Notes And Questions For NCERT Class 10 Science Heredity And Evolution

Students should refer to the below Heredity And Evolution Class 10 notes prepared as per the latest curriculum issued by CBSE and NCERT. These notes and questions are really useful as they have been developed based on the most scoring topics and expected questions in upcoming examinations for Class 10. Chemical Reactions and Equations is an important topic in Science Class 10 which if understood properly can help students to get very good marks in class tests and exams.

Heredity And Evolution Class 10 Notes and Questions PDF Download

Read the notes below which will help you to understand all important and difficult topics in this chapter. There are some topics in Heredity And Evolution chapter which you should understand carefully as many questions can come from those parts. Our team of teachers have designed the revision notes so that its helpful for students to revise entire course prior to the class tests.

INTRODUCTION

• Variation is the degree of difference found in morphological, physiological & other traits found among individuals belonging to the same family race & species.
• Most of the variation are due to mutations caused by error in DNA replication.

ACCUMULATION OF VARIATIONS DURING REPRODUCTION

• Variation appear during reproduction whether organisms are multiplying asexually or sexually.
• Each generation provides the next generation with a common basic body design and some subtle changes or variations
• The variations accumulate and pass on to more and more individuals with each generation.
• The diversity is small in case of asexually reproducing organism as it is caused by only errors of DNA copying mechanism.

SIGNIFICANCE OF VARIATIONS

Preadaptations :

• Some of the neutral variations function as preadaptation to changing environment. For example, heat wave may kill most bacteria except a few which have a pre-adaptation or variation to tolerate high temperature. 

Evolution :
Variations are raw materials for evolutionary processes. Picking up suitable variants by nature or environment forms the basis of evolution. 

Struggle for Existence :
Useful variation provide advantage to the individuals in the struggle for existence and hence survival in nature.

Individuality :

• Variations provide a distinct identity to each and every individual. 

Artificial Selection :

• Picking up of certain variations by breeders have resulted in development of a number of breeds and varieties of domesticated plants and animals 

HEREDITY
Heredity (L. hereditas – heirship or inheritance) is the transmission of genetic characters from parents to offspring or one generation to the next
Inherited Traits :  They are those traits which are controlled by getnetic material of the individuals and are obtained from the parents in inheritance. 

MENDELISM
Mendel was a monk in Austria.
Mendel performed a number of experiments on the gardan pea plants (Pisum sativum).

Mendel’s Law :

• The theoretical explanation of mendel’s results are now firmly establisas Mendel’s laws of inheritance.
• These are as follows :

Mendel’s law of Dominance :

• According to this law, if a cross is made between plants with contrasting pair of charactess, the character that appears in the first generation is dominant and the other is recessive.

Mendel’s monohybrid cross between a homozygous red flowered (RR) and a homozygous white flowered (rr) plant to show that red colour is dominant over white (recessive)
Mendel’s law of segregation (or law of purity of gametes) :

• Both parental allels (i.e. dominant as well as recessive) segregate or separate and are expressed phenotypically in F2 generation, and this is called mendel’s Law of segregation.

Mendel’s Law of Independent Assortment :

• Which states that most of the characters of parents can appear in any combination in their offsprings

Fig: Result of Dihybrid cross between pea plants having round yellow seeds and wrinkled green seeds.

SEX DETERMINATION
It is a process by which the sex of a person is determined. Genetics is involved in the determination of the sex of a person, which is explained as follows :

• A male has one X chromosome and one Y chromosome, i.e., half of the male gamete or sperms will have X chromosome and the other half will have Y chromosome.
• A female has two X chromosomes, i.e., all the female gametes or ova will have only X chromosomes.
• Sex of a child depends on what happens at fertilisation.
• If a sperm carrying X chromosome fertilises an ovum which carries X chromosome, then the child born will be a girl.
• If a sperm carrying Y chromosome fertilises an ovum which carries X chromosome, then the child born will be a boy.
• Thus, the sperm determines the sex of the child.
• Sex determination is also controlled by the environmental factors in some animals.
• For example, in some reptiles like turtle, high incubation temperature leads to the development of female offsprings, while in case of lizard. high incubation temperature results in male offsprings.

