Among the given examples: wings of an insect, wings of bat are examples of analogous organs whereas forelimbs of frog and human are examples of homologous organs. 

Long Answer:

Question: Define evolution. How does it occur ? Explain how fossils provide evidences in support of evolution 
Answer :
Evolution is the gradual accumulation of variations and its selection by nature leading to the formation of new species. Variations may arise in a population due to mutations or sexual reproduction. Sub-populations are formed due to genetic drift and geographical isolation.
When natural selection acts on them, most suitable variation survives leading to evolution of a new species.
Fossils provide missing link between the species, who has evolved from whom. They provide information about prehistoric organisms.

Question: What are fossils ? How are fossils formed and how is their age determined ? State the importance of fossils in the study of evolution.
Answer : Fossil are the preserved traces of living organisms. Fossils are formed when the body parts of the dead organisms do not decompose completely and are caught up in mud and eventually harden to retain the impression of the body parts. Age of the fossil can be determined by :
(a) Relative method
(b) Carbon dating method
Importance of fossils in the study of evolution :
(a) They help us in knowing about the species which are no longer alive.
(b) Provide evidence of missing links between two groups of organisms.

Question: (a) Why did Mendel choose garden pea for his experiments ? Write two reasons.
(b) List two contrasting visible characters of garden pea Mendel used for his experiment.
(c) Explain in brief how Mendel interpreted his results to show that the traits may be dominant or recessive .
Answer : (a) Mendel choosed garden pea for his experiments because :
(i) Pea plant is small and easy to grow.
(ii) A large number of true breeding varieties of pea plant are available.
(iii) Short life cycle.
(iv) Both self and cross pollination can be made possible.
(b) Contrasting characters which mendal used for his experiment are round or wrinkled seeds and tall or short plants.
(c) When Mendel crossed two pea plants with a pair of contrasting characters only one character appeared in all the members of F₁ progeny, the others remain hidden . On selfing F₁, the hidden characters reappeared in just 75% of the offsprings and the other 75% shared the characters expressed in F₁. Mendel concluded that the character which expresses itself in F₁ and in 75% of the individuals of F₂ is dominating while the other is recessive .

Question: How do Mendel’s experiments show that
(a) Traits may be dominant or recessive ?
(b) Inheritance of two traits is independent of each other ?
Answer :

In this experiment Mendel concluded that the F1 tall plants were not true breeding, they were carrying both short and tall height traits. They appeared tall only because the tall traits were dominant over the dwarf trait. This shows that traits may be dominant or recessive.
(b) Mendel demonstrated that traits are inherited independently through his dihybrid cross. He considered two traits at a time, seed colour and seed shape in which yellow colour (YY) and round shape (RR) are dominant over green colour (yy) and wrinkled (rr).

Mendel observed that the F2 progeny of dihybrid cross had a phenotypic ratio of 9 : 3 : 3 : 1 and produces 9 plants with round yellow seeds, three plants with round green seed, 3 plants with wrinkled yellow and one plant with wrinkled green seeds.
In this experiment, he found that round yellow and wrinkled green are parental combination whereas round green and wrinkled yellow are new combination. 
In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes are produced (RY, Ry, ry, rY). Each of these gametes segregates independently of each other and each has a frequency of 25% of the total gametes produced.
From this experiment, he concluded that when two pairs of traits are combined together in a hybrid, one pair of character segregates independent of the other pairs of character. This is known as the law of independent assortment.

