58. A Drosophila male carrying an X-linked temperature sensitive recessive mutation that is lethal at 29⁰C but viable at 18⁰C is mated to:
A. a normal female
B. a female containing attached X-chromosome
If the eggs laid in both the cases are reared at 29⁰C, what will be male-female ratio in the given progeny?
(1) A-1:2, B-1:1    (2) A-1:1, B-only females
(3) A-0:1, B-1:1    (4) A-1:0, B-1:2

Introduction:
Drosophila genetics provides a powerful model to study X-linked mutations, especially temperature-sensitive lethal alleles that kill males at a restrictive temperature such as 29°C but allow viability at permissive temperatures like 18°C. Crosses of males carrying such mutations to normal or attached-X females reveal distinct male-to-female progeny ratios at the restrictive temperature. Understanding these ratios requires examining the inheritance dynamics of the X chromosome and how lethality influences survival of males and females.

Explanation of the Problem and Key Genetic Principles:

• The mutation is recessive and X-linked, lethal to hemizygous males at 29°C, but females heterozygous for the mutation (carrying one mutant and one normal X chromosome) are viable.

• A normal female has two separate X chromosomes; an attached-X female has both X chromosomes physically attached and inherited together.

• Male progeny inherit their single X chromosome from the mother and Y from the father.

• Female progeny inherit one X chromosome from each parent.

Cross A: Male with mutant X crossed with normal female:

• Females inherit one normal X from the mother and one mutant X from the father → heterozygous and viable.

• Males inherit Y from father and X from mother → normal X → viable.

• Males inheriting mutant X from father do not occur because males get Y from father.

• At 29°C, mutant X males would be lethal if existed, but sons inherit normal X from mother → all males survive.

• Expected ratio: For every 2 females (heterozygotes), 1 male (normal) → sex ratio 1:2 (male:female).

Cross B: Male with mutant X crossed with attached-X female:

• Females receive both attached X chromosomes from mother (both normal), males receive Y from father.

• Sons receive attached-X from attached-X female? Actually, attached-X females produce eggs carrying both X chromosomes or no X (nondisjunction), but when crossed with male, only females inherit the attached-X, males inherit no X or Y.

• Males inherit Y from father and no X from mother → nonviable or not produced.

• All progeny with attached-X are females.

• At restrictive temperature, no male progeny survive.

• Female progeny will be heterozygous or carry normal X (attached), all viable.

• Expected ratio: only females → sex ratio male:female 0:1 or no males.

By examining the options:
(1) A-1:2, B-1:1
(2) A-1:1, B-only females
(3) A-0:1, B-1:1
(4) A-1:0, B-1:2

Correct interpretation is option (2):

• Cross A produces equal males and females 1:1 (males normal, females heterozygous).

• Cross B produces only females (attached-X chromosomes prevent male progeny).

Detailed Explanation of Each Option:

• Option 1 suggests more females than males in A and equal ratio in B, incorrect because males survive normally in A and males not produced in B.

• Option 2 matches the expected sex ratios given the genetics.

• Option 3 says no males in A and equal ratio in B, opposite of actual expected.

• Option 4 says only males in A and more females in B, which contradicts genetics.

Hence, option (2) is the correct answer based on X-linked temperature sensitive lethal mutation inheritance and the effect of attached-X chromosome females.

This explanation highlights the importance of genetic cross design and X chromosome behavior in Drosophila affecting sex ratios under temperature sensitive lethal conditions. These principles are fundamental in genetics for understanding mutation effects on progeny survival and sex determination.​