# Sex linkage

**Sex linkage** is the pattern of inheritance associated with genes on sex chromosomes; inheritance patterns depend on the chromosome system and can differ according to the sex of the transmitting parent and offspring. In humans the term usually refers to genes on the [X chromosome](https://www.edgepedia.org/x-chromosome), which is far larger and carries far more genes than the Y, so many recessive alleles on a male's single X can be expressed because there is no second X-linked allele to mask them.<sup>[1](https://www.genome.gov/genetics-glossary/Sex-Linked)</sup><sup> • </sup><sup>[2](https://www.britannica.com/science/heredity-genetics/Sex-linkage)</sup>

| Key fact | Detail |
|---|---|
| Definition | Traits or disorders influenced by genes on the sex chromosomes; in humans, mostly the X chromosome<sup>[1](https://www.genome.gov/genetics-glossary/Sex-Linked)</sup> |
| Chromosome sizes | X: 155.27 Mb (5% of the 3.10-billion-bp genome); Y: 59.37 Mb (1.9%); the X is 2.6 times the size of the Y<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> |
| Gene content | X: 1,965 total genes, 825 known protein-coding; Y: 421 total, 47 known protein-coding; gene density 17.8-fold higher on X<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> |
| Disease burden | The X holds about 4% of human genes but almost 10% of mendelian disease loci (307 of 3,199 in OMIM circa 2005); later counts reach at least 533 X-linked disorders<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2665286/)</sup><sup> • </sup><sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK557383/)</sup> |
| Recurrence risk | Carrier mother × unaffected father: each son has a 50% chance of being affected, each daughter a 50% chance of being a carrier; an affected father passes his X-linked allele to all daughters and none of his sons<sup>[6](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)</sup> |
| De novo mutations | One-third of isolated cases of affected males with X-linked disorders are new mutations<sup>[7](https://www.ncbi.nlm.nih.gov/books/NBK11593/)</sup> |
| Dosage compensation | One X is inactivated in each cell of people with two X chromosomes; about 50% of females show inactivation skews of 65:35 or greater in blood<sup>[8](https://medlineplus.gov/genetics/chromosome/x/)</sup><sup> • </sup><sup>[9](https://genome.cshlp.org/content/34/11/1954)</sup> |

## What sex linkage means

Genes outside pseudoautosomal regions can show inheritance patterns that differ from those of autosomal genes. Because a father passes his [Y chromosome](https://www.edgepedia.org/y-chromosome) to his sons and his X chromosome to his daughters, X-linked traits follow a crisscross pattern: a male's X-linked allele is passed to his daughters and can be passed on by them to their sons, but is not passed to his sons.<sup>[2](https://www.britannica.com/science/heredity-genetics/Sex-linkage)</sup>

The concept dates to [Thomas Hunt Morgan](https://www.edgepedia.org/thomas-hunt-morgan)'s 1910 work on white eyes in *Drosophila*. White-eyed females crossed to normal red-eyed males produce red-eyed daughters and white-eyed sons in the F1 generation, the classic sex-linked result.<sup>[2](https://www.britannica.com/science/heredity-genetics/Sex-linkage)</sup> [Calvin Bridges](https://www.edgepedia.org/calvin-bridges) then supplied the proof. He proposed that the rare exceptional progeny arose from meiotic nondisjunction, a failure of the two homologous X chromosomes to segregate at the first meiotic division, producing eggs with either both maternal X chromosomes or none, and verified cytologically that half of regular females and all exceptional females had the XXY constitution, which had never been observed previously.<sup>[10](https://academic.oup.com/genetics/article-abstract/202/1/15/5930267)</sup> His 1916 data were consistent with the assumption that females producing secondary exceptions do so because of an additional, aberrant sex chromosome constitution.<sup>[11](http://www.esp.org/foundations/genetics/classical/holdings/b/cbb-1916b.pdf)</sup>

## The mechanism: X, Y, and meiosis

The size and gene-content asymmetry between X and Y drives everything else. The X chromosome is 155.27 Mb, about 5% of the 3.10-billion-bp genome, and the Y is 59.37 Mb, about 1.9%, making the X 2.6 times the size of the Y.<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> The X carries 1,965 total genes against 421 on the Y, with 825 known protein-coding genes on X versus 47 on Y, and a gene density 17.8-fold higher: one gene per 7,901 bp on X versus one per 141,029 bp on Y.<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> Other counts differ by source and genome build: the X has been reported as bearing 1,669 genes and about 5% of the haploid genome,<sup>[12](https://www.nature.com/articles/hdy2011106)</sup> and as 156,040,895 base pairs with 857 coding genes, while the Y is given as 57,227,415 base pairs with 64 coding genes and no essential genes for viability.<sup>[13](https://bio.libretexts.org/Courses/University_of_Massachusetts_Boston/Bio_252_254%3A_Genetics/14%3A_SPOC_XIV_-_Sex-Linkage/14.01%3A_Exceptions_to_autosomal_inheritance)</sup>

**Recombination is the key.** The male X chromosome fails to recombine along virtually its entire length during meiosis; recombination is restricted to short regions at the tips of the chromosome arms, the pseudoautosomal regions.<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2665286/)</sup> PAR1, at the ends of Xp/Yp, is 2.64 Mb with 24 genes and transcripts, and PAR2, at the ends of Xq/Yq, is 330 kb with 8 genes and transcripts.<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> Because genes in these regions have homologous partners on both the X and the Y and recombine like ordinary pairs, they do not follow sex-linked inheritance.

Outside the PAR, a male's X has no partner, so he is hemizygous for every X-linked gene: one copy, so a recessive allele is not masked by a second copy of the gene. The Y, meanwhile, is gene poor and largely heterochromatic; its male-specific region bears 156 transcription units including 78 protein-coding units, many amplified, which collectively code for only 27 distinct proteins, 20 of which have X-derived partners.<sup>[12](https://www.nature.com/articles/hdy2011106)</sup> Of the over 600 genes the human Y once shared with the X, only 17 retain functionality.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup>

## Patterns of inheritance

**X-linked recessive.** A carrier mother has a 50% chance of passing the gene on with each pregnancy; each son has a 50% chance of being affected and each daughter a 50% chance of being a carrier.<sup>[6](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)</sup> With a carrier mother and an unaffected father, a son is affected with 50% probability and a daughter with 0%.<sup>[13](https://bio.libretexts.org/Courses/University_of_Massachusetts_Boston/Bio_252_254%3A_Genetics/14%3A_SPOC_XIV_-_Sex-Linkage/14.01%3A_Exceptions_to_autosomal_inheritance)</sup> An affected father transmits the condition to none of his sons, who receive only his Y, but all of his daughters inherit his X-linked allele.<sup>[6](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)</sup> Affected homozygous females are exceptionally rare in X-linked recessive disorders.<sup>[7](https://www.ncbi.nlm.nih.gov/books/NBK11593/)</sup>

**De novo mutation and counseling.** One-third of isolated cases of affected males are due to new mutations, with the mutation inherited from a heterozygous mother in the remaining two-thirds.<sup>[7](https://www.ncbi.nlm.nih.gov/books/NBK11593/)</sup> This matters for families with no prior history: an isolated affected boy may reflect a new mutation in that child or a maternal carrier state, which changes the recurrence risk for future pregnancies. De novo mutations on the paternal X are always found in daughters, not sons, who receive their father's Y.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup>

**A classification in retreat.** A 2025 review of penetrance, expressivity, and X chromosome inactivation data for 55 X-linked genes and 57 X-linked disorders concluded that the conventional division of X-linked inheritance into dominant and recessive subtypes is biologically flawed and should be retired, proposing four new groups instead.<sup>[15](https://www.gimjournal.org/article/S1098-3600(25)00031-0/pdf)</sup> The same analysis found that X-linked disorders with intermediate or high female penetrance greatly outnumber those with low female penetrance, and GeneReviews entries have already removed references to X-linked dominant and recessive inheritance.<sup>[15](https://www.gimjournal.org/article/S1098-3600(25)00031-0/pdf)</sup>

## Classic examples and their frequencies

Red-green color blindness is the common textbook case, affecting at least 10% of men and only about 1% of women, and may be partial or complete.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK557383/)</sup> Hemophilia A results from a mutation in the factor VIII gene and may be inherited or occur by spontaneous mutation.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK557383/)</sup> An XY individual hemizygous for a mutant F8 allele has hemophilia type A, whereas heterozygous carrier females are typically unaffected because roughly half their liver cells still export [Factor VIII](https://www.edgepedia.org/factor-viii).<sup>[13](https://bio.libretexts.org/Courses/University_of_Massachusetts_Boston/Bio_252_254%3A_Genetics/14%3A_SPOC_XIV_-_Sex-Linkage/14.01%3A_Exceptions_to_autosomal_inheritance)</sup> Duchenne muscular dystrophy and color blindness are listed among the classic X-linked recessive examples, while X-linked dominant disorders tend to be lethal in male offspring, as in incontinentia pigmenti.<sup>[6](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)</sup>

**Females get these disorders too.** An electronic health record cohort identified 39 women with X-linked disorders classified as X-linked recessive or X-linked dominant male-lethal, most commonly [Fabry disease](https://www.edgepedia.org/fabry-disease) (14 cases), Duchenne/[Becker muscular dystrophy](https://www.edgepedia.org/becker-muscular-dystrophy) (6), and X-linked intellectual disability (5); 4 of the 20 disorders affecting females in the cohort had previously not been known to affect females.<sup>[16](https://doi.org/10.1002/ajmg.a.63527)</sup> In the same cohort, female-to-male ratios were 0.06 for X-linked recessive disorders and 1.63 for X-linked dominant disorders, or 2.97 excluding fragile X.<sup>[16](https://doi.org/10.1002/ajmg.a.63527)</sup> Intellectual disability overall is 30% more common in males, and over 100 X-linked intellectual disability genes have been identified.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup>

## Dosage compensation and X-inactivation

Females carry roughly 1,000 X-linked genes in two copies against the male's one, a dosage imbalance compensated by inactivation of one of the two X chromosomes in early female development, as Mary Lyon hypothesized in 1961.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup> [X-inactivation](https://www.edgepedia.org/x-inactivation) generally leaves one X chromosome active in people with two X chromosomes, although some X-linked genes escape silencing, and the inactivated X is usually chosen at random.<sup>[8](https://medlineplus.gov/genetics/chromosome/x/)</sup> The result is that heterozygous females are mosaics of two cell populations, one with the mutation on the active X and one with the wild-type allele on the active X.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup>

**Skewing changes severity.** Carrier women may be unaffected or have symptoms; skewed X-inactivation that keeps the abnormal X active in most cells can contribute to severity.<sup>[6](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)</sup> Skewed XCI can arise from selection against cells expressing the trait, from stochastic events, or from selection against other alleles on the X.<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup> It is common: an estimated 50% of females display skews of 65:35 or greater in blood.<sup>[9](https://genome.cshlp.org/content/34/11/1954)</sup> In a 2024 study of 16 females with X-linked inherited retinal disease, skewed X-inactivation was overrepresented, and in all severely affected females the skew orientation was deleterious; skewing correlated with severity of RPGR-associated retinitis pigmentosa in female carriers.<sup>[9](https://genome.cshlp.org/content/34/11/1954)</sup>

Not every X gene is silenced. An estimated 15% to 25% of all genes on the X chromosome are postulated to escape X inactivation.<sup>[16](https://doi.org/10.1002/ajmg.a.63527)</sup> Inactivation also tolerates aneuploidy: XXX and XXY karyotypes are viable largely because only one X remains active, while more than 95% of XO ([Turner syndrome](https://www.edgepedia.org/turner-syndrome)) fetuses rapidly abort, indicating strict dosage requirements for many genes on the therian X.<sup>[12](https://www.nature.com/articles/hdy2011106)</sup>

## By the numbers

The X chromosome's outsized disease share is the central quantitative fact: it contains only about 4% of all human genes, yet almost 10% of diseases with a mendelian pattern of inheritance, 307 out of 3,199 in OMIM at the time of the X chromosome sequence publication, have been assigned to it; the molecular basis for 168 X-linked phenotypes had then been determined.<sup>[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC2665286/)</sup> A later reference work counts at least 533 disorders due to genes on the X chromosome, most involving tissues such as bone, neural, blood, hepatic, renal, retina, ears, cardiac, skin, and teeth.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK557383/)</sup>

