VBQs Class 12 Biology Principles of Inheritance and Variation

VBQs For Class 12

Please refer to VBQs for Class 12 Biology Principles of Inheritance and Variation. All value based questions for Biology Class 12 have been provided with solutions. We have provided below important values questions and answers. Students should learn these solved VBQs for Class 12 Biology as these will help them to gain more marks and help improve understanding of important topics.

Principles of Inheritance and Variation VBQs Class 12 Biology with Answers

Mendel’s Laws of Inheritance

Very Short Answer Type Questions

Question. Mention any two contrasting traits with respect to seeds in pea plant that were studied by Mendel? 
Answer.Seed traits studied by Mendel in pea plant were:
(i) Seed shape – Smooth/Round (R) Wrinkled (r)
(ii) Seed (cotyledon) – Yellow (Y) Colour Green (y)

Question. Mention two contrasting flower related traits studied by Mendel in his pea plant experiments.
Answer.Traits related to flower studied by Mendel were flower colour [violet/red (V or R) dominant over white (v or r)] and flower position [Axial (A) dominant over terminal (a)].

Question. A garden pea plant (A) produced inflated yellow pod, and another plant (B) of the same species produced constricted green pods. Identify the dominant traits? 
Answer. Inflated and green colour pod are dominant traits over constricted and yellow colour pod which are recessive traits.

Question. A geneticist interested in studying variations and patterns of inheritance in living beings prefers to choose organisms for experiments with shorter life cycle. Provide a reason?
Answer.Living beings with shorter life cycles are preferred by geneticists for studying variations and patterns of inheritance because such organisms complete their life cycle in short duration and produce large number of progenies in less time span e.g., pea plant used in Mendel’s experiments.

Question. Name the contrasting pod-related traits studied by Mendel in pea plant experiment?
Answer.Traits related to pod studied by Mendel were pod shape and pod colour. Inflated (I) shaped and green coloured (G) pod were dominant traits while constricted (i) pod and yellow coloured (g) pod were recessive traits.

Inheritance of One Gene

Short Answer Type Questions

Question. Write the percentage of the pea plants that would be homozygous recessive in F2 generation when tall F1 heterozygous pea plants are selfed?
Answer. 25% of homozygous recessive is obtained in F2 generation. It can be illustrated as

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. How many kinds of phenotypes would you expect in F2 generation in a monohybrid cross exhibiting co-dominance?
Answer.Three types of phenotypes are obtained in a monohybrid cross exhibiting co-dominance. E.g., coat colour in cattle. In F2 generation three types of phenotypes were obtained – red, roan and white coat colour.

Question. When a tall plant was self-pollinated, one-fourth of the progeny were dwarf. Give the genotype of the parent and dwarf progenies.
Answer.Genotype of parent = Tt
Genotype of dwarf progeny = tt

Question. Name the respective pattern of inheritance where F1 phenotype
(a) does not resemble either of the two parents and is in between the two.
(b) resembles only one of the two parents.
Answer.(a) Incomplete dominance is the condition in which the F1 phenotype does not resemble both the parents and is in between the two.
(b) Complete dominance is the condition in which the F1 phenotype resembles the dominant parent i.e., one of the two parents.

Question. State a difference between a gene and an allele.
Answer.Differences between a gene and an allele are as follows :

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. Differentiate between dominance and codominance.
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. How would you find the genotype of an organism exhibiting a dominant phenotype trait? 
Answer.By performing a test cross, one can find the genotype of an organism exhibiting a dominant phenotypic trait.

Question. In a test cross progeny of pea plants, all were bearing violet flowers. Give the genotypes of the parent pea plant.
Answer.Genotypes of the parent pea plant : Homozygous dominant (violet – VV) and homozygous recessive (white – vv).

Question. On what basis is the skin colour in humans considered polygenic? 
Answer.The skin colour of human is controlled by three genes where the dominant alleles have cumulative effect. Each dominant allele expresses a part or unit of the trait (skin colour). Such type of genes are called polygenes and their inheritance is called as polygenic inheritance. So, the skin colour of human is a polygenic trait.

