Sex linkage describes the sex-specific patterns of inheritance and presentation when a gene mutation (allele) is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). In humans, these are termed X-linked recessive, X-linked dominant and Y-linked. The inheritance and presentation of all three differ depending on the sex of both the parent and the child. This makes them characteristically different from autosomal dominance and recessiveness.
There are many more X-linked conditions than Y-linked conditions, since humans have several times as many genes on the X chromosome than the Y chromosome. Only females are able to be carriers for X-linked conditions; males will always be affected by any X-linked condition, since they have no second X chromosome with a healthy copy of the gene. As such, X-linked recessive conditions affect males much more commonly than females.
In X-linked recessive inheritance, a son born to a carrier mother and an unaffected father has a 50% chance of being affected, while a daughter has a 50% chance of being a carrier, however a fraction of carriers may display a milder (or even full) form of the condition due to a phenomenon known as skewed X-inactivation, in which the normal process of inactivating half of the female body's X chromosomes preferably targets a certain parent's X chromosome (the father's in this case). If the father is affected, the son will not be affected, as he does not inherit the father's X chromosome, but the daughter will always be a carrier (and may occasionally present with symptoms due to aforementioned skewed X-inactivation).
In X-linked dominant inheritance, a son or daughter born to an affected mother and an unaffected father both have a 50% chance of being affected (though a few X-linked dominant conditions are embryonic lethal for the son, making them appear to only occur in females). If the father is affected, the son will always be unaffected, but the daughter will always be affected.
A Y-linked condition will only be inherited from father to son and will always affect every generation.
The inheritance patterns are different in animals that use sex-determination systems other than XY. In the ZW sex-determination system used by birds, the mammalian pattern is reversed, since the male is the homogametic sex (ZZ) and the female is heterogametic (ZW).
Background In humans (and mammals in general), biological sex is determined by genetics. However this is not the case for all organisms. For instance, the biological sex of select reptiles is environmentally determined, and the sex of select worms is dependent on location.
22 of the 23 pairs of human chromosomes are autosomal (not involved in sex determination), while the 23rd pair of human chromosomes are the sex chromosomes. The possession of two X-chromosomes defines a biological female, while the possession of one X and one Y chromosome defines a biological male. The two sex chromosomes differ in size and gene content, and unlike the sets of autosomal chromosomes, are not homologous. The X-chromosome contains an estimated 1400 genes, most of which are involved in tissue development and the development of human disorders. The Y-chromosome is host to the SRY gene, which is involved in the development of several male sex characteristics, while the identified functions of many of the remaining approximately 200 genes on the Y-chromosome are associated with human disease. Sex linkage thus refers to the association of a trait encoded by one of the genes on these sex chromosomes. There are many more X-linked conditions than Y-linked conditions due to the larger size of the X-chromosome and greater number of genes encoded within it.
In classical genetics, a mating experiment called a reciprocal cross is performed to test if an animal's trait is sex-linked.
X-linked patterns of inheritance A disease or trait determined by a gene on the X-chromosome demonstrates X-linked inheritance. Historically, X-linked inheritance has been divided into the patterns of X-linked dominant inheritance and X-linked recessive inheritance as synonymous with classical Mendelian inheritance of genes on autosomal chromosomes. However, more recently scholars have suggested the discontinuation of the use of the terms dominant and recessive when referring to X-linked inheritance, stating that the highly variable penetrance of X-linked traits in females as a result of mechanisms such as skewed X-inactivation or somatic mosaicism is difficult to reconcile with standard definitions of dominance and recessiveness. (A) (B) (C) 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.Genetics home reference (2006), genetic conditions illustrations, National Library of Medicine.
X-linked dominant inheritance
X-linked dominant inheritance is the pattern by which a trait encoded by an allele on the X-chromosome is passed down through generations, wherefore only one copy of that allele is sufficient for an individual to be affected (dominance). Unlike with X-linked recessive traits, females are more often affected by X-linked dominant traits than males as females have two X-chromosomes as opposed to one. Moreover, some X-linked dominant conditions (for example, Aicardi syndrome) are embryonically or neonatally lethal in hemizygous males (XY), and thus can be seen to only affect heterozygous females, who experience less severe symptoms.
Frequency and patterns of inheritance In X-linked dominant inheritance, the transmission of the gene depends on the genotype of each of the parents. A mother heterozygous for an X-linked dominant trait will be affected, and half of her children (whether female or male) will inherit her affected X-chromosome and be affected themselves, assuming an unaffected father. Of the children of a father affected by an X-linked dominant trait and an unaffected mother, all daughters will be affected (having inherited their father's only affected X-chromosome), while no sons will be affected (having received their father's Y-chromosome). A child affected by an X-linked dominant condition will always have at least one affected parent, and an affected son will always have an affected mother.