Other anchors: the X is 155.27 Mb against the Y's 59.37 Mb,<sup>[3](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)</sup> though the Y is also reported as about 55 Mb<sup>[14](https://academic.oup.com/hmg/article/27/R2/R242/4986911)</sup> or 60 Mb;<sup>[12](https://www.nature.com/articles/hdy2011106)</sup> about half of females show X-inactivation skews of 65:35 or greater;<sup>[9](https://genome.cshlp.org/content/34/11/1954)</sup> one-third of isolated affected male cases are de novo;<sup>[7](https://www.ncbi.nlm.nih.gov/books/NBK11593/)</sup> and red-green color blindness affects at least 10% of men and about 1% of women.<sup>[5](https://www.ncbi.nlm.nih.gov/books/NBK557383/)</sup>

## How it compares with other patterns and other species

Against autosomal inheritance, sex linkage differs in two visible ways: no male-to-male transmission, because a man never passes his X to his sons;<sup>[7](https://www.ncbi.nlm.nih.gov/books/NBK11593/)</sup> and hemizygous expression in males, which makes recessive alleles behave as dominant.<sup>[2](https://www.britannica.com/science/heredity-genetics/Sex-linkage)</sup>

The mammalian model is not universal. All therian mammals, eutherian and marsupial, use an XY male/XX female system in which maleness is normally determined by the dominant Y-borne SRY gene, whereas birds, snakes, and many lizards have a female heterogametic system with ZZ males and ZW females.<sup>[12](https://www.nature.com/articles/hdy2011106)</sup> In birds, female birds are the ones with a single copy of Z-linked genes, reversing which sex shows Z-linked traits.<sup>[13](https://bio.libretexts.org/Courses/University_of_Massachusetts_Boston/Bio_252_254%3A_Genetics/14%3A_SPOC_XIV_-_Sex-Linkage/14.01%3A_Exceptions_to_autosomal_inheritance)</sup> X chromosome inactivation is a whole-X dosage compensation system unique to therian mammals; in birds, reptiles, fish, and even monotreme mammals, dosage compensation is usually partial and gene-specific, with some examples of regional control.<sup>[17](https://www.nature.com/articles/nrg.2015.2)</sup> Mammalian X inactivation is initiated by coating the X to be inactivated with a non-coding RNA transcribed from an X locus, independently evolved in placental mammals and marsupials, and in placentals the silencing is locked in by [DNA methylation](https://www.edgepedia.org/dna-methylation).<sup>[17](https://www.nature.com/articles/nrg.2015.2)</sup>

The Y's role also varies. In *Drosophila*, Bridges concluded that the Y chromosome plays no role in sex determination and apparently contains no genes other than those required for male fertility; XO males are normal males but sterile.<sup>[10](https://academic.oup.com/genetics/article-abstract/202/1/15/5930267)</sup> Beyond classical inheritance, accumulating evidence shows that sex chromosomes exert distinct, cell-autonomous effects on health, aging, and disease, with XX and XY complements altering gene dosage, epigenetic programming, and immune function beyond gonadal hormone effects.<sup>[18](https://www.science.org/doi/10.1126/science.aeh0145)</sup>

## What has changed since 2023

**Carrier screening expanded.** The 2024 ACMG technical standard divides carrier screening into four tiers, with tier 3 comprising 97 autosomal recessive genes and 16 X-linked genes, including [Duchenne muscular dystrophy](https://www.edgepedia.org/duchenne-muscular-dystrophy) (DMD) and Fragile X Messenger Ribonucleoprotein 1 (FMR1), and recommends that all pregnant patients and those planning a pregnancy be offered tier 3 screening.<sup>[19](https://www.gimjournal.org/article/S1098-3600(24)00070-4/fulltext)</sup> The same standard notes that routinely screening for conditions with a carrier frequency below 1/200 adds a "diminishingly small" number of at-risk couples.<sup>[19](https://www.gimjournal.org/article/S1098-3600(24)00070-4/fulltext)</sup>

**Classification and prognosis moved.** The 2025 proposal to retire the X-linked dominant/recessive classification, already implemented in GeneReviews entries, replaces pedigree-based labels with groups based on penetrance and X-inactivation behavior.<sup>[15](https://www.gimjournal.org/article/S1098-3600(25)00031-0/pdf)</sup> Adaptive nanopore sequencing now measures X-inactivation skew, and its correlation with RPGR retinitis pigmentosa severity in female carriers supports investigating its use in prognostics as gene therapies for RPGR and CHM emerge.<sup>[9](https://genome.cshlp.org/content/34/11/1954)</sup> [Electronic health record](https://www.edgepedia.org/electronic-health-record) data are also reshaping expectations: the 2024 EHR cohort documented female cases of disorders previously thought not to affect females.<sup>[16](https://doi.org/10.1002/ajmg.a.63527)</sup>

## References

1. [Sex Linked, NHGRI Genetics Glossary](https://www.genome.gov/genetics-glossary/Sex-Linked)
2. [Heredity: Sex Linkage, Encyclopaedia Britannica](https://www.britannica.com/science/heredity-genetics/Sex-linkage)
3. [Sex Chromosomes and Sex Chromosome Abnormalities, Clinics in Laboratory Medicine](https://www.labmed.theclinics.com/article/S0272-2712(11)00079-5/abstract)
4. [The DNA sequence of the human X chromosome (Ross et al., Nature 2005)](https://pmc.ncbi.nlm.nih.gov/articles/PMC2665286/)
5. [Genetics, X-Linked Inheritance, StatPearls/NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK557383/)
6. [Modern Genetics in Obstetrics and Gynecology, ACOG](https://www.acog.org/clinical/clinical-guidance/technology-assessment/articles/2018/09/modern-genetics-in-obstetrics-and-gynecology)
7. [Chapter 7: General aspects of X-linked diseases, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK11593/)
8. [X chromosome, MedlinePlus Genetics](https://medlineplus.gov/genetics/chromosome/x/)
9. [Measuring X-Chromosome inactivation skew for X-linked diseases with adaptive nanopore sequencing, Genome Research (2024)](https://genome.cshlp.org/content/34/11/1954)
10. [Centenary of GENETICS: Bridges to the Future, Genetics (Oxford Academic)](https://academic.oup.com/genetics/article-abstract/202/1/15/5930267)
11. [Bridges (1916), secondary nondisjunction paper, ESP classical holdings](http://www.esp.org/foundations/genetics/classical/holdings/b/cbb-1916b.pdf)
12. [The origin and evolution of vertebrate sex chromosomes and dosage compensation, Heredity](https://www.nature.com/articles/hdy2011106)
13. [Exceptions to autosomal inheritance, Biology LibreTexts](https://bio.libretexts.org/Courses/University_of_Massachusetts_Boston/Bio_252_254%3A_Genetics/14%3A_SPOC_XIV_-_Sex-Linkage/14.01%3A_Exceptions_to_autosomal_inheritance)
14. [The eXceptional nature of the X chromosome, Human Molecular Genetics](https://academic.oup.com/hmg/article/27/R2/R242/4986911)
15. [Patterns of X-linked inheritance: A new approach for the genome era, Genetics in Medicine (2025)](https://www.gimjournal.org/article/S1098-3600(25)00031-0/pdf)
16. [Data from electronic healthcare records expand our understanding of X-linked genetic diseases, Am J Med Genet](https://doi.org/10.1002/ajmg.a.63527)
17. [Evolution of vertebrate sex chromosomes and dosage compensation, Nature Reviews Genetics](https://www.nature.com/articles/nrg.2015.2)
18. [X and Y chromosomes as determinants of aging and disease, Science](https://www.science.org/doi/10.1126/science.aeh0145)
19. [Laboratory testing for preconception/prenatal carrier screening: ACMG technical standard (2024)](https://www.gimjournal.org/article/S1098-3600(24)00070-4/fulltext)

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*Topic: Encyclopedia › Life and health › Biological foundations › Genetics and genomic reference › Classical and non-Mendelian inheritance*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license