Sex chromosome

Previously we mentioned that chromosomes come in homologues pairs. Homologues chromosomes are not identical, but they are very similar and contain the same genes in the same sequence. The previous part handled about the exchange between two homologues chromosomes. There's one pair of chromosomes that is an exception to this general rule: the sex chromosomes or allosomes.

There are two different (non homologous!) chormosomes. They are often called the X-chromosome and the Y-chromosome. The Y-chromosome is much smaller then the X-chromosome and carries only a few genes. Because of this, the Y-chromosome is sometimes referred to as the empty chromosome, but empty is exaggerated.

A cock carries two X-chromosomes in each cell. These X-chromosomes are homologous and can exchange parts through recombination, just as a normal chromosome pair. The hen on the other hand only has one X-chromosome and one Y-chromosome. When compaired to the other (normal) chromosome pairs, this XY pair is very strange, since it does not consist of two homologous chromosomes. Since both chromosomes are for the majority not homologous they cannot go through recombination (It has however been suggested that the small homologous parts of the X and Y chromosome can exchange genetic information through recombination).

This difference between the cock an the hen, and moreover, the difference between the X and Y-chromosome has big implications.

First of all, the X-chromosome carries genes that are not present on the Y-chromosome. The cin gene (cin stands for cinnamon) is such a gene. This gene comes in two alleles:

cin+	normal: black melanin
cin	cinnamon: brown melanin

The cin allele is recessive to its wild type. So, for the cock (XX), the following combinations are possible:

Xcin+/Xcin+	normal cock
Xcin+/Xcin	normal cock/cinnamon
Xcin/Xcin	cinnamon cock

The X is added for clarity.
But what about the hen (XY)?
For the hen there are only two possibilities: Xcin⁺/Y and Xcin/Y. What is the physical appearance (phenotype) for these two genotypes?
Since the Y-chromosome does not carry a cin gene, all is controled by the one cin gene on X-chromosome. In other words, the gene on the X-chromosome solely determines the phenotype:

Xcin+/Y	normal hen
Xcin/Y	cinnamon hen

Because the hen only carries one gene in stead of a pair, it can never be split for cinnamon!

The first pairing is quite trivial: cock x hen.

Pairing 5.0: cock x hen The genotypes are: XX and XY. The gametes produced by the cock all have one X-chromosome. The gametes produced by the hen have an X or a Y. The Punnet square: Cock X Hen X XX Y XY Not surprisingly, the offspring is 50% male and 50% female.

For a real example can be found here.