X-linked recessive inheritance
X-linked recessive inheritance describes one of the patterns of inheritance of genetic traits or disorders encoded by an allele (version of a gene) situated on the X-chromosome. In X-linked recessive inheritance, females (XX) must have two copies of the allele (homozygous), while males (XY) require only one copy of the allele (hemizygous) to display the phenotype. Thus, it is far more common for males to be affected by X-linked recessive traits.
A female heterozygous for an X-linked recessive trait is considered a carrier. While a carrier female most often does not display the phenotype, rare cases of skewed X-chromosome inactivation, can lead to varied levels of expression. This would occur when, during the normal process of inactivating half of a female's X-chromosomes, inactivation preferentially targets X-chromosomes inherited from a single parent, thus the remaining active X-chromosomes from the other parent are disproportionately expressed.
There are characteristic patterns for X-linked recessive inheritance. As each parent contributes one sex chromosome to their offspring, sons cannot receive the X-linked trait from affected fathers, who provide only a Y-chromosome. Consequently, affected males must inherit the X-linked recessive trait from their mothers. Sons of a healthy carrier female have a 50% change of being affected, while sons of an affected female will always be affected. Females must inherit a X-linked recessive allele from each parent. All daughters of an affected father and healthy non-carrier mother will be carriers. Daughters of an affected father and carrier mother will have a 50% chance of being affected, and daughters of two affected parents will always be affected. While there is no male-to-male transmission of X-linked recessive traits, an affected male can pass his recessive X allele to a grandson through his unaffected carrier daughter.
X-linked diseases
X-linked dominant diseases Aicardi syndrome Alport syndrome (majority of cases) Coffin–Lowry syndrome (CLS) Fragile-X syndrome Goltz syndrome Idiopathic hypoparathyroidism Incontinentia pigmenti Porphyria (2-10% of cases) Rett syndrome (5% of cases) Vitamin D resistant rickets (X-linked hypophosphatemia)
Alport syndrome Alport syndrome is a genetic disorder that primarily affects the kidneys by damaging the glomeruli, which are tiny filtering units inside the kidneys.
It is caused by inherited mutations in the COL4A3, COL4A4, and COL4A5 genes, which are responsible for producing type IV collagen. These mutations disrupt the formation of basement membranes, particularly in the kidneys, inner ears, and eyes, where this collagen network plays a key structural role. The specific gene affected determines the form of Alport syndrome: mutations in COL4A5 cause the X-linked form (XLAS), while mutations in COL4A3 or COL4A4, located on chromosome 2, are associated with autosomal recessive (ARAS) or autosomal dominant (ADAS) forms.
Common signs and symptoms of Alport syndrome include hematuria (blood in urine), proteinuria (protein in urine), hearing loss, and eye abnormalities.
Diagnosis typically involves urine and blood tests, hearing and vision assessments, and genetic testing to confirm the mutation and identify the specific type of Alport syndrome. Kidney biopsy can reveal a characteristic 'basketweave' appearance of the glomerular basement membrane under electron microscopy. In some cases, skin or kidney tissue may be tested for the absence of type IV collagen proteins. Genetic testing is especially useful when biopsy is inconclusive or unavailable.
While there is no cure, treatments focus on slowing the progression of the kidney disease and managing symptoms. ACE inhibitors are commonly used to control blood pressure and reduce proteinuria. If kidney failure develops, dialysis or kidney transplant is often effective. Rarely, transplant recipients may develop anti-GBM disease due to antibodies against type IV collagen.
Fragile X syndrome Fragile X syndrome is a genetic neurodevelopmental disorder caused by a CGG trinucleotide repeat expansion in the FMR1 gene on the X chromosome. It is inherited in an X-linked dominant pattern and is the most common known inherited genetic cause of autism spectrum disorder. The condition affects approximately 1 in 3,600 males and 1 in 4,000 to 6,000 females.
Fragile X syndrome occurs when the CGG repeat expansion exceeds 200 repeats, causing methylation and silencing of the FMR1 gene. This leads to a deficiency of FMRP, a protein essential for normal synaptic development and brain plasticity.
Fragile X syndrome typically presents with learning disabilities to severe intellectual impairment, along with distinct physical features such as a long face, large or protruding ears, flat feet, and low muscle tone. Many individuals, especially males, also exhibit behavioural traits including social anxiety, hyperactivity, hand-flapping, and self-injurious behaviors such as biting. Females, who have two X chromosomes, are generally less severely affected due to X-inactivation.