Drosophila Grey and Yellow Body Colour Cross - CSIR NET LIFE SCIENCE COACHING | NTA NET LIFE SCIENCE

56. Consider the following crosses involving grey(wild-type) and yellow body colour true breeding Drosophila: Cross F1 Progeny F2 Progeny Cross 1 Grey female X yellow male All male : grey All female: grey Grey female : 98 yellow males : 45 Grey males : 49 Cross 2 Yellow female X grey males All males : yellow All female : grey ? Assuming 200 F2 offspring are produced in cross 2, which one of the following outcome is expected? (1) 97 grey males, 54 yellow females, 49 grey males (2) 102 yellow males, 46 yellow females. 52 grey females (3) 52 grey males, 49 yellow males, 48 yellow females, 51 grey females (4) 98 grey males, 94 yellow females, 2 yellow males, 6 grey females

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December 1, 2025
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Consider the following crosses involving grey(wild-type) and yellow body colour true breeding Drosophila:

	Cross	F1 Progeny	F2 Progeny

Cross 1	Grey female X yellow male	All male : grey All female: grey	Grey female : 98 yellow males : 45 Grey males : 49
Cross 2	Yellow female X grey males	All males : yellow All female : grey	?

Assuming 200 F2 offspring are produced in cross 2, which one of the following outcome is expected?
(1) 97 grey males, 54 yellow females, 49 grey males
(2) 102 yellow males, 46 yellow females. 52 grey females
(3) 52 grey males, 49 yellow males, 48 yellow females, 51 grey females
(4) 98 grey males, 94 yellow females, 2 yellow males, 6 grey females

Introduction

This problem on grey (wild type) and yellow body colour in Drosophila melanogaster tests understanding of X‑linked recessive inheritance and how reciprocal crosses change phenotypic ratios in F₂. Using the given F₁ and F₂ data from one cross, the inheritance pattern can be deduced and then applied to the reciprocal cross to predict the expected numbers among 200 F₂ offspring.

Genetics of body colour in Drosophila

Grey body is controlled by a dominant X‑linked allele $X^{+}$ , whereas yellow body is due to a recessive X‑linked allele $X^{y}$ . Because males are XY, a single recessive $X^{y}$ makes a male yellow, while females must be homozygous $X^{y} X^{y}$ to be yellow. True‑breeding grey or yellow lines are therefore $X^{+} X^{+}$ (grey female), $X^{+} Y$ (grey male), $X^{y} X^{y}$ (yellow female), and $X^{y} Y$ (yellow male).

Interpreting cross 1 (given data)

Cross 1 is grey female × yellow male, both true breeding.

Parental genotypes: female $X^{+} X^{+}$ , male $X^{y} Y$ .
F₁: all females $X^{+} X^{y}$ (grey), all males $X^{+} Y$ (grey), matching the table.

F₁ intercross: $X^{+} X^{y}$ females × $X^{+} Y$ males.
Gametes:

Female: $X^{+}$ , $X^{y}$
Male: $X^{+}$ , $Y$

Punnett square gives four genotypes in equal proportion:

$X^{+} X^{+}$ grey female
$X^{+} X^{y}$ grey female
$X^{+} Y$ grey male
$X^{y} Y$ yellow male

Thus F₂ expectation: 50% grey females, 25% grey males, 25% yellow males; no yellow females. The observed F₂ data in the question (98 grey females, 45 yellow males, 49 grey males; total ≈ 192) fits these proportions within sampling error and confirms the X‑linked recessive model.

Setting up cross 2 and its F₁

Cross 2 is the reciprocal: yellow female × grey male, both true breeding.

Female $X^{y} X^{y}$ , male $X^{+} Y$ .
Gametes:
Female: $X^{y}$
Male: $X^{+}$ , $Y$

F₁ genotypes:

Females: $X^{+} X^{y}$ (grey; heterozygous).
Males: $X^{y} Y$ (yellow).

This matches the table that “all males: yellow, all females: grey” in F₁ for cross 2.

The F₁ generation of cross 2 is then intercrossed (grey heterozygous females × yellow males):

Female: $X^{+} X^{y}$
Male: $X^{y} Y$ .

F₂ prediction for cross 2

Gametes:

Female: $X^{+}$ , $X^{y}$
Male: $X^{y}$ , $Y$

Punnett square for F₂:

Female gamete ↓ / Male gamete →	$X^{y}$ (from male)	$Y$ (from male)
$X^{+}$ (from female)	$X^{+} X^{y}$ grey female	$X^{+} Y$ grey male
$X^{y}$ (from female)	$X^{y} X^{y}$ yellow female	$X^{y} Y$ yellow male

Each of the four genotypes has probability 1/4, so:

25% grey females
25% yellow females
25% grey males
25% yellow males.

Among 200 F₂ offspring, the expected numbers are therefore about 50 of each category, with small deviations from 50 likely in real counts.

Matching with the given options

The options (all totals around 200) are:

97 grey males, 54 yellow females, 49 grey males (incomplete and strongly male‑biased; does not give ~1:1 sex ratio or equal grey–yellow).
102 yellow males, 46 yellow females, 52 grey females (skewed toward yellow, especially in males; not consistent with 1:1:1:1 expectation).
52 grey males, 49 yellow males, 48 yellow females, 51 grey females (≈50 of each class; matches 25% for each phenotype and 1:1 sex ratio).
98 grey males, 94 yellow females, 2 yellow males, 6 grey females (extreme bias of sex and phenotype; incompatible with Punnett result).

Since the genetic analysis predicts equal frequencies of grey and yellow in both sexes, option (3) is the only one consistent with X‑linked recessive inheritance and the F₂ data from cross 1.

Key takeaways for exam preparation

Yellow body in Drosophila is an X‑linked recessive trait, while grey is the dominant wild type.
Reciprocal crosses involving X‑linked traits give different F₁ and F₂ patterns because male and female parents contribute X chromosomes differently.
For cross 2 F₁ ( $X^{+} X^{y}$ females × $X^{y} Y$ males), the F₂ ratio is 1 grey female : 1 yellow female : 1 grey male : 1 yellow male, which corresponds to option (3).

Introduction

Genetics of body colour in Drosophila

Interpreting cross 1 (given data)

Setting up cross 2 and its F₁

F₂ prediction for cross 2

Matching with the given options

Key takeaways for exam preparation

Tags :

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