A sex-linked gene is simply a gene that is located on a sex chromosome and a sex-linked trait is one that is determined by a sex-linked gene. These are distinguished from autosomal genes which are found on autosomes (non-sex chromosomes).

So before going further we should make sure we know what a sex chromosome is. A sex chromosome is often defined as a chromosome that is involved with sex determination. However, this is not a very good definition since many autosomal genes play a role in sex determination. Sex chromosomes were originally discovered and named because of a different chromosome composition between the sexes but this does not necessarily mean that there is a role in sex determination for the chromosomes that differ. Therefore, a far better way to define a sex chromosome is any chromosome that is part of the homologous pair that differs in the heterogametic sex of the species. The heterogametic sex is defined below: this is the sex in which the sex chromosomes are not the same, such as XY in humans.

In humans, and mammals in general, sex linkage refers to either X-linked or Y-linked genes. Note that sex-linkage and X-linkage are NOT synonymous. Although there are very few Y-linked genes in humans, meaning that most sex-linked genes are X-linked, make sure you distinguish sex-linkage and X-linkage. Well known X-linked traits in humans include Haemophilia A (which is a result of a deficiency of clotting Factor VIII, a protein coded by an X-linked gene and Haemophilia B (a deficiency in clotting Factor IX) - though Haemophilia C is NOT X-linked - color blindness, Fragile X syndrome and various traits associated with deficiencies in MAOA. One of the few commonly known Y-linked traits is Hairy Ears, and the SRY gene, responsible for triggering testes development, is also Y-linked.

There is nothing different about sex-linked genes (in relation to autosomal genes) other than the fact that they are located on sex chromosomes and they do not necessarily have anything to do with sex determination.

In terms of allele designations the same rules apply. However, alleles for sex-linked genes are generally designated by a superscript of the sex chromosome that carries them. For example, X^a indicates a recessive allele (to wild type) on the X chromosome and X^a⁺ or simply X⁺ indicates the wild type allele for this gene.

You should be sure to differentiate sex-linked traits from sex-limited traits, which are traits that can be coded by any gene, autosomal or sex-linked, but which are expressed in only one sex.

You should be familiar with the following terms:

Homogametic: This refers to the sex that produces gametes that all have the same chromosome content. In humans this sex is females: since they are XX they only produce eggs with an X chromosome.

Heterogametic: This refers to the sex that produces gametes with different chromosome contents. In humans this sex is males: since they are XY they produce some sperm (50%) with a Y chromosome and the other half with an X chromosome. Of course, this results from segregation of X and Y during meiosis.

Hemizygous: The heterogametic sex carries only a single copy of most sex-linked genes. Instead of being homozygous or heterozygous, these individuals are referred to as hemizygous for whatever allele they carry for these genes.

In birds and some species of fish, as well as various other species representing crustacea and insects, sex is determined by the ZW sex-determination system. In these species, the male is homogametic (ZZ) while the female is heterogametic (ZW). The basic concepts of sex-linkage hold for these species allowing for the fact that females are heterogametic. The letters Z and W are used simply to make it easily distinguishable from the XY system. In terms of general inheritance patterns, Z-linked genes behave like X-linked genes.

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