EVOLUTION
Evolution (L.evolvere – to unfold) or organic evolution (Spencer, 1852) is the unfolding of nature wherein newer types of organisms develop from the pre-existing ones through modification.

Acquired Traits :
Acquired traits are those variations which an individual develops during its life time due to effect of environmental factors, use and disuse of organs and conscious efforts.
Theory of Inheritance of Acquired Characters (Lamarckism). It is the first theory of evolution that was proposed by French biologist Jean Baptiste de Lamarck. It is based on the following

• Internal Vital Force : There is a vital force in organisms which rends to change them, generally making them larger and more complex.
• Environment : A change in environment brings about direct changes in plants. It produces new needs in animals.
• New needs (Doctrine of Desires), New need being about new desires that result in changes of older and formation of new organs.
• Use and Disuse of Organs, Repeated use of an organ makes it more complex and efficient. Non-use of an organ brings about its degeneration.
• Inheritance of Acquired Characters. The traits acquired by an individual during its life time are passed on to the next generation. After several generations the accumulation changes results in the formation of new species. 

Inherited traits :

• Inherited traits are those characteristics which are passed from parents to their offspring, generation after generation because they are controlled by genes. 
• There is also reshuffling of inherited variations during gemetogenesis and fertilization.

SPECIATION
It is the formation of new species from an existing one due to reproductive isolation of a section of its population.
Development of reproductive isolation is basic to formation of new species. This can occur by the following methods

Physical Barrier :

• A physical barrier like valley, mountain, water body, etc. develop between two populations of a species.

Two Ends of Long Range :

• Subpopulations at the two ends of a long range seldom interbreed.

Mutations :

• A large mutation can make some members reproductively isolated from the rest.

Genetic Drift (Wright Effect) : 

• It is random change in gene frequency that occurs in a small population due to fixation of certain alleles and elimination of others.

EVIDENCE OF EVULOTION

Homologus Organs :

• These organs are similar in structure but disimilar in functions.
• Homologous organs are found in forms showing adaptive radiation from a common ancestor so these give evidence of ‘divergent evolution’.
• Ex. forelimbs of mammals, bat’s wing, a cat’s paw.
• In plants homologous organs may be a thorn or a tendril as they arise in the axillary position. If homology is seen at the molecular level. This is called molecular homology.

Analogous Organs :

• These organs are perform similar function but quite different in origin and development.
• Analogous organs are also called homoplastic organs.
• Analogous organs like wings of insects, birds and bats illustrate ‘convergent evolution’.
• Ex. – Wings of birds, wings of insects show the same function but their origin and development is different.

Vestigial Organs :

• Vestigial organs are useless organs found in the body. These are present in reduced form.
• Ex. wings of kiwi, limbs of snakes, the nictitating membrane of the eye, body hair and the muscles that move the ears.
• During the period of evolution vestigial organs have become functionless.

Fossil Evidences :

• Fossils are the remains of the organisms that found in the ancient period. We can determine fossil’s age through the period of rock by radioactive material. Histological time scale is a chronological order of evolution history based upon the study of fossils.
• Study of fossils known as palaeontology.

Petrification :

• Hard parts are preserved in this type of fossil.

Archaeopteryx :

• A fossil bird show both reptilian and avian features – It had wing like birds, thecodent dentition and tail like reptiles. Hence, archeopteryx is a connecting link between birds and reptiles.

Connecting links :

Embryological Evidences:
The study of embryo from variours organisms reveals similarity in the early stages of embryo development & this theory suggests that these organism have evolved from common ancestors.
Eg. Embryos of fish, turtle, bird, pig & man shows the similarity during embryo development.

Important Questions Heredity And Evolution Class 10 Science

Very Short Answer Type Questions :

Question. What indication do we get by appearance of dwarf plant in F₂ generation?
Answer: After obtaining F₂ progeny from F₁ generation in the dihybrid cross, Mendel concluded that when two pairs of traits are combined in a hybrid, one pair of character segregates independently from the other pair of character.

