Introduction

In this classical Drosophila melanogaster genetics problem, a male with normal wings and grey body is crossed with a female having vestigial wings and yellow body; both mutant traits are recessive. Analysis of the sex‑wise F2 phenotypic classes allows distinction between X‑linked and autosomal inheritance, which is a frequently asked concept in CSIR‑NET and other competitive exams.

Stepwise genetic analysis

• Yellow body vs grey body

  • Yellow body is a known X‑linked recessive mutation in Drosophila; grey (wild type) is dominant and represented as X⁺, while yellow is represented as Xʸ.

  • In the given cross, the P generation is:

    • Male: normal wing, grey body = X⁺Y for body colour; wings are normal (dominant autosomal allele, say Vg⁺Vg⁺).

    • Female: vestigial wing, yellow body = XʸXʸ; wings vestigial (recessive autosomal allele, vgvg).

• F1 progeny

  • Sons receive Xʸ from the mother and Y from the father, so their body colour is yellow, and they receive one Vg⁺ from father and one vg from mother, so wings are normal (Vg⁺vg).

  • Daughters receive X⁺ from father and Xʸ from mother, so they are grey (X⁺Xʸ) with normal wings (Vg⁺vg).

  • Thus F1: all males yellow with normal wings; all females grey with normal wings, which is characteristic of an X‑linked body‑colour gene and autosomal wing gene.

• F2 after sib‑mating F1

  • Cross: grey, normal F1 female (X⁺Xʸ; Vg⁺vg) × yellow, normal F1 male (XʸY; Vg⁺vg).

  • For body colour:

    • Female genotypes: X⁺Xʸ (grey), XʸXʸ (yellow).

    • Male genotypes: X⁺Y (grey), XʸY (yellow).

    • This gives both grey and yellow in both sexes, but the proportions differ because males have only one X.

  • For wings (autosomal):

    • Gametes from each parent for wing gene: Vg⁺ and vg.

    • F2 genotypes: Vg⁺Vg⁺ (normal), Vg⁺vg (normal), vgvg (vestigial) in a 3:1 phenotypic ratio among both sexes combined.

  • Because the wing gene is autosomal and unlinked to the X‑linked body‑colour gene, each sex shows a 3:1 ratio of normal:vestigial wings irrespective of body colour, matching the given table (3 normal:1 vestigial for each sex).

• Interpretation of the F2 table in the image

  • For males: normal grey (3), normal yellow (3), vestigial grey (1), vestigial yellow (1).

  • For females: normal grey (3), normal yellow (3), vestigial grey (1), vestigial yellow (1).

  • Each sex shows 3:1 for wings (normal:vestigial) and 1:1 for body colour (grey:yellow), consistent with autosomal recessive wings and X‑linked recessive body colour.

Explanation of each option

• Option (1): “Yellow body colour X‑linked, while vestigial wing an autosomal character.”

  • Yellow body shows sex‑linked inheritance: in the original P cross, all F1 males are mutant (yellow) while all F1 females are wild‑type grey, which is typical of an X‑linked recessive when the mutant female is crossed with a wild‑type male.

  • In F2, both sexes show grey and yellow, with proportions determined by X‑linked segregation, while wings follow a simple autosomal 3:1 pattern; this exactly fits the observations, so option (1) is correct.

• Option (2): “Yellow body colour is autosomal, while vestigial wing is an X‑linked character.”

  • If yellow body were autosomal and vestigial wings X‑linked, the P cross would produce F1 in which both sexes show identical body‑colour ratios, and sex‑biased wing phenotypes; that is not what is observed.

  • The experimental F1 has sex‑specific body colour (all sons yellow, all daughters grey) with both sexes having normal wings, so this option contradicts the data and is incorrect.

• Option (3): “Both yellow body and vestigial wing are X‑linked characters.”

  • If both genes were X‑linked, wing phenotype would differ between males and females depending on X chromosome combinations, not display the same 3:1 normal:vestigial ratio in each sex.

  • Moreover, autosomal 3:1 wing segregation in both sexes and equal wing distribution between males and females strongly rules out X‑linkage for the vestigial wing gene, so option (3) is wrong.

• Option (4): “Both yellow body and vestigial wing are autosomal and unlinked.”

  • If both traits were autosomal and unlinked, no sex bias would appear in body colour or wing phenotype; males and females would show identical 9:3:3:1 or 3:1 dihybrid ratios depending on the cross.

  • The observed F1 sex‑specific body colour (sons mutant, daughters wild type) proves body colour is not autosomal, so this option is also incorrect.

Key takeaway for exams

This Drosophila problem demonstrates how sex‑specific F1 patterns combined with F2 ratios help distinguish X‑linked recessive traits (yellow body) from autosomal recessive traits (vestigial wing), even when both are recessive mutants. Remember that:

• X‑linked recessive: mutant phenotype appears in all F1 sons when mutant mother is crossed with wild‑type father.

• Autosomal recessive unlinked: 3:1 ratio in each sex independently, with no sex bias in expression.