It's not one gene that makes a sex-link; there are a number of them. However the most commonly used is the Silver/Gold gene. But others can be used instead.
The sex chromosomes in chickens are Z and W. The W chromosome is shorter than the Z chromosome, and therefore does not have space for a copy of some of the genes that are on W. A female chicken is ZW, a male chicken is ZZ.
For a chick to be a male, the mother must provide a Z chromosome; likewise, for a chick to be female, she must provide a W chromosome. The sex-linked genes are those located on the Z chromosome, but not on the W. For these genes, hens carry only one copy of the gene whereas roosters carry two.
To create a sex-linked cross, the hen must have a dominant allele of a sex-linked gene while the male has a recessive allele. The offspring will all receive the recessive form from the father. The cockerels will receive the dominant form from their mother--which will override the recessive allele received from the mother. The pullets will not receive a gene from their mother, thus the gene received from their father will show, regardless of the fact that it is recessive.