Question. Mention the type of allele that expresses itself only in homozygous state in organism.
Answer.Recessive allele, e.g., tt represents dwarf plant.

Short Answer Type Questions

Question. With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population? 
Answer.In the ABO system, there are four blood groups A, B, AB and O. ABO blood groups are controlled by gene I. The gene I has three alleles IA, IB and i. This phenomenon is known as multiple allelism. IA and IB are completely dominant over i. When IA and IB are present together they both express themselves and produce blood group AB. This phenomenon is known as codominance.

Question. In a typical monohybrid cross the F2 population ratio is written as 3:1 for phenotype but expressed as 1 : 2 : 1 for genotype. Explain with the help of an example. 
Answer.
Monohybrid cross is the cross that considers
only one trait, e.g., cross of a yellow seeded pea plant with green seeded pea plant. If pea plant with yellow seed coat is crossed with pea plant having green seed coat then in the F1 generation all the plants will produce yellow seeds. On selfing F2 generation will be as shown below:

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. Give an example of a gene responsible for multiple phenotypic expressions. What are such genes called ? State the cause that is responsible for such an effect?
Answer.Multiple alleles is the presence of more than two alleles of a gene. They are produced due to repeated mutation of the same gene but in different directions and show meristic type of germinal variations, e.g., eye colour in Drosophila. Multiple alleles occur on the same gene locus of the same chromosome or its homologue and are responsible for multiple phenotypic expression.
For example, the wild type of allele for red eye colour (w+ or W) in Drosophila melanogaster mutated to form allele for white eye (w). Further mutations in both have produced as much as 15 alleles which are recessive to wild type and dominant over white eye (w) but have incomplete intermediate dominance over one another.

Question. When does a geneticist need to carry a test cross? How is it carried?
Answer.To determine the genotype of a plant i.e., whether the individual is exhibiting dominant character is homozygous or heterozygous, a test cross is carried out by a geneticist. The individual having dominant phenotype is crossed with its homozygous recessive parent. If heterozygous tall is crossed with homozygous recessive parent, tall and dwarf will be produced, in equal proportion while if homozygous tall is crossed with homozygous recessive, the upcoming progenies will contain all tall plant.

Question. A cross was carried out between two pea plants showing the contrasting traits of height of the plants. The result of the cross showed 50% parental characters.
(a) Work out the cross with the help of a Punnett square.
(b) Name the type of the cross carried out.
Answer.Two contrasting characters of height are tall and dwarf. In the given cross, if 50% of the progeny shows parental characters, then it must be a cross between a heterozygous tall and a homozygous recessive dwarf parent.

VBQs Class 12 Biology Principles of Inheritance and Variation

(b) This type of cross is known as test cross.

Question. How does the gene ‘I’ control ABO blood groups in humans? Write the effect the gene has on the structure of red blood cells.
Answer.ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface. The kind of sugar is controlled by the gene I. The gene I has three alleles I A , I B and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar. Each person possesses any two of the three I gene alleles. I A and IB are completely dominant over i. When I A and IB are present together they both express their own types of sugar because of codominance. Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes are there in human ABO blood group.

Question. State and explain the law of segregation as proposed by Mendel in a monohybrid cross?
Answer.Principle of segregation states that, “when a pair of contrasting factor or gene are brought together in a hybrid; these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation”, i.e, allele pairs segregate during gamete formation and the paired condition is restored by random fusion of gametes during fertilisation. The above law is also known as “law of purity of gametes” because each gamete is pure in itself.

Question. In snapdragon a cross between true-breeding
red flowered (RR) plants and true-breeding
white flowered (rr) plants showed a progeny
of plants with all pink flowers.
(a) The appearance of pink flowers is not known as blending. Why?
(b) What is this phenomenon known as?
Answer.(a) When a cross is made between a red flowered plant with a white flowered plant of snapdragon, the F 1 hybrid has pink flowers. When the F 1 individual was self pollinated F 2 individuals were obtained bearing red, pink and white flowers in the ratio 1 : 2 : 1. It is not a case of blending inheritance because the parental characters appear in the F 2 generation without any change. It is due to law of seggregation which states that the members of the allelic pair that remained together in the parent, segregate during gamete formation and only one factor enters a gamete.
(b) In this neither of the two alleles of a gene is completely dominant over the other, hence the phenomenon is known as incomplete dominance.