Question. Why is it that asexual reproduction produces exact copies but sometimes minor variations are seen in next progeny?
Answer: Bio-chemical reactions are not fully reliable, therefore, it may cause slight difference which causes these variations.

Question. Name the term used for traits that are exhibited externally. 
Answer: Phenotype.

Question. What is F₂ generation? 
Answer: The generation produced by the offsprings of F₁ generation is called F₂ or second generation.

Question. Give an example where sex determination is regulated by environmental factors. 
Answer: In snail, sex is determined by temperature i.e., environmental factor.

Question. In order to ensure than Mendel gets pure breeding plants in his experiments, what did he do?
Answer: Mendel allowed each variety to self fertilise for several generations.

Question. Where is DNA found in the cell? 
Answer: DNA is found in the nucleus of the eukaryotic cells and cytoplasm of prokaryotic cells. It is also present in plastids and mitochondria.

Question. What is heredity? 
Answer: Heredity is a process in which traits are passed onto the offspring from parents.

Question. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny? 
Answer: Violet flowers

Question. How does the creation of variations in a species promote survival? 
Answer: Variations lead to adaptation of an organism, which is then able to survive in difficult and unfavourable environmental conditions. It means variations promote survival.

Question. What is variation?
Answer: Variation is the difference in characteristics or traits among the individuals in a particular species.

Question. A tall plant is crossed with a dwarf plant. What will be the ratio of homozygous tall and heterozygous dwarf plant in F2 generation?
Answer: The ratio of homozygous tall and heterozygous dwarf plant in F2 generation is 3 : 1.

Question. What is the scientific name of man and garden pea?
Answer: Man — Homo sapiens
Garden pea — Pisum sativum

Question. Name the human traits which show variations. 
Answer: (i) Colour of eye, (ii) Height, (iii) Colour of skin

Question. How can chromosomes be identified? 
Answer: Individual chromosomes can be identified by their lengths, position of centromere and binding pattern of staining and shape.

Question. What is sex of the baby that inherits Y-chromosome from the father?
Answer: Male

Question. Give the respective scientific names used for studying:
(i) the mechanism by which variations are created and inherited by organism from the previous generation.
(ii) the development of new type of organisms from the existing ones. 
Answer: (i) Heredity (ii) Evolution

Question. What is a gene? 
Answer: Gene is the unit of heredity present in chromosomes, DNA which directs specific protein synthesis, when it is passed from the parent to the offspring.

Question. Name the information source for making proteins in the cells. 
Answer: DNA

Question. What is DNA? 
Answer: DNA stands for deoxyribonucleic acid. It is self replicative, molecule present in all living organisms as the main constituent of chromosomes. It passes on genetic characteristics to offsprings.

Question. State Mendel’s first law of inheritance.
Answer: Mendel’s first law, also known as the law of segregation states that ‘during gamete formation, the alleles of the character segregate in such a way that each gamete carries only one allele for each gene’.

Question. If YYRR is round yellow, what do the following represent? 
Answer: yyrr – wrinkled, green seeds
yyRR – Round, green seeds.

Question. Where are genes located? What is the chemical nature of genes?
Answer: Genes are located on chromosomes at fixed positions. They are made of nucleic acid (DNA) therefore acidic in nature.

Short Answer Type Questions :

Question. In a pea plant, find the contrasting trait if
(i) The position of flower is terminal.
(ii) The flower is white in colour
(iii) Shape of pod is constricted.
Answer: (a) Axial position of flower.
(b) The flower is violet in colour.
(c) Shape of pod is inflated.

Question. What do you understand by the following terms:
(i) Phenotype (ii) Genotype
(iii) Dominant trait (iv) Recessive trait
(v) Factors or Genes
Answer: (i) Phenotype: The set of characteristics observed in an individual as a result of interaction of its genotype with the environment.
(ii) Genotype: The genetic make up of the cell which determines the characteristics (phenotype) of an organism.
(iii) Dominant trait: The trait of the parent that expresses itself in the offspring.
(iv) Recessive trait: The trait of the parent appearing in the offspring which cannot express itself in the presence of dominant trait.
(v) Factors or Genes: The units of inheritance responsible for a trait in an individual.

Question. A cross was carried out between pure breed tall pea plant with pure dwarf pea plant and F₁ progeny was obtained. Later, F₁, progeny was selfed to obtain F2 progeny. Answer the following questions:
(a) What is the phenotype of the F1 progeny and why?
(b) Give the phenotypic ratio of the F2 progeny.
(c) Why is F₂ progeny different from the F1 progeny?
Answer: (a) Tall because it is a dominant trait whereas dwarf is a recessive trait.
(b) 3 : 1 is ratio between tall to dwarf.
(c) In F₂ generation, recessive genes are also expressed in homozygous condition.

Question. ‘DNA is the carrier of the genetic information’. Justify.
Answer: Most of the characters or traits of an organism are controlled by the genes. Genes are actually segments of DNA guiding the formation of proteins by the cellular organelles. These proteins may be enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA (genes) is responsible for structure and
functioning of a living body. Genotype of an individual controls its phenotype.
In human beings there are 23 pairs of chromosome. We inherit half of our chromosomes (genes) from our two parents. This means half of DNA in each cell has been inherited from each parent, hence we resemble both of them or their side of family members.

Question. A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything whether the light eye colour trait is dominant or recessive? Why or why not? 
Answer: We cannot predict because information is not sufficient. We can decide dominant or recessive trait if we have the data for at least three generations. More information is required.

Question. (a) On what rules inheritance is based? 
(b) Is each trait influenced by both parental and maternal DNA?
Answer: (a) (i) Law of dominance. (ii) Law of segregation (iii) Law of independent assortment
(b) Yes, each trait is influenced by DNA of both parents.

Question. In a monohybird cross between tall pea plants denoted by TT and short pea plant by tt, Sehaj Anant obtained only tall plants denoted by Tt in F1 generation. However in F2 generation she obtained both tall and short plants. Using the above information explain the law of dominance. 
Answer: According to law of dominance, dominant trait is expressed in F1 generation although dominant as well as recessive traits are inherited. Single copy of dominant trait is sufficient for expression of tall dominant trait in F1 generation.

Question. List two differences in tabular form between dominant traits and recessive traits.
Answer:

Question. Study the following cross showing self pollination in F1, fill in the blank and answer the question that follow:
Parents RRYY × rryy
Round, yellow Wrinkled, green
F1 Rr Yy × ……………………..
What are the combinations of character in the F2 progeny? What are their ratios?
Answer: F1 progeny is Rr Yy-Round, Yellow
Combinations of character in the F2 progeny are:
(i) Round yellow — 9 (Both dominant traits)
(ii) Round green — 3 (One recessive, one dominant)
(iii) Wrinkled yellow — 3 (One dominant, one recessive)
(iv) Wrinkled green — 1 (both recessive traits)
The ratio is 9 : 3 : 3 : 1

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: Chromosomes are long thread like structures which contain heriditary information of individual and are carrier of genes.
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, a type of cell division which reduces chromosome number to half. When these haploid gametes fuse during fertilisation, the two nuclei of these 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. After self-pollination in pea plants with round, yellow seeds, following types of seeds were obtained by Mendel:

Analyse the result and describe the mechanism of inheritance which explains these results
Answer: The ratio obtained is 9:3:3:1 in which parental as well as new combinations are observed. This indicates that progeny plants have not inherited a single whole gene set from each parent. Every germ cell takes one chromosome from the pair of maternal and paternal chromosomes. When two germ cells combine, segregation of one pair of characters is independent of other pair of characters.

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: Justification: Women produce only one type of ovum (carrying X chromosome) and males produce two types of sperms (carrying either X or Y chromosome) in equal proportions. So the sex of a child is a matter of chance depending upon the type of sperm fertilising the ovum.