Q30 — Linked Genes Controlling Fruit Fly Wing Pigmentation
In some fruit flies, wing pigmentation is controlled by genes A, B, and D, which are linked on the same chromosome. One of the genes codes for an ENZYME, converting yellow pigment to orange. The other two genes code for an INHIBITOR of ENZYME and an ACTIVATOR of ENZYME. ACTIVATOR acts by inhibiting INHIBITOR. Moreover, one of them has a redundant role in larval development with the gene E which is located on a separate chromosome (larvae do not survive when both of the genes are in recessive state). Gene E has no role in wing pigmentation. The recessive allele of all these genes results in a non-functional protein product. The parental cross scheme and the results of the F2 generation are shown below.
Cross scheme: Parental P1-1 (AABBDD) × P1-2 (aabbdd) → F1 × P1-2 → F2
| Wing pigmentation | Genotype | Offspring number (both P1 parents EE) | Offspring number (both P1 parents ee) |
|---|---|---|---|
| Orange | ABD/abd | 200 | 200 |
| Yellow | abd/abd | 200 | 0 |
| Orange | AbD/abd | 33 | 30 |
| Yellow | aBd/abd | 31 | 0 |
| Orange | Abd/abd | 16 | 0 |
| Yellow | aBD/abd | 16 | 19 |
| Yellow | ABd/abd | 1 | 0 |
| Yellow | abD/abd | 1 | 1 |
Table 1. Phenotype and genotype distribution of the F2 generation.
On your answer sheet, indicate “T” for true statements and “F” for false ones.
Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF