Principles of Inheritance and Variation

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Principles of Inheritance and Variation — solved practice questions

8 NEET Biology questions with step-by-step solutions. Attempt each, then reveal the answer.

1. Q1easy

   In a monohybrid cross between two heterozygous tall pea plants (Tt × Tt), what is the expected phenotypic ratio in the F2 generation?

   • A1 : 1
   • B3 : 1
   • C9 : 3 : 3 : 1
   • D1 : 2 : 1
   Show answer & solution

   Correct answer: (B) 3 : 1

   A Tt × Tt cross gives a genotypic ratio of 1 TT : 2 Tt : 1 tt, which corresponds to a phenotypic ratio of 3 tall : 1 dwarf.

2. Q2medium

   A colour-blind man marries a woman who is homozygous normal for colour vision. What is the expected outcome regarding colour blindness in their children?

   • AAll sons will be colour blind
   • BHalf the daughters will be colour blind
   • CAll daughters will be carriers and all children will have normal vision
   • DAll children will be colour blind
   Show answer & solution

   Correct answer: (C) All daughters will be carriers and all children will have normal vision

   Colour blindness is X-linked recessive. The father (XcY) passes Xc only to daughters, who become carriers; sons get his Y and the mother's normal X. All children have normal vision, but all daughters are carriers.

3. Q3medium

   In a dihybrid test cross, a plant heterozygous for two independently assorting genes (RrYy) is crossed with a fully recessive plant (rryy). What phenotypic ratio is expected in the offspring?

   • A9 : 3 : 3 : 1
   • B3 : 1
   • C1 : 2 : 1
   • D1 : 1 : 1 : 1
   Show answer & solution

   Correct answer: (D) 1 : 1 : 1 : 1

   A test cross of a dihybrid (RrYy × rryy) yields four phenotypic classes in equal proportion, giving a 1 : 1 : 1 : 1 ratio, which is why test crosses reveal the genotype of the dominant parent.

4. Q4medium

   The ABO blood group system in humans is an example of which phenomenon, given that alleles I^A and I^B are both expressed in an AB individual?

   • ACodominance
   • BIncomplete dominance
   • CComplete dominance
   • DEpistasis
   Show answer & solution

   Correct answer: (A) Codominance

   In blood group AB, both I^A and I^B are fully and simultaneously expressed, producing both A and B antigens; this simultaneous expression of both alleles is codominance.

5. Q5easy

   In Mirabilis jalapa, a cross between a red-flowered (RR) and a white-flowered (rr) plant produces pink-flowered (Rr) offspring. This is an example of:

   • ACodominance
   • BIncomplete dominance
   • CPleiotropy
   • DMultiple allelism
   Show answer & solution

   Correct answer: (B) Incomplete dominance

   The heterozygote (Rr) shows an intermediate pink phenotype rather than red, so neither allele is completely dominant; this is incomplete dominance, and the F2 phenotypic ratio equals the genotypic ratio 1 : 2 : 1.

6. Q6medium

   Two genes located on the same chromosome show 20 percent recombination frequency. The map distance between them is:

   • A2 map units
   • B10 map units
   • C20 map units
   • D40 map units
   Show answer & solution

   Correct answer: (C) 20 map units

   One map unit (centiMorgan) equals 1 percent recombination frequency. Therefore 20 percent recombination corresponds to a map distance of 20 map units (20 cM).

7. Q7hard

   A woman heterozygous for haemophilia (carrier) marries a normal man. What is the probability that a given son will be haemophilic?

   • A1/2
   • B1/4
   • C1
   • D0
   Show answer & solution

   Correct answer: (A) 1/2

   Haemophilia is X-linked recessive. The carrier mother (XHXh) passes Xh to half her sons, who then express the disease since they have only one X; thus each son has a 1/2 probability of being haemophilic.

8. Q8easy

   Down syndrome in humans is most commonly caused by:

   • ADeletion in chromosome 21
   • BMonosomy of chromosome 21
   • CAn extra X chromosome
   • DTrisomy of chromosome 21
   Show answer & solution

   Correct answer: (D) Trisomy of chromosome 21

   Down syndrome results from trisomy of chromosome 21 (an extra copy, giving 47 chromosomes), usually due to non-disjunction during meiosis.

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