42. A virgin Drosophila female was crossed with a wild type male. The Fi progeny obtained had four types of males as shown below.

Phenotype	White eyed	Wild type	Cross veinless	White eyed and crossveinless
Number	50	3	44	3

Assuming that white eye and crossveinless mutations are X-linked and recessive, the following statements were made:
A. F1 females were also of four types as that of males.
B. The white eyed crossveinless male flies appeared due to independent assortment.
C. The map distance between the genes for white eye and crossveinless is estimated to be 12 cM.
D. The map distance between white eye and crossveinless is estimated to be 6 cM.
E. All Ft females are expected to be Wild type.
F. The F1 wild type males appeared due to crossing over.
The combination with correct statements is:
(1) C, E, F       (2) A, B, D
(3) A, D, F       (4) B, D, E

Introduction

This article provides a step-by-step, SEO-optimized solution for the Drosophila genetics problem from CSIR NET June 2018, focusing on X-linked white eye and crossveinless mutations. Learn how to analyze inheritance patterns, calculate map distance, interpret progeny types, and critically evaluate each answer option.

Problem Statement Overview

A virgin Drosophila female was crossed with a wild type male. The F1 progeny included four male phenotypes: white-eyed (50), wild type (3), crossveinless (44), and white-eyed crossveinless (3). The mutations are assumed X-linked and recessive. Several statements about the cross were presented, and the task is to identify which combination is correct.

Option Explanations

Statement A

A. F1 females were also of four types as that of males.

Since the mutations are X-linked and recessive, F1 females would inherit one mutant and one wild-type allele for both genes. All F1 females will be heterozygous and express the wild-type phenotype for both genes, not four types. This statement is incorrect.

Statement B

B. The white-eyed crossveinless male flies appeared due to independent assortment.

The appearance of double mutant males is due to recombination (crossing over) between two genes on the same chromosome, not independent assortment, which applies to genes on different chromosomes. Therefore, this statement is incorrect.

Statement C

C. The map distance between the genes for white eye and crossveinless is estimated to be 12 cM.

Map distance (cM) = (Number of recombinant progeny / Total progeny) × 100
Recombinants = wild type (3) + white-eyed crossveinless (3) = 6, total males = 100
Map distance = (6/100) × 100 = 6 cM, not 12 cM. This statement is incorrect.

Statement D

D. The map distance between white eye and crossveinless is estimated to be 6 cM.

As shown above, the correct calculation yields 6 cM. Hence, this statement is correct.

Statement E

E. All F1 females are expected to be wild type.

F1 females will be heterozygous for both traits (white eye and crossveinless) and display the wild type phenotype since the mutations are recessive and X-linked. This is correct.

Statement F

F. The F1 wild type males appeared due to crossing over.

Wild type males (3 observed) resulted from recombination between the two loci during meiosis, as only crossing over can produce F1 males with both wild-type alleles from a double mutant female and wild type male cross. This statement is correct.

Correct Statement Combination Table

Statement	Correct/Incorrect	Explanation
A	Incorrect	All F1 females are wild type; only males show all four types.
B	Incorrect	Double mutants arise from recombination, not independent assortment.
C	Incorrect	Map distance calculated as 6 cM, not 12 cM.
D	Correct	Proper map distance calculation gives 6 cM.
E	Correct	All F1 females are heterozygous, wild type phenotype.
F	Correct	Wild type F1 males arise via crossing over (recombination).

Conclusion: Correct Answer

The correct combination is (3): A, D, E, as only D, E, and F are correct according to genetics principles and the observed data.

This comprehensive analysis assists CSIR NET aspirants in mastering the logic behind X-linked Drosophila genetic crosses and map distance calculations.