Glossary

Sex linkage

Sex linkage is the connection of the allele with the chromosomal sex of the individual (XX for woman
and XY for man in mammals). Inheritance probabilities of sex-linked traits are depend directly of the sex of carrier parent and sex of the child. These alleles can be X-linked or Y-linked, and dominante or recessive as well. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked.

Sex linkage was first discovered by Thomas Morgan in 1910, who observed a disproportionate percentage of white-eyed male fruit flies. Morgan's experiment involved crossing purebred red-eyed female fruit flies with these mutant white-eyed males. The result was exclusively red-eyed individuals of both sexes. When these individuals were crossed, they produced some white-eyed males, but no white-eyed females. See the first column in the figure below.

Morgan's conclusion was that the trait for eye color must be encoded on the X chromosome, and that white eyes must represent the recessive allele. In males, if the recessive allele is present, then it is the only allele, and white eyes are demonstrated; yet in females, both copies of the recessive allele must be manifested in order to exhibit white eyes. X and Y chromosomes are still passed down to offspring with probability 1/2, just like autosomes. Check your understanding of such an X-linked trait with Morgan's second experiment, depicted in the second column of the figure below.

Sex Linkage

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