VBQs Principles of Inheritance and Variation Class 12 Biology

VBQs Principles of Inheritance and Variation Class 12 Biology with solutions has been provided below for standard students. We have provided chapter wise VBQ for Class 12 Biology with solutions. The following Principles of Inheritance and Variation Class 12 Biology value based questions with answers will come in your exams. Students should understand the concepts and learn the solved cased based VBQs provided below. This will help you to get better marks in class 12 examinations.

Principles of Inheritance and Variation VBQs Class 12 Biology

Very Short Answer Type Questions

Question. Write technical term used in human ABO blood groups for IA, IB and i 
Answer : IA, IB and i are the alleles of the gene I.

Question. Give an example of a polygenic trait in humans.
Answer : Skin colour/height in humans (any other suitable example)

Question. If two genes are located far apart from each other on a chromosome, what will be its effect on the frequency of recombination ?
Answer : Frequency of recombination will be higher.
Detailed Answer :
Frequency of recombinations will be higher because of greater chances of crossing over and also because of the lesser strength of linkage as the strength of the linkage is inversely proportional to the distance between the genes located on the chromosome.

Question. Name the type of cross that would help to find the genotype of a plant bearing violet flowers.
Answer : Crossing of an F₁ hybrid with it’s recessive plant is called test cross. It is used to find out the unknown genotype.

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 : Many generations can be obtained/variations can be exhibited/selected faster. 
Detailed Answer:
A geneticist choose organism with a short life cycle because the organism will produce the offspring quickly in a short period of time. These offsprings can be studied by the geneticist and can also be further mated to produce more generation of offsprings.

Question. What are true breeding lines’ that are used to study inheritance pattern of traits in plants ?
Answer : Self pollination continuous for several generations / homozygous. 
Detailed Answer :
True breeding lines for a trait are those plants that have been generated through repeated continuous self-pollination and have become homozygous for a particular trait. They show stability in the inheritance of trait for several future generation. 

Question. How many types of phenotypes would you expect in F₂ generation in a monohybrid cross ? 
Answer : Two types in the ratio of 3 : 1. 1

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, green pods. 

Question. Write the percentage of the pea plants that would be heterozygous tall in F₂ generation when tall heterozygous F₁ pea plants are selfed.
Answer : 50% heterozygous tall pea plants would be obtained in F₂ generation when tall heterozygous pea plants are selfed. 

Question. Mention two contrasting flower-related traits studied by Mendel in his pea plant experiments.
Answer : Contrasting flower-related traits :
(i) Flower colour : violet/white or red/white.
(ii) Flower position : Axial/Terminal. 

Question. Name the respective pattern of inheritance where F₁ phenotype.
(i) does not resemble either of the two parents and is in between the two.
(ii) resemble only one of the parents.
Answer : (i) Incomplete dominance
(ii) Dominance. 

Question. AaBb was crossed with aabb. What would be the phenotypic ratio of the progeny ? Mention the term used to denote this kind of cross ?
Answer : Phenotypic ratio would be 1:1:1:1. It is test cross.

Short Answer Type Questions

Question. State the Mendelian principle which can be derived from a dihybrid cross and not from monohybrid cross.
Answer : From the dihybrid cross, law of independent assortment can be derived which states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

Question. Write the scientific name of the fruit-fly. Why did Morgan prefer to work with fruit-flies for his experiments ? State any three reasons.
Answer : Drosophila melanogaster 
Grown in simple synthetic medium, complete the life cycle in two weeks / short life cycle, single mating produce more progeny, dimorphism, many heritable variations / easy to handle.
Detailed Answer :
Morgan preferred to work with fruit flies because of the following reasons:
(i) It can be grown in simple synthetic medium.
(ii) It can complete its life cycle within two weeks i.e., it has short life cycle.
(iii) Single mating produce more progenies.
(iv) It shows dimorphism and many heritable variations.
(v) It is easy to handle. 

Question. In Snapdragon, a cross between true breeding red flower (RR) plants and true breeding white flower (rr) plants showed a progeny of plants with all pink flowers.
(i) The appearance of pink flowers is not known as blending. Why ?
(ii) What is the phenomenon known as ?
Answer : (i) R (dominant allele red colour) is not completely dominant over r (recessive allele white colour) / r maintains its originality and reappear in F₂ generation. 
(ii) Incomplete dominance. 
Detailed Answer :
(i) The appearance of pink flowers in snapdragon in F₁ generation is not due to blending of genes Rr which are although together in hybrid, because the parental characters i.e. red and white flowers appear again in F₂ generation without any change.
(ii) Neither of the two alleles are completely dominant over each other therefore the phenomenon is called as incomplete dominance.

Question. Study the figures given below and answer the question.

Identify in which of the crosses the strength of linkage between the genes is higher. Give reasons in support of your answer.
Answer : Cross A, because they are tightly linked / due to close physical association / they are closely located.
Detailed Answer :
Strength of linkage between genes is higher in cross A than that of cross B because the two genes yw are located closely on the same chromosome. Whereas in case of cross B the genes w and m are located far apart on the same chromosome. Therefore, in the latter cross the chances of recombination are higher for crossing over because lesser the distance between genes greater the strength of linkage.

Question. With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population.
Answer : ABO blood group in human being is an example of multiple allelism.
Three alleles for the gene I i.e. I^A, I^B, i.
When I^A and I^B are present together the blood group is AB.
Both A and B are expressed and is called co-dominance.
Detailed Answer :
ABO blood group in human population is an example of multiple allelism and co-dominance.
There are three alleles of gene I, i.e., IA, IB and i. thus exhibiting multiple allelism. Out of these three alleles any two alleles may occur in human being, one alleles is contributed from each parent. The alleles I^A and I^B are dominant over allele i and produce A and B phenotypes respectively. But however when alleles I^A and I^B are present together both of them express equally and independently producing AB phenotype. This is a phenomenon called co-dominance.

Question. Differentiate between multiple allelism and pleiotropy with the help of an example each.
Answer : Multiple alleles : More than two alternate forms of a gene present on the same locus of a homologous pair of chromosomes in a population are called multiple alleles. They control the single trait. For example, ABO blood group in humans.
Pleiotropic genes : The gene having a multiple phenotypic effect because of its ability to control the expression of a number of characters is called pleiotropic gene. For example, skin pigmentation and phenylketonuria. 

Question. List all three different allelic forms of gene I in humans. Explain different phenotypic expressions controlled by these three forms.
Answer : The three different forms of gene I are I^A, I^B and i.
ABO blood grouping is controlled by these three alleles, hence it is an example of multiple allelism.
Each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over allele
(i) while I^A and I^B are co-dominant because both of them express themselves equally and independently when present together. As there are three alleles of gene I there would be following six types of allelic combinations of genotypes resulting in four phenotypic expressions as follows :

Question. ABO blood groups is a good example of codominance. Justify.
Answer : (i) ABO blood group in humans is contributed by gene ‘I’ that has 3 alleles ‘I^A‘ ‘I^B‘ and ‘i.’
(ii) Because human beings are diploid and each person has two of the three alleles.
(iii) I^A and I^B produce two different types of sugar while allele i does not produce sugar on the plasma membrane of RBC.
(iv) When I^A and I^B present they produce their own type of sugar-this is called co-dominance.

Long Answer Type Questions

Question. (i) A couple with blood group ‘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.
(ii) Explain the genetic basis of blood groups in human population.
Answer : (i) Father = I^Ai
Mother = I^Bi
Child = ii

Phenotypes of all offsprings = AB, B, A and O blood group
(ii) Genetic basis of blood group :
Three alleles of one gene/multiple alleles/gene I^A, I^B, i^A and B are co-dominant/expressed together.

Question. (i) State the cause and symptoms of colour blindness in humans.
(ii) Statistical data has shown that 8% of the human males are colour-blind whereas only 0.4% of females are colour-blind. Explain giving reasons how is it so. 
Answer : (i) Cause-sex-linked recessive disorder.
Symptoms-failure to discriminate between red and green colour.
(ii) Since males have only one X chromosome gene for colour blindness, if present in any one parent will always be expressed, whereas in female it will be expressed only if it is present on both the X chromosomes, when both parents are carrying gene for colour blindness. 
Detailed Answer :
(i) The colour blindness is due to recessive sex linked disorder. In this disorder of vision the patient is unable to distinguish between red and green colour. The gene for colour blindness
is located on the X chromosome.
(ii) The colour blindness is found in about 8% of the males and only 0.4% of the females. The greater prevalence of the colour blindness in males is due to the presence of only one X chromosome and the hemizygous (X & Y) expression of the allele for colour blindness i.e. if gene for colour blindness is present on X-chromosome of male then it will always express while in case of females the incidence of disease of colour blindness is possible only in homozygous condition (X^CX^C) i.e. if both the X-chromosomes carry the allele (X^CX^C) for colour blindness. Occurrence of allele for colour blindness on one of the X chromosome makes the female a carrier X^CX.

Question. (i) How does a chromosomal disorder differ from a Mendelian disorder ?
(ii) Name any two chromosomal aberrations associated disorders.
(iii) List the characteristics of the disorders mentioned  above that help in their diagnosis.
Answer : (i) Mendelian disorders are mainly determined by alteration or mutation in the single gene, chromosomal disorders are due to absence / excess / abnormal arrangement of one or more chromosomes. 
(ii) Turner’s syndrome, Klinefelter’s syndrome, Down’s syndrome. (Name any two) 
(iii) Turner’s syndrome : Such a disorder is caused due to the absence of one of the X chromosomes i.e. 44 with XO, Such females are sterile as ovaries are rudimentary besides
other features including lack of other secondary sexual characters.
Klinefelter’s syndrome : This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY.
Such an individual has overall masculine development, however, the feminine development (development of breast i.e. Gynaecomastia) is also expressed. Such individuals are sterile.
Down’s syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21)
This disorder was first described by Langdon Down (1866).
The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded. 

Question. (i) Why is haemophilia generally observed in human males ? Explain the condition under which a human female can be haemophilic.
(ii) A pregnant human female was advised to undergo M.T.P. It was diagnosed by her doctor that the foetus she is carrying has developed from a zygote formed by an XX-egg fertilized by Y-carrying sperm. Why was she advised to undergo M.T.P. ?
Answer : (i) Haemophilia is caused due to the recessive gene on X chromosome. 
Y has no allele for this / if a male is XhY then he is haemophilic / if male inherits Xh from the mother he is haemophilic (with the genotype X^hY). 
If female inherits X^hX^h, one from the carrier mother and one from her haemophilic father then she can be haemophilic. 
(ii) Embryo has (trisomy of sex chromosome) XXY / Klinefelter’s syndrome. 
Advised MTP since child will have the following problems :
Male with feminine traits / like gynecomastia / under developed testes / sterile.