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. Explain pleiotropy with the help of an example. 
Answer.The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of two or more characters is called pleiotropic gene. For example, in cotton a gene for the lint also influences the height of plant, size of the boll, number of ovules and viability of seeds.

Question. A woman with blood group O married a man with AB group. Show the possible blood groups of the progeny. List the alleles involved in this inheritance. 
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Possible blood groups of the progeny are A and B.The gene for blood group exists in three allelic forms IA, IB and i. Blood grouping is an example of multiple allelism.

Question. Work out a cross to find the genotype of a tall pea plant. Name the type of cross.
Answer.
Test cross is a cross used to identify whether an individual is homozygous or heterozygous for dominant character. The individual is crossed with homozygous recessive parent for the trait being investigated.
Tall plant could have two possible genotypes : TT and Tt
Case I : Tall (homozygous) pea plant crossed with dwarf pea plant :

VBQs Class 12 Biology Principles of Inheritance and Variation

If plant produces tall plants as offspring, then the genotype of plant is TT i.e., homozygous tall plant.
Case II : Tall (heterozygous) pea plant is crossed with dwarf pea plant.

VBQs Class 12 Biology Principles of Inheritance and Variation

If plant produces both tall and dwarf plants in the ratio of 1 : 1 then genotype of plant is TT, i.e., heterozygous tall pea plant.

Question. A man with blood group A married a woman with B group. They have a son with AB blood group and a daughter with blood group O.Workout the cross and show the possibility of such inheritance. 
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Parents must be heterozygous since blood group O appears in progeny. The progeny can have all the four blood groups A, B, AB and O. There are three alleles of the gene controlling blood group character, i.e., IA,IB and i. IA and IB are dominant over i and together they are codominant to each other.

Question. Tallness of pea plant is a dominant trait, while dwarfness is the alternate recessive trait.When a pure-line tall is crossed with pure-line dwarf, what fraction of tall plant in F2 shall be heterozygous? Give reasons. 
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Fraction of heterozygous tall plants in F2 = 1/2.
This can be explained by law of segregation which states that the members of the allelic pair that remained together in the parent, segregate during gamete formation and only one factor enters a gamete.

Question. Inheritance pattern of ABO blood groups in humans shows dominance, codominance and multiple allelism. Explain each concept with the help of blood group genotypes.
Answer.Dominance :
 The alleles IA and IB both are dominant over allele i as IA and IB form antigen A and antigen B respectively but i does not form any antigen.
Codominance : Both the alleles IA and IB are codominant as both of them are able to express themselves in the presence of each other in blood group AB (IAIB) by forming antigens A and B. Multiple allelism : It is the phenomenon of occurrence of a gene in more than two allelic forms on the same locus. The ABO blood groups in humans are determined by three different allelic forms IA, IB and i.

Question. Identify a, b and c in the table given below:

VBQs Class 12 Biology Principles of Inheritance and Variation

Answer.
(a) Both the forms of a trait are equally expressed in F1 generation.
(b) Dominance.
(c) Phenotypic expression of F1 generation is somewhat intermediate between the two parental forms of a trait.
Question.

VBQs Class 12 Biology Principles of Inheritance and Variation

Look at the above diagram and answer the following questions.
(a) Write the genotypes of A, B, C, D.
(b) Write the phenotypes of A, B, C, D.
(c) Write phenotypic ratio of progeny.
(d) Write genotypic ratio of progeny.
Answer.
(a) A – Tt
B – TT
C – Tt
D – tt
(b) A – Tall
B – Tall
C – Tall
D – Dwarf
(c) Phenotypic ratio is 3 : 1; Tall : Dwarf
(d) Genotypic ratio of progeny is
TT    Tt    tt
1  :  2  :  1
Homozygous : Heterozygous : Dwarf
tall                     tall

Question. In a cross between two tall pea plants some of the offsprings produced were dwarf. Show with the help of Punnett square how this is possible. 
Answer. Tall plants may either have genotype TT or Tt. Two tall pea plants that produce some dwarf plants among their progenies must be heterozygous with the genotype Tt, because TT plants cannot produce dwarf offsprings as they lack the allele for dwarfness (t) and hence cannot transfer it to the progeny. Besides, both of them should have a ‘t’ allele as dwarfness is expressed in homozygous (tt) condition only. It can be expressed using Punnett square as follows:

VBQs Class 12 Biology Principles of Inheritance and Variation

Short Answer Type Questions

Question. Human population shown variations in blood groups. Explain the genetic basis for this variation seen in the population. 
Answer.ABO blood groups are controlled by gene I. The gene I has three alleles IA, IB and i. This phenomenon is known as multiple allelism.
The blood groups and their possible genotypes are given below in the table :

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. A pea plant with purple flowers was crossed with white flowers producing 50 plants with only purple flowers. On selfing, these plants produced 482 plants with purple flowers and 162 with white flowers. What genetic mechanism accounts for these results? Explain.
Answer. In pea plant purple colour is dominant over the white colour. The cross between the two can be shown as below :

VBQs Class 12 Biology Principles of Inheritance and Variation

Mendel’s law of dominance and law of seggregation can be derived from this cross. Law of dominance states that when individuals differing in a pair of contrasting characters are crossed, the character that appears in the F1 hybrid is dominant over the alternate form that remain hidden. Principle of segregation states that, “when a pair of contrasting factor or gene are brought together in a hybrid; these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation”, i.e, allele pairs segregate during gamete formation and the paired condition is restored by random fusion of gametes during fertilisation.

Question. (a) Write the conclusions Mendel arrived at on dominance of traits on the basis of monohybrid crosses that he carried out in pea plants.
(b) Explain why a recessive allele is unable to express itself in a heterozygous state.
Answer.(a) Whenever Mendel carried out a cross between plants for a contrasting trait he found that only one trait out of the two appears in the F1 generation. He concluded that the trait which is expressed in F1 is dominant while the one which remains hidden is recessive. He also said that characters are controlled by discrete unit called factors which occur in pair.
(b) In a diploid organism, there are two copies of each gene, i.e., pair of alleles. These two alleles are not always identical, as in a heterozygote. One of them may be modified due to mutation. The unmodified functional allele that represents the original phenotype behaves as dominant allele and codes for functional protein. The mutated nonfunctional allele behaves as recessive allele and codes for mutant or non-functional protein. The phenotype of the organism will only be dependent on the functioning of the unmodified allele. Hence, in a heterozygote, the dominant allele will express itself whereas recessive allele will remain hidden.

Question. In pea plants, the colour of the flower is either violet or white whereas human skin colour shows many gradations. Explain giving reasons how it is possible. 
Answer.Flower colour in pea plant is a case of Mendelian inheritance where only one of the parental trait appears at F1. The contrasting trait did not show any blending. Human skin colour is an example of polygenic inheritance. The inheritance is controlled by three genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait.

Question. A teacher wants his/her students to find the genotype of pea plants bearing purple coloured flowers in their school garden. Name and explain the cross that will make it possible.
Answer.The genotype of pea plant that whether an individual for purple coloured flowers is homozygous or heterozygous, can be determined by test cross. Purple coloured flowers in pea plant is a dominant trait. If the individual is homozygous dominant, then all offsprings will be 100% dominant. In case of heterozygous individual, offsprings will be 50% dominant and 50% recessive. This can be explained as follows:
When plant is homozygous dominant.

VBQs Class 12 Biology Principles of Inheritance and Variation

When plant is heterozygous dominant.

VBQs Class 12 Biology Principles of Inheritance and Variation


Question. “Multiple alleles can be found only when population studies are made.” Explain with the help of an example in humans.
Answer.Human population is characterised by the presence of different blood groups although this trait is controlled by a single gene which indicates that more than two alleles of the gene are involved in the inheritance of this trait i.e., multiple allelism. Since,an individual contains only two alleles of a gene,therefore to study multiple allelism, population survey is required.
The gene involved in the inheritance of human blood group possesses four alleles which are present in different combinations that results in six genotypes and four phenotypes.

Question. The F2 progeny of a monohybrid cross showed phenotypic and genotypic ratio as 1 : 2 :1,unlike that of Mendel’s monohybrid F2 ratio.With the help of a suitable example, work out a cross and explain how it is possible.
Answer.The phenotypic and genotypic ratio of 1 : 2 : 1 in F 2 progeny of a monohybrid cross is seen in incomplete dominance. Incomplete dominance is the phenomenon where dominant allele does not completely express itself. This phenomenon has been observed in flower colour of Mirabilis jalapa or four O’ clock plant. The phenotypic as well as genotypic monohybrid ratio in F 2 generation in incomplete dominance is 1 : 2 : 1 i.e., pure dominant : hybrid : pure recessive. F 1 generation expresses a phenotype which is intermediate between those of the parent.

VBQs Class 12 Biology Principles of Inheritance and Variation

When a cross is made between a red flowered plant and a white flowered plant of Mirabilis jalapa, the Fhybrid has pink flowers. When the F1 individual was self pollinated, Findividuals were obtained bearing red, pink and white flowers in the ratio 1 : 2 : 1.

Question. How are dominance, codominance and incomplete dominance patterns of inheritance different from each other? 
Answer.In dominance, F1 is similar to the dominant parent, phenotypic ratio is different from genotypic ratio. In incomplete dominance, F1 is different from either of the two parents. Phenotypic and genotypic ratios are the same. In codominance, the effect of both the alleles is equally conspicuous. Both the alleles produce their effect independently.

Long Answer Type Questions

Question. (a) What is polygenic inheritance? Explain with the help of a suitable example.
(b) How are pleiotropic inheritance different form polygenic pattern of inheritance?
Answer.
(a)The skin colour of human is controlled by three genes where the dominant alleles have cumulative effect. Each dominant allele expresses a part or unit of the trait (skin colour). Such type of genes are called polygenes and their inheritance is called as polygenic inheritance. So, the skin colour of human is a polygenic trait.
(b) Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present. Here a cross between two pure breeding parents does not produce dominant trait of one parent but instead an intermediate trait is exhibited. Similarly in F2 generation apart from the two parental types there are several intermediate types which link the two parental traits. E.g., kernel colour in wheat, cob length in maize, skin colour in human beings, etc.
Pleiotropy is the ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously. Pleiotropy is due to effect of the gene on two or more inter-related metabolic pathways that contribute to formation of different phenotypes. It is not essential that all the traits are equally influenced. Sometimes the effect of a pleiotropic gene is more evident in case of one trait (major effect) and less evident in case of others (secondary effect). E.g., in cotton a gene for the lint also influence the height of the plant, size of the boll, number of ovules and viability of seeds.

Question. (a) Differentiate between dominance and co-dominance.
(b) Explain co-dominance taking an example of human blood groups in the population.
Answer.(a)

VBQs Class 12 Biology Principles of Inheritance and Variation

(b) Codominance is a phenomenon in which alleles do not show dominance-recessive relationship and are able to express themselves independently when present together. In human blood groups both the alleles IA and IB are codominant as both of them are able to express themselves in the presence of each other in blood group AB (IAIB) by forming antigens A and B.

Question. (a) A couple with blood groups ‘A’ and ‘B’ respectively have a child with blood group ‘O’.Work out a cross to show how it is possible and the probable blood groups that can be expected in their other off-springs.
(b) Explain the genetic basis of blood groups in human population.
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Parents must be heterozygous since blood group O appears in progeny. The progeny can have all the four blood groups A, B, AB and O. There are three alleles of the gene controlling blood group character, i.e., IA,IB and i. IA and IB are dominant over i and together they are codominant to each other.

