Principles of Inheritance and Variation — Questions & Answers

Sample questions from Principles of Inheritance and Variation

1. Q1. A woman who is a known carrier for an X-linked recessive disorder marries an unaffected man. Considering all possible offspring, what proportion of her sons are expected to be affected?

   • A.0%
   • B.25%
   • C.50%✓
   • D.100%
   Solution

   A carrier mother (XHXh) passes either the normal (XH) or the recessive (Xh) allele to each son with equal probability; since sons receive their only X chromosome from the mother, 50% of her sons are expected to be affected (XhY).

2. Q2. A dihybrid cross showing dominant epistasis typically produces a modified F2 phenotypic ratio of:

   • A.9:3:3:1
   • B.12:3:1✓
   • C.9:7
   • D.15:1
   Solution

   Dominant epistasis modifies the standard 9:3:3:1 ratio into 12:3:1, since the dominant epistatic allele masks one gene's effect, merging the 9 and one 3 class into a single 12 class, while the remaining 3 and 1 classes stay distinct.

3. Q3. A key distinction between structural chromosomal aberrations (like deletions and duplications) and numerical aberrations (aneuploidy) is that structural aberrations involve:

   • A.A change in the total number of chromosomes
   • B.A change in chromosome structure (loss, gain, or rearrangement of a chromosome segment) without necessarily changing the total chromosome number✓
   • C.Identical mechanisms to non-disjunction
   • D.No effect on phenotype whatsoever
   Solution

   Structural aberrations (deletions, duplications, inversions, translocations) alter the structure or arrangement of genetic material within or between chromosomes, often without changing the total chromosome count, distinguishing them from numerical aberrations (aneuploidy/polyploidy), which change the actual chromosome number.

4. Q4. In the polygenic model of human skin colour with three additive gene pairs, two AaBbCc parents are crossed. What fraction of the children carry exactly three dominant (dark-contributing) alleles, that is the same shade as the parents?

   • A.5 out of 16✓
   • B.20 out of 27
   • C.1 out of 8
   • D.15 out of 64
   Solution

   The six alleles contributed by the two parents behave like six independent coin tosses, so the number of dominant alleles follows the binomial expansion of one half to the sixth power. The class with three dominant alleles has 20 of the 64 equally likely combinations, that is 5 out of 16.

5. Q5. Which of the following statements about mutation, as described in NCERT, is correct?

   • A.Mutation is a change in DNA sequence that is never inherited by the offspring
   • B.Loss or gain of a segment of DNA can result in mutation, and physical or chemical agents such as UV radiation that induce mutation are called mutagens✓
   • C.Mutations arise only through crossing over during meiosis
   • D.Only whole-chromosome changes are called mutations; single base changes are not
   Solution

   Mutation is a heritable change in DNA that alters genotype and phenotype. It may involve loss or gain of a DNA segment or a single base change, and chemical or physical factors (e.g., UV radiation) that induce it are termed mutagens.

Other Class 12 Biology chapters