3.2 Sex Linked Genes
We have learned that the male has two X chromosomes and is denoted by XX. The female has one X chromosome and a shorter Y chromosome. A chromosome is made up of several genes. So far we have seen that all genes come in pairs or have two copies. However, in the figure below we see that the female bird has some genes on the X chromosome for which there is no corresponding gene on the Y chromosome. The same gene has two copies in the male bird but only one copy in the female bird. Such genes are known as sex-linked genes.
In female birds, the sex-linked genes are unpaired. As a result, only a single copy of the gene is sufficient to dominate and produce a visual bird. A female bird therefore cannot be split to a sex-linked gene. If it has a sex-linked gene it will show it visually. If it is not showing a sex-linked trait then it is safe to conclude that it is not carrying the sex-linked gene.
We have mentioned previously that the ino gene is sex-linked recessive. We now understand what is meant by sex-linked but why the recessive? In a male bird the sex-linked genes still occur in pairs. In order to produce a visual male we still need two copies of the ino gene. A single copy of the ino gene will be recessive to the normal gene and only produce a male that is split to ino and visually green in color. Fig 3 shows how male and female birds may carry sex-linked genes.