Sex-linked characteristics:
As well as being carried by normal, autosomal chromosomes, genetic disorders can also be sex-linked. A large majority of sex-linked diseases are linked to the X chromosome. This is because the Y chromosome is much smaller, and thus cannot house as many genes (36). A majority of these genes are concerned with basic cellular activities (26) and the production of sperm (9), leaving little room for variability. Also, the Y chromosome basically just recombines with the X chromosome from generation to generation as it has no identical chromosome to pair up with and cross over. Thus, it is very hard to get rid of any mutations on the Y chromosome, and as a result, a majority of the genes of the Y chromosome are unneccesary genetic junk. Also, it is impossible for this chromosome to contain any genes that are vital to an individual's health as women function normally without one. Thus, the only sex-linked diseases carried on the Y chromosome have to do with masculine issues, such as sperm motility.
Therefore, nearly all genetic disorders that are sex-linked are linked to the X chromosome. Sex-linked characteristics can either be classified as X-linked recessive or X-linked dominant. X-linked recessive traits will always be evident in males as there is no other X chromosome to mask the effects of the gene. Females will only experience an X-linked recessive genetic disorder if both of their X chromosomes are linked to a recessive version of the gene. Otherwise, the other X chromosome masks the effect of the gene, and the female remains unaffected, but becomes a carrier for the disease. If a man is affected by a X-linked recessive trait, then all his daughters will inherit the X-linked recessive mutant gene from him, but there is no father to son transmission, as the disease is linked to the X chromosome only. As a result, more males than females suffer from X-linked recessive disorders. Examples of such disorders are red-green colourblindness and haemophilia.
X-linked dominant traits cannot be masked by any other allele. Thus, as it is an X-linked disorder, females are more frequently affected than males, as they have two X chromosomes and thus have a greater chance of receiving an affected gene. However, females are usually more mildly affected than males. For an affected male, all his daughters and none of his sons will inherit the condition, while the children of an affected female, regardless of sex, will have a 50% chance of inheriting the mutated gene.
When characteristics located on the X chromosome were studied, a pattern of inheritance emerged between the two sexes. When representing sex-linked characteristics diagrammatically, the X and Y chromosomes must be shown with the characteristic in question represented as a superscript, but following normal allele rules. If a woman is a carrier of an X-linked recessive disorder, and her husband is free of this disorder, then none of their daughter's will inherit the condition, but half of them will be a carrier. Their sons have a 50% chance of inheriting the disorder.
If a man has an X-linked recessive disorder and his wife does not carry the allele at all, then none of the boys will inherit the gene, while all their daughters will be carriers.
Therefore, nearly all genetic disorders that are sex-linked are linked to the X chromosome. Sex-linked characteristics can either be classified as X-linked recessive or X-linked dominant. X-linked recessive traits will always be evident in males as there is no other X chromosome to mask the effects of the gene. Females will only experience an X-linked recessive genetic disorder if both of their X chromosomes are linked to a recessive version of the gene. Otherwise, the other X chromosome masks the effect of the gene, and the female remains unaffected, but becomes a carrier for the disease. If a man is affected by a X-linked recessive trait, then all his daughters will inherit the X-linked recessive mutant gene from him, but there is no father to son transmission, as the disease is linked to the X chromosome only. As a result, more males than females suffer from X-linked recessive disorders. Examples of such disorders are red-green colourblindness and haemophilia.
X-linked dominant traits cannot be masked by any other allele. Thus, as it is an X-linked disorder, females are more frequently affected than males, as they have two X chromosomes and thus have a greater chance of receiving an affected gene. However, females are usually more mildly affected than males. For an affected male, all his daughters and none of his sons will inherit the condition, while the children of an affected female, regardless of sex, will have a 50% chance of inheriting the mutated gene.
When characteristics located on the X chromosome were studied, a pattern of inheritance emerged between the two sexes. When representing sex-linked characteristics diagrammatically, the X and Y chromosomes must be shown with the characteristic in question represented as a superscript, but following normal allele rules. If a woman is a carrier of an X-linked recessive disorder, and her husband is free of this disorder, then none of their daughter's will inherit the condition, but half of them will be a carrier. Their sons have a 50% chance of inheriting the disorder.
If a man has an X-linked recessive disorder and his wife does not carry the allele at all, then none of the boys will inherit the gene, while all their daughters will be carriers.