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NEET Biology · Genetics and Evolution

Mendelian inheritance and its deviations

Taught in NCERT Class 12 Biology, Chapter 4: Principles of Inheritance and Variation.

5 verified questions: Easy 0 · Medium 4 · NEET level 1

Sample questions

Sample 1Medium

A Punnett square shows the selfing of pink-flowered F1 snapdragons (Rr). Gametes R and r head each row and column, and the four boxes read RR, Rr, Rr and rr, where RR plants are red and rr plants are white. What are the F2 phenotypic and genotypic ratios?

  1. A

    Phenotypic 3 red : 1 white; genotypic 1 : 2 : 1

  2. B

    Phenotypic 1 red : 2 pink : 1 white, the same as the genotypic ratio 1 RR : 2 Rr : 1 rr

  3. C

    Phenotypic 1 : 2 : 1; genotypic 3 : 1

  4. D

    Phenotypic 9 : 3 : 3 : 1

Show the answer
The answer is B.

In snapdragon, flower colour shows incomplete dominance, so the heterozygote Rr is pink. The F2 phenotypic ratio 1 : 2 : 1 is therefore the same as the genotypic ratio.

  • A.

    A 3 : 1 ratio needs complete dominance; here Rr is pink, a distinct phenotype.

  • C.

    A monohybrid cross always gives a 1 : 2 : 1 genotypic ratio.

  • D.

    That is a dihybrid ratio; only one gene is involved here.

NCERT: Class 12 Biology, Chapter 4, 4.2

Sample 2NEET level

In garden pea, round seed (R) is dominant over wrinkled (r) and yellow seed (Y) is dominant over green (y), the two genes assorting independently. In a cross RrYy × Rryy, what fraction of the progeny will have round, green seeds?

  1. A

    1/8

  2. B

    3/8

  3. C

    1/4

  4. D

    3/16

Show the answer
The answer is B.

For seed shape, Rr × Rr gives 3/4 round; for seed colour, Yy × yy gives 1/2 green (yy). Since the genes assort independently, round green = 3/4 × 1/2 = 3/8.

  • A.

    This is the fraction of wrinkled green seeds (1/4 × 1/2).

  • C.

    Multiplies 1/2 by 1/2, treating Rr × Rr as giving only half round.

  • D.

    This is the round green fraction of a dihybrid RrYy × RrYy cross, not of this cross.

NCERT: Class 12 Biology, Chapter 4 (Principles of Inheritance and Variation), Inheritance of two genes

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