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 the father.
In classical genetics, a reciprocal cross is performed to test if a trait is sex-linked.
|Illustration of some X-linked heredity outcomes (A) the affected father has one X-linked dominant allele, the mother is homozygous for the recessive allele: only daughters (all) will be affected. (B) the affected mother is heterozygous with one copy of the X-linked dominant allele: both daughters and sons will have 50% probability to be affected. (C) the heterozygous mother is called "carrier" because she has one copy of the recessive allele: sons will have 50% probability to be affected, 50% of unaffected daughters will become carriers like their mother.|
Each child of a mother affected with an X-linked dominant trait has a 1/2 chance of inheriting the mutation and thus being affected with the disorder. All female children of an affected father will be affected (daughters possess their fathers' X-chromosome). No male children of an affected father will be affected (sons do not inherit their fathers' X-chromosome).
- 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
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. (Males have a single X-chromosome and therefore have only one copy of X-linked genes.) All offspring of a carrier female have a 1 in 2 chance of inheriting the mutation. All female children of an affected father will be carriers. (Daughters possess their father's X-chromosome.) No male children of an affected father will be affected. (Sons do not inherit their father's X-chromosome.)
- 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
- Various failures in the SRY genes
- 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— 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 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.
See also 
- Morgan, Thomas Hunt 1919. The physical basis of heredity. Philadelphia: J.B. Lippincott Company.
- Genetics home reference (2006), genetic conditions illustrations, National Library of Medicine.
- Morgan T.H. 1910. Sex-limited inheritance in Drosophila. Science 32: 120-122
- Doncaster L. & Raynor G.H. 1906. Breeding experiments with Lepidoptera. Proceedings of the Zoological Society of London. 1: 125-133
- Zirkle, Conrad 1946. The discovery of sex-influenced, sex limited and sex-linked heredity. In Ashley Montagu M.F. (ed) Studies in the history of science and learning offered in homage to George Sarton on the occasion of his sixtieth birthday. New York: Schuman, p167–194.
- King R.C; Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press. ISBN 0-19-539761-5
- Mallet J. & Joron M. (1999). "The evolution of diversity in warning color and mimicry: polymorphisms, shifting balance, and speciation". Annual Review of Ecology and Systematics 30: 201–33. doi:10.1146/annurev.ecolsys.30.1.201.
- Ford E.B. 1965. Genetic polymorphism. p17-25. MIT Press 1965.
- Joron M, Papa R, Beltrán M, et al. (2006). "A conserved supergene locus controls colour pattern diversity in Heliconius butterflies". PLoS Biol. 4 (10): e303. doi:10.1371/journal.pbio.0040303. PMC 1570757. PMID 17002517.