Sex linkage



Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.

In mammals, the female is the homogametic sex, with two X chromosomes (XX), while the male is heterogametic, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex-linked.

In birds, the opposite is true: the male is the homogametic sex, having two Z chromosomes (ZZ), and the female (hen) is heterogametic, having one Z and one W chromosome (ZW).

X-linked recessive traits are expressed in all heterogametics, but are only expressed in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes hemophilia. Hemophilia is much more common in males than females because males are hemizygous - they only have one copy of the gene in question - and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. Tsarevich Alexei of Russia was the most famous sufferer of X-linked hemophilia.

The incidence of recessive X-linked phenotypes in females is the square of that in males (squaring a proportion less than one gives an outcome closer to 0 than the original). If 1 in 20 males in a human population are red-green color blind, then 1 in 400 females in the population are expected to be color-blind (1/20)*(1/20). (The term 'color-blind' is not completely accurate. There are degrees of weakness in color vision and it is now called 'color vision deficiency')

X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele. There are a few Y-linked traits; these are inherited from father.

In classical genetics, a reciprocal cross is performed to test if a trait is sex-linked.

X-linked dominant inheritance
Each child of a mother affected with an X-linked dominant trait has a 50% chance of inheriting the mutation and thus being affected with the disorder. If only the father is affected, 100% of the daughters will be affected, since they inherit their father's X-chromosome, and 0% of the sons will be affected, since they inherit their father's Y-chromosome.

Examples

 * Alport's syndrome
 * Aarsog's syndrome
 * Coffin-Lowry syndrome (CLS)
 * idiopathic hypoparathyroidism
 * incontinentia pigmenti
 * Ornithine carbamoyltransferase deficiency
 * Rett syndrome (RS)
 * vitamin D resistant rickets
 * fragile X syndrome

X-linked recessive inheritance
Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder. All males possessing an X-linked recessive mutation will be affected, since males have only a single X-chromosome and therefore have only one copy of X-linked genes. All offspring of a carrier female have a 50% chance of inheriting the mutation if the father does not carry the recessive allele. All female children of an affected father will be carriers (assuming the mother is not affected or a carrier), as daughters possess their father's X-chromosome. No male children of an affected father will be affected, as males only inherit their father's Y-chromosome.

Examples

 * ALD
 * Duchenne muscular dystrophy
 * Hunter syndrome
 * Menkes disease (kinky hair syndrome)
 * Glucose-6-phosphate dehydrogenase deficiency
 * Hemophilia A and B
 * Fabry's disease
 * Wiskott-Aldrich syndrome
 * Bruton's agammaglobulinemia
 * Color blindness
 * Complete androgen insensitivity syndrome
 * Congenital aqueductal stenosis (hydrocephalus)
 * Inherited nephrogenic diabetes insipidus

Y-linked

 * Various failures in the SRY genes

Sex-linked traits in other animals

 * Fur color in domestic cats: the gene that causes orange pigment is on the X chromosome; thus a Calico or tortoiseshell cat, with both black (or gray) and orange pigment, is nearly always female.
 * White eyes in Drosophila melanogaster flies&mdash; the first sex-linked gene discovered in Drosophila.
 * The first sex-linked gene ever discovered was the "lacticolor" X-linked recessive gene in the moth Abraxas grossulariata by Leonard Doncaster.

Related terms
It is important to distinguish between sex-linked characters, which are controlled by genes on sex chromosomes, and two other categories.

Sex-influenced traits
Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body. Even in a homozygous dominant or recessive female the condition may not be expressed fully. Example: baldness in humans.

Sex-limited traits
These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes. Examples: female sterility in Drosophila; and many polymorphic characters in insects, especially in relation to mimicry. Closely linked genes on autosomes called "supergenes" are often responsible for the latter.