A pair of sex chromosomes is present in all sexually reproducing organisms. In humans, females carry XX sex chromosomes and males carry XY sex chromosomes. Human sex is determined by the presence or absence of a Y chromosome. Genes that are carried on the sex chromosomes show inheritance patterns which are described as being sex-linked, and it can be detected from phenotypes that segregate differently between males and females. It is not possible to be certain about sex linkage from a pedigree chart, as autosomal traits could produce the same pattern of inheritance. X-linked inheritance can be eliminated as a possibility if a father passes a trait to his son. Fathers cannot pass X-linked traits to their sons as their X chromosomes are passed to their daughters; they only pass their Y chromosomes to their sons. Conversely, mothers pass their X chromosomes to their sons as well as their daughters, making it possible for X-linked traits to pass from mother to sons or daughters. As males contain only a single X chromosome, the chance of X-linked inheritance of disease is higher in males than in females. This is as the dominant or recessive allele on the single X chromosome will always be expressed; there is no other allele present. If the allele locus was on an autosome, there would be a homologous chromosome with a paired allele that could affect expression.

X-linked Recessive Patterns

The allele locus for an X-linked recessive condition is on the X chromosome/s. Males who have the recessive allele on their X chromosome will always express the phenotype as they only have one X chromosome. Females will only express the phenotype when both X chromosomes have the affected allele. For a female child to be affected, the father must be affected and the mother must be either affected or a carrier. A heterozygous female will be a carrier. Males show X-linked recessive phenotypes at a higher frequency in comparison to females as a result. In this type of inheritance, a male with the phenotype cannot pass the trait to his sons, as they inherit his Y chromosome only; his daughters will get the affected X chromosome, but they will only be present in the phenotype if they inherit another affected X chromosome from their mother. This type of inheritance can skip generations. X-linked Dominant Patterns

The allele locus for an X-linked dominant condition is on the X chromosome. This type of inheritance is similar to X-linked recessive inheritance, except that heterozygous females will always show the phenotype, and any individuals with the phenotype must have a parent with the phenotype. Males showing the phenotype will not pass the affected allele on to their sons, but will pass it on to all their daughters who will present the trait in their phenotype; this is as daughters always inherit their father's X chromosome. A heterozygous female is expected to pass on the allele to 50% of her offspring, regardless of the sex; the condition should appear in every generation. An affected male receives the dominant allele from an affected mother.

Y-linked Patterns If a trait is carried on the Y chromosome, it is said to be Y-linked. Only males are affected. Y-linked conditions or traits are rare as the Y chromosome is short and has a limited number of genes, most of which code for male sexual development. Inheritance of genes that appear on the Y chromosome is along the male line, from father to son.