Sex linkage

 

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.^{[citation needed]}

There are fewer X-linked dominant conditions than X-linked recessive, because dominance in X-linkage requires the condition to present in females with only a fraction of the reduction in gene expression of autosomal dominance, since roughly half (or as many as 90% in some cases) of a particular parent's X chromosomes are inactivated in females.^{[citation needed]}

• Alport syndrome
• Coffin–Lowry syndrome (CLS)
• Fragile X syndrome
• Idiopathic hypoparathyroidism
• Incontinentia pigmenti
• Rett syndrome (RS)
• Vitamin D resistant rickets (X-linked hypophosphatemia)

Females possessing one X-linked recessive mutation are considered carriers and will generally not manifest clinical symptoms of the disorder, although differences in X chromosome inactivation can lead to varying degrees of clinical expression in carrier females since some cells will express one X allele and some will express the other. 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. If the mother is not a carrier, no male children of an affected father will be affected, as males only inherit their father's Y chromosome.^{[citation needed]}

The incidence of X-linked recessive conditions in females is the square of that in males: for example, 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).

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

• Various failures in the SRY genes

• White eyes in Drosophila melanogaster flies was one of the earliest sex-linked genes discovered.^[3]
• 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.
• The first sex-linked gene ever discovered was the "lacticolor" X-linked recessive gene in the moth Abraxas grossulariata by Leonard Doncaster.^[4]

It is important to distinguish between sex-linked characters, which are controlled by genes on sex chromosomes, and two other categories.^[5]

Sex-influenced or sex-conditioned traits are phenotypes affected by whether they appear in a male or female body.^[6] Even in a homozygous dominant or recessive female the condition may not be expressed fully. Example: baldness in humans.

These are characters only expressed in one sex. They may be caused by genes on either autosomal or sex chromosomes.^[6] 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.^[7][8][9]

• X-linked dominant inheritance
• X-linked recessive inheritance