23. Two Yellow mice with straight hair were crossed and the following progeny was obtained:
1/2 Yellow, straight hair
1/6 Yellow, fuzzy hair
1/4 gray, straight hair
1/12 gray, fuzzy hair
In order to provide genetic explanation for the and assign genotypes to the parents and progeny of this cross the following statements were given:
(A) The 6:2:3:1 ratio obtajned here indicate’, recessive epistasis.
(B) This cross concerns two independent body colour and type of hair.
(C) The deviation of dihybrid ratio from 9:3:3:1 to 6:2:3:1 may be due to one of the genes being a recessive lethal
(D) The lethal allele is associated With straight hair
The most appropriate combination of statements to provide genetic explanation for the result is:
(1) B and C (2) A only
(3) B, C and D (4) A, C and D
Explanation of the 6:2:3:1 Ratio and Genetic Basis
The observed phenotypic ratio of 6:2:3:1 from crossing two yellow, straight-haired mice deviates from the expected 9:3:3:1 Mendelian ratio typical of two independently assorting genes. This deviation is best explained by the presence of a recessive lethal allele affecting one of the genes, eliminating some genotypes and altering the phenotypic proportions. The genes controlling body color and hair type are independent, and the lethal allele affects viability associated with body color, but specifically not the hair type gene.
Analysis of the Statements
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(A) The 6:2:3:1 ratio indicates recessive epistasis: Incorrect. Classical recessive epistasis usually produces a 9:3:4 ratio, not 6:2:3:1. The deviation here is due to lethality, not epistasis.
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(B) The cross concerns two independent body color and hair type genes: Correct. Body color and hair type segregate independently as per Mendel’s law.
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(C) The deviation from 9:3:3:1 to 6:2:3:1 is due to one gene being recessive lethal: Correct. The lethal allele removes certain genotypes from the progeny pool, causing the altered ratio.
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(D) The lethal allele is associated with straight hair: Incorrect. Lethality is linked to the body color gene, not the hair type gene.
Summary Table
| Statement | Correctness | Explanation |
|---|---|---|
| A | Incorrect | 6:2:3:1 ratio is not recessive epistasis |
| B | Correct | Body color and hair type segregate independently |
| C | Correct | Recessive lethal causes deviation from Mendelian ratio |
| D | Incorrect | Lethality linked to body color, not hair type allele |
The answer is (1) B and C because the 6:2:3:1 phenotypic ratio reflects two independently assorting genes with a recessive lethal allele affecting the body color gene, causing deviation from the standard 9:3:3:1 ratio.
