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Sex linkage
Sex linkage describes the sex-specific patterns of inheritance and expression when a gene is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). Genes situated on the X-chromosome are thus termed X-linked, and are transmitted by both males and females, while genes situated on the Y-chromosome are termed Y-linked, and are transmitted by males only. As human females possess two X-chromosomes and human males possess one X-chromosome and one Y-chromosome, the phenotype of a sex-linked trait can differ between males and females due to the differential number of alleles (polymorphisms) possessed for a given gene. In humans, sex-linked patterns of inheritance 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 sex-linked patterns of inheritance characteristically different from autosomal dominance and recessiveness. This article will discuss each of these patterns of inheritance, as well as diseases that commonly arise through these sex-linked patterns of inheritance. Variation in these inheritance patterns arising from aneuploidy of sex chromosomes, sex-linkage in non-human animals, and the history of the discovery of sex-linked inheritance are briefly introduced.
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.
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.
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.
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 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.
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Sex linkage AI simulator
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Sex linkage
Sex linkage describes the sex-specific patterns of inheritance and expression when a gene is present on a sex chromosome (allosome) rather than a non-sex chromosome (autosome). Genes situated on the X-chromosome are thus termed X-linked, and are transmitted by both males and females, while genes situated on the Y-chromosome are termed Y-linked, and are transmitted by males only. As human females possess two X-chromosomes and human males possess one X-chromosome and one Y-chromosome, the phenotype of a sex-linked trait can differ between males and females due to the differential number of alleles (polymorphisms) possessed for a given gene. In humans, sex-linked patterns of inheritance 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 sex-linked patterns of inheritance characteristically different from autosomal dominance and recessiveness. This article will discuss each of these patterns of inheritance, as well as diseases that commonly arise through these sex-linked patterns of inheritance. Variation in these inheritance patterns arising from aneuploidy of sex chromosomes, sex-linkage in non-human animals, and the history of the discovery of sex-linked inheritance are briefly introduced.
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.
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.
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.
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 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.