Sex-linked Genes

Drosophila: Thomas Hunt Morgan


Eye colors (clockwise): brown, cinnabar, sepia, vermilion, white, wild. Also, the white-eyed fly has a yellow body, the sepia-eyed fly has a black body, and the brown-eyed fly has an ebony body. White-eyed flies have a gender imbalance and occur mostly in males.

Around 1908, Thomas Hunt Morgan began to explore the genetics of what was to become a model organism, Drosophila melanogaster (Fruit fly). This small organism had a relatively short life cycle, great fecundity and was easily managed. From these flies that normally have red eye coloring, he and his students found white-eyed mutants. The lab noted that white-eyed flies were almost exclusively male. This gender imbalance lead Morgan to believe that the trait was sex-linked. In 1911, Morgan published a paper that described the inheritance patterns of 5 eye-colors in Drosophila (Morgan, 1911).

Drosophila XY sex-determination

Drosophila follow a sex determination based on the the ratio of X:A chromosomes and not by the presence of a Y as in mammals. A 1:1 ratio results in a female and a 1:2 ratio results in a male where the Y is ignored. The standard 2N = 8. Credit: GYassineMrabet [CC-BY-SA 4.0]

While DNA was not yet known as the source of genetic information, Morgan’s studies revealed that the location of genes most likely resided on the chromosomes. By cataloging many mutations in the lab, he was able to construct a map of gene locations. His 1922 paper specifically stated that some traits were sex-linked and therefore residing on the sex chromosome. When performing crosses of white-eyed males to wild-type females, he continued to find white-eyed trait only in males. However, in the subsequent cross of females from that generation with white-eyed males, the presence of white-eyed males and females were revealed. This indicated that the white-eyed trait was recessive and resided on the X chromosome.

Analysis of the transmission of “White-Eyed” color in Drosophila Credit: Jeremy Seto (CC-BY-SA)

Morgan received the Nobel Prize in Physiology or Medicine in 1933 for his inference of chromosomes being a physical mechanism for packaging genetic information in the cells.