Intro
Thomas Hunt Morgan, (born Sept. 25, 1866, Lexington, Ky., U.S.—died Dec. 4, 1945, Pasadena, Calif.), American zoologist and geneticist, famous for his experimental research with the fruit fly (Drosophila) by which he established the chromosome theory of heredity. He showed that genes are linked in a series on chromosomes and are responsible for identifiable, hereditary traits. Morgan’s work played a key role in establishing the field of genetics. He received the Nobel Prize for Physiology or Medicine in 1933.

a Define ‘sex linkage’. b Describe a specific example of the work by Thomas Hunt Morgan that led him to hypothesise about the existence of sex linkage.

A6.
Bk Ch6 S6.3 A6 a Sex linkage refers to genes that are carried on the X chromosomes. Sex-linked characteristics occur more often in one sex than the other because they are inherited on the X chromosome. b Thomas Hunt Morgan bred fruit-flies (Drosophila) in the early 20th century in an attempt to replicate the work of Gregor Mendel. However, the characteristic of eye colour that he chose to investigate led him to the hypothesis that some characteristics are linked to the sex of individuals and are therefore sex-linked. When he mated a white-eyed male with a red-eyed female all of the F1 were red-eyed suggesting that red eye colour is dominant to white. However, when he crossed the red-eyed offspring the results he obtained were quite unexpected. Instead of red-eyed flies and white-eyed flies turning up in the 3 : 1 ratio he observed a spread of red-eyed males, red-eyed females and white-eyed males but no white-eyed females. W: red eye colour (dominant); w: white eye colour (recessive). Parents: XwY  XWXW F1: XWXw red-eyed females, XWY red-eyed males F2:

XW
Xw
XW
XWXW
XWXw
Y
XWY
XwY

When he carried out a test cross of the original white-eyed male with an F1 red-eyed female, both red eye and white eye occurred in the male and female offspring.

XW
Xw
Xw
XW Xw
Xw Xw
Y
XWY
XwY
This led him to conclude that the gene for eye colour in Drosophila is carried on the X chromosome and since the Y chromosome is an unmatched pair for the X chromosome it would not carry the same genes. Since females have two X chromosomes this would account for the non-appearance of white-eyed females in the F1—they would have genotype XWXW or XWXw, both resulting in the red-eyed phenotype. Only when the test cross was performed did white-eyed females occur, resulting from the combination of X chromosomes carrying the white-eye allele from both the mother and the father. Morgan’s hypothesis reliably accounted for his