Question. (a) State and explain the law of dominance as proposed by Mendel.
(b) How would phenotypes of monohybrid F1 and F2 progeny showing incomplete dominance in snapdragon and co-dominance in human blood group be different from Mendelian monohybrid F1 and F2 progeny? Explain.
Answer.(a) Law of dominance : According to this law,characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other dissimilar pair. This law explains expression of only one of the parental character in F1 generation.
This can be explained by the following cross :

VBQs Class 12 Biology Principles of Inheritance and Variation

In the given cross, the trait producing yellow seeds is dominant over the trait producing green seeds. In F1 generation all offsprings showed yellow colour of seed (dominant character) and no green seeds plants were obtained.
(b)

VBQs Class 12 Biology Principles of Inheritance and Variation
VBQs Class 12 Biology Principles of Inheritance and Variation

Question. What is the inheritance pattern observed in the size of starch grains and seed shape of Pisum sativum? Workout the monohybrid cross showing the above traits. How does this pattern of inheritance deviate from that of Mendelian law of dominance? 
Answer.The starch synthesis in pea plants is controlled by a single gene. It has two alleles B and b. Homozygous for BB produced large starch grains as compared to that produced by plants which are homozygous for bb. After maturation it was observed that BB seeds were round and bb were wrinkled. When they were crossed the result and progeny were intermediate size Bb seeds showing round seeds. The cross involved is:

VBQs Class 12 Biology Principles of Inheritance and Variation

Phenotype : 1 Large : 2 Intermediate : 1 Small But in case of seed shape the phenotype is 3 : 1;
Round : Wrinkled.
Deviation from Mendel’s law of dominance : If starch grain size is considered as the phenotype, then the alleles show incomplete dominance. Thus,
dominance is not an autonomous feature of a gene, it depends on gene product and production of particular phenotype from this product.

Question. How do “pleiotropy”, “incomplete dominance”,“co-dominance” and “polygenic inheritance” deviate from the observation made by Mendel?Explain with the help of one example for each. 
Answer. Mendel discovered laws of inheritance. According to law of dominance when two individuals of a species, differing in a pair of contrasting forms of a trait are crossed, the form of the trait that appears in the F1 hybrid is dominant and the alternate form that remains hidden, is called recessive. Incomplete dominance and co-dominance are exception to this law.
Incomplete dominance is the phenomenon where none of the two contrasting alleles or factors is dominant. The expression of the character in a hybrid or F1 individual is intermediate or a fine mixture of the expression of the two factors. As seen in Mirabilis jalapa where when two types of plants having flower colour in pure state red and white are crossed, the hybrid or F1 generation have pink flowers.
Co-dominance is the phenomenon of expression of both the alleles in a heterozygote, i.e., both alleles are able to express themselves independently when present together. E.g., hair colour in cattle. When red cattle are crossed with white cattle, the hybrid of F1 generation are of roan colour i.e., having a dark coat interspered with white hair.
According to Mendel one gene control the expression of one character only. Pleiotropy is exception to this. The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy. The gene having a multiple phenotypic effect because of its ability to control expression of two or more characters is called pleiotropic gene. For example, in cotton a gene for the lint also influences the height of plant, size of the boll, number of ovules and viability of seeds.
Polygenic Inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present.
The genes involved in quantitative inheritance are called polygenes and inheritance called as polygenic inheritance. E.g., human skin colour. Human skin colour is caused by pigment called melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). If black or very dark (AABBCC) and white or very light (aabbcc) individuals marry, the offspring show intermediate colour called mulatto (AaBbCc). When two such individuals of intermediate colour marry, the skin colour of the children will vary from very dark or black to very light or white. A total of eight allele combinations is possible in the gametes forming 27 distinct genotypes distributed into 7 phenotypes.

Question. Work out a monohybrid cross upto F2 generation between two pea plants and two Antirrhinum plants both having contrasting traits with respect to colour of flower. Comment on the pattern of inheritance in the crosses carried above.
Answer. In case of pea plant

VBQs Class 12 Biology Principles of Inheritance and Variation
VBQs Class 12 Biology Principles of Inheritance and Variation

The inheritance pattern of flower colour in garden pea plant is an example of complete dominance whereas inheritance pattern of flower colour in Antirrhinum is an example of incomplete dominance.

Question. (a) How are Mendelian inheritance, polygenic inheritance and pleiotropy different from each other?
(b) Explain polygenic inheritance pattern with the help of a suitable example. 
Answer.(a) Mendelian Inheritance : Mendelian inheritance is a type of inheritance controlled by one or more genes in which only dominant trait was expressed in the F1 generation while at the F2 stage both the traits were expressed. The contrasting traits did not show any blending at either F1 or F2 stage on the basis of this Mendel proposed three laws: (a) Law of Dominance, (b) Law of Segregation and (c) Law of Independent Assortment.
Polygenic Inheritance : Polygenic inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each dominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present.
Pleiotropy: The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotropy.
Pleiotropy is due to effect of the gene on two or more inter-related metabolic pathways that contribute to formation of different phenotypes.
(b) Human skin colour is an example of polygenic inheritance. Human skin colour is caused by pigment melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). If black or very dark (AABBCC) and white or very light (aabbcc) individuals marry, the offsprings or individuals of F1 generation show intermediate colour often called mulatto (AaBbCc). When two such individuals of intermediate colour marry, the skin colour of the children will vary from very dark or black to very light or white. A total of eight allele combinations is possible in the gametes forming 27 distinct genotypes distributed into 7 phenotypes-1 very dark, 6 dark, 15 fairly dark, 20 intermediate, 15 fairly light, 6 light and 1 very light.

Sex Determination

Very Short Answer Type Questions

Question. How many chromosomes do drones of honeybee possess? Name the type of cell division involved in the production of sperms by them.
Answer. Drones of honeybees are haploid and possess 16 chromosomes. Mitosis is involved in the production of sperms.

Question. Identify the correct statement.
(a) Female of many birds has a pair of dissimilar ZW chromosomes, while the males possess a pair of similar ZZ chromosomes.
(b) Female of many birds has a pair of similar ZZ chromosomes, while the males possess a pair of dissimilar ZW chromosomes.
Answer. Statement (a) is correct. In birds, the male has two homomorphic sex chromosomes (ZZ) and is homogametic, and the female has two heteromorphic sex chromosomes (ZW) and is heterogametic.

Question. Identify and write the correct statement :
(a) Drosophila male has one X and one Y chromosome.
(b) Drosophila male has two X chromosomes.
Answer. Drosophila male has one X and one Y chromosome.

Question. A male honeybee has 16 chromosomes whereas its female has 32 chromosomes. Give one reason. 
Answer. In honeybees an unfertilised egg develops into a male and a fertilised egg develops into a female.
Therefore, the female is diploid (2n), and the male is haploid (n).

Question. Identify and write the correct statement :
(a) In grasshopper males two sex chromosomes are X and Y types.
(b) In grasshopper males there exist XO type of sex – determinants. 
Answer. In grasshopper males, there exist XO type of sex-determinants.

Short Answer Type Questions

Question. Explain why it is scientifically incorrect to blame the mother for bearing female child.
Answer. In humans, sex of the child is determined at the time of fertilisation. The female parent produces only one type of egg with X-chromosome. The male gametes are of two types with X-chromosome and Y-chromosome. Fertilisation of the egg with sperm carrying X-chromosome produces a female child while fertilisation with sperm carrying Y-chromosome give rise to male child. Thus sex of the child is determined by father and not by the mother.
Hence, it is scientifically incorrect to blame the mother for bearing female child.

Question. Explain mechanism of sex-determination in birds. 
Answer. Birds have ZW – ZZ type of sex determination mechanism. In this type the male has two homomorphic sex chromosomes (ZZ) and is homogametic, and the female has two heteromorphic sex chromosomes (ZW) and is heterogametic. There are, thus, two types of eggs: with Z and with W, and only one type of sperms, i.e., each with Z. Fertilisation of an egg with Z chromosome by a sperm with Z chromosome gives a zygote with ZZ chromosomes (male). Fertilisation of an egg with W chromosome by a sperm with Z chromosome yields a zygote with ZW chromosomes (female).

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. Differentiate between ‘ZZ’ and ‘XY’ type of sex-determination mechanisms.
Answer.

VBQs Class 12 Biology Principles of Inheritance and Variation

Question. Differentiate between male and female heterogamety. 
Answer. The type of sex determination mechanism shown in female XX with male XY is called male heterogamety because male produces two different types of gametes. Example – Drosophila
The type of sex determination mechanism shown in female ZW with male ZZ is female heterogamety because female produces two different types of gametes. Example – Birds

Question. The male fruit fly and female fowl are heterogametic while the female fruit fly and the male fowl are homogametic. Why are they called so? 
Answer. Male fruit fly and female fowl produce two types of gametes whereas female fruit fly and male fowl produce only one type of gamete.
(i) The type of sex determination mechanism shown in female XX with male XY is called male heterogamety because male produces two different types of gametes. Example – Drosophila.
(ii) The type of sex determination mechanism shown in female ZW with male ZZ is female heterogamety because female produces two different types of gametes.
Example – Birds

Question. Write the types of sex-determination mechanisms the following crosses show. Give an example of each type.
(a) Female XX with male XO
(b) Female ZW with male ZZ. 
Answer. (a) The type of sex determination mechanism shown in female XX with male XO is called male heterogamety. In this case male are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.
Example – Grasshopper
(b) The type of sex determination mechanism shown in female ZW with male ZZ is female heterogamety because female produces two different types of gametes.
Example – Birds

Question. Do you agree to the perception in our society that the woman is responsibile for the gender.Substantiate your answer scientifically.
Answer. No, the perception in our society that the woman is responsible for the gender of the child is totally wrong. It is the male who is responsible for this. 

Long Answer Type Questions

Question. (a) Why are grasshopper and Drosophila said to show male heterogamety? Explain.
(b) Explain female heterogamety with the help of an example.
Answer. (a) In male heterogamety, males produce two different types of gametes. In human and Drosophila the males have one X and one Y chromosome, whereas in grasshopper the male have only one X-chromosome
(XO type). Thus, the males of these organisms show male heterogamety as they produce (i) gametes either with or without X-chromosome or (ii) some gametes
with X-chromosome and some with Y-chromosome.
(b) In some organisms female produce two different types of gametes. This is termed as female heterogamety. In birds and some reptiles female has two different sex chromosomes (one Z and one W chromosome) whereas male has a pair of same chromosome (a pair of Z-chromosomes).

Question. Explain the mechanism of sex determination in insects like Drosophila and grasshopper.
Answer.In Drosophila, XX–YY type of sex determination has been found, in which the female is homogametic and produces only one type of eggs (22 + X). The male gametes are of two types, androsperms (22 + Y) and gynosperms (22 + X). They are produced in equal proportion. Fertilisation of the egg (22 + X) with a gynosperm (22 + X) will produce a female child (44 + XX) while fertilisation with an androsperm (22 + Y) gives rise to male child (44 + XY). In roundworms and some insects (true bugs, grasshoppers, cockroaches), the type of sex determination is XX – X0 type. The females have two sex chromosomes, XX, while the males have only one sex chromosome. There is no second sex chromosome. Therefore, the males are designated as X0.

Question. Explain the sex determination mechanism in humans. How is it different in birds?
Answer. Chromosomal determination of sex in human beings is of XX-XY type. Human beings have 22 pairs of autosomes and one pair of sex chromosomes. The female possess two homomorphic (= isomorphic) sex chromosomes, named XX. The males contain two heteromorphic sex chromosomes, i.e., XY.
All the ova formed by female are similar in their chromosome type (22 + X). Therefore, females are homogametic. The male gametes or sperms produced by human males are of two types, gynosperms (22 + X) and androsperms (22 + Y). Human males are therefore, heterogametic. Sex of the offspring is determined at the time of fertilisation. Fertilisation of the egg (22 + X) with a gynosperm (22 + X) will produce a female child (44 + XX) while fertilisation with an androsperm (22 + Y) gives rise to male child (44 + XY). As the two types of sperms are produced in equal proportions, there are equal chances of getting a male or female child in a particular mating. As Y-chromosomes determines the male sex of the individual, it is also called androsome.

VBQs Class 12 Biology Principles of Inheritance and Variation

In case of birds, the type of sex determination is ZW-ZZ type. Female has two different sex chromosomes (Z and W) whereas as male has a pair of same chromosomes (ZZ), therefore, in birds, sex is determined by female.