The Inheritance of Sex and Sex-Linked Traits in Poultry Like all living creatures, biological traits in birds are controlled by genes located on chromosomes. A gene may have slightly different forms, called alleles, which cause traits such as comb shape or feather attributes to differ between individuals. Each species has a characteristic set of chromosomes, a copy of which exists in the nucleus of every non-reproductive cell in the body.
Chromosomes occur in pairs.
Unlike other chromosome pairs, the sex chromosomes come in two different forms, one carrying genes and the other rudimentary with little genetic material. These sex chromosomes are generally designated as X and Y, respectively. The way the sex chromosomes are paired determines the sex of an individual. Unlike mammals, in which individuals having two X chromosomes are female, and those with single X and Y chromosomes are male, birds with two X chromosomes are male while those with X and Y chromosomes are female. Biological traits controlled by genes on a sex chromosome are said to be sex-linked, because the way they are inherited is linked to the inheritance of the sex chromosome itself.