Q.96 A genetic experiment was performed to map the gene(s) for eye colour in a newly-discovered moth species.
Sex determination in this moth species: XY – male and XX – female. When blue-eyed males were mated to green-eyed females,
all of both male and female progeny had green eyes. When these progeny were mated among themselves, about half of the males
of the resulting second generation had blue eyes; however, all females were green-eyed.
Which one of the following is consistent with the above data?
Answer: Gene(s) for eye colour is located on the X chromosome
This X-linked recessive inheritance pattern perfectly matches the observed data:
blue-eyed males (XbY) × green-eyed females (XBXB) produce all green-eyed F1,
with blue eyes reappearing only in ~50% F2 males.
Options Explained
(A) Multiple genes control eye colour
False: Single gene hypothesis suffices: simple 1:1 male ratio in F2 rules out polygenic inheritance, which would produce continuous variation, not discrete blue/green phenotypes.
✅ (B) Gene(s) on X Chromosome
Correct: Blue = recessive (b), green = dominant (B). Parental cross:
XbY (blue male) × XBXB (green female) → F1:
XBY (green males), XBXb (green females).
F1 intercross → F2 males: 50% XBY (green), 50% XbY (blue);
females all XB_ (green). Perfect crickets pattern.
(C) Gene(s) on Y Chromosome
Impossible: Y-linked traits pass father-to-son exclusively. Blue-eyed father produces no blue F1 sons, eliminating Y-linkage. Daughters don’t inherit Y from father.
(D) May Not Be Sex-Linked
Autosomal recessive impossible: blue male × green female should yield 50% blue F2 (both sexes), not male-only blues. Data shows obligate sex-linkage.
Punnett Square Analysis
P0: Blue ♂ × Green ♀
| XB | XB | |
|---|---|---|
| Xb | XBXb Green ♀ |
XBXb Green ♀ |
| Y | XBY Green ♂ |
XBY Green ♂ |
All F1 green-eyed
F1 × F1: Carrier ♀ × Green ♂
| XB | Y | |
|---|---|---|
| XB | XBXB Green ♀ |
XBY Green ♂ |
| Xb | XBXb Green ♀ |
XbY Blue ♂ |
F2: Females all green; Males 50% blue ✅
X-Linked Recessive Diagnostic Features
- Affected ♂ × Normal ♀ → All normal F1
- F2: Trait reappears in 50% sons only
- No male-to-male transmission
Why NEET Students Must Master This
Moth eye color X chromosome inheritance reveals classic X-linked recessive pattern—blue reappears only in F2 males, definitive for NEET genetics mastery. This exact pattern appears repeatedly in medical entrance exams.
Key Takeaway
Criss-cross inheritance: Father → Daughters (carriers) → Grandsons (affected). Perfect X-linked recessive signature.
Single X-linked locus explains all data perfectly. Option (B) is definitively correct.