Question: Consider the following situations and explain with example how characteristics of a population changes over the years :
(a) To get survival advantage.
(b) Due to accidental survival
(c) Due to temporary changes in characteristics.
Answer : (a) To get survival advantage :
For example : Suppose a colour variation occurs in beetle during reproduction. One beetle develops green colour instead of normal red colour. This variance will be inherited to its offsprings and a population of green beetles will be formed. These green-coloured beetles could not be recognised by crows as they would be camouflaged with green bushes. So, the population of green-coloured beetles would increase as compared to red ones, because red ones are easily identified by the crows. So in this situation colour variation gives survival advantage.
(b) Due to accidental survival : For example : Suppose another colour variation occurs in beetles during reproduction. Blue beetles are produced in this situation instead of normal red ones. They will pass this variation to their progeny. Both blue and red-coloured beetles are easily identified by crows. Initially the population of blue beetles would be less as compared to red ones. Suppose an elephant come and stamped most of the beetles under bushes. By chance the beetles that survive are blue ones and these blue ones will now increase their population. In this situation survival advantage is due to accident and not due to colour variation.
(c) Due to temporary changes in characteristics :
For example, Suppose a plant disease spreads which leads to reduced number of leaves in bushes. Hence expanding beetle population will be not properly nourished. So, weight of beetles will decrease as compared when leaves were plenty in the bushes. After few time when plant diseases are eliminated again leaves become plentily available to beetles, their weight is increased. So, here increase or decrease in weight is an acquired trait, it does not bring any changes in DNA hence it cannot be passed from one generation to other. This is an example of temporary changes in characteristics.

Question: Give the basic features of the mechanism of inheritance.
Answer : The basic features of mechanism of inheritance are :
(a) Traits are controlled by genes.
(b) Genes are present on chromosomes.
(c) Each gene controls one character.
(d) There may be two or more forms of a gene.
(e) One form may be dominant over the other.
(f) An individual possesess two forms of genes which may be similar or dissimilar.
(g) The two forms of gene separate at the time of gamete formation.
(h) The two forms of gene are brought together in a zygote after fertilisation.

Question: (a) Why did Mendel choose garden pea for his experiments ? Write two reasons.
(b) List two contrasting visible characters of garden pea Mendel used for his experiment. 
Answer:  (a) Mendel chose garden pea for his experiments because:
(i) It was easy to grow and it shows some clear contrasting traits like some pea plants are tall whereas others are dwarf.
(ii) Pea plants are self pollinating and many generation of pea plants can be produced in comparatively less time.
(b) The contrasting characters of garden pea plant studied by Mendel are:

(c) Mendel crossed the pea plant for twocontrasting characters under consideration.
The trait that expressed itself in F₁ generation was dominant and the one not expressed in F₁ generation was recessive. He later selfed the plants of F₁ generation and recovered, both parental traits in a deffnite proportion in F₂ generation. Mendel interpreted his results as, the trait that expressed itself in F₁ was dominant and the one that reappeared in F₂ generation was recessive. It can be demonstrated by the following cross :

Question: How do Mendel’s experiments show that traits are inherited independently ?
Answer : When Mendel crossed pure pea plants with round, yellow seeds with pure plants with wrinkled, green seeds in F1 generation all pea plants with round and yellow seeds were produced. This shows that round and yellow are dominant characters whereas green and wrinkled are recessive characters. Again when these F1 plants were crossed round, yellow pea plants as well as green, wrinkled seeds pea plants were produced. But in addition to these two new characters were produced i.e., round and green, wrinkled and yellow seeds pea plants were produced.
This shows that two pair of characters combine in F1 generation but they get separated and behave independently in F2 generation 

Question: How do Mendel’s experiments show that (a) traits may be dominant or recessive?
(b) inheritance of two traits is independent of each other? 
Answer:  (a) Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the first generation (F₁). He then self crossed the tall pea plants of the F1 generation and found that tall plants and dwarf plants were obtained in the second generation or (F₂) in the ratio of 3:1. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost, it was merely concealed
or suppressed in the first generation to re-emerge in the second generation. He called the suppressed trait of ‘dwarfness’ as ‘recessive trait’ and the expressed trait of ‘tallness’ as the ‘dominant trait’. In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant or recessive. 

(b) Mendel observed that he had started with two combinations of characteristics in seeds, round-yellow and wrinkled -green, and two new combinations of characteristics had appeared in the F₂ generation, round-green and wrinkled-yellow.
According to Mendel’s second law of inheritance more than one pair of traits are considered in a cross simultaneously, the factors responsible for each pair of trait are distributed independently to the gametes.
The cross given below showing dihybrid cross explains that the inheritance of two traits is independent of each other.

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