Single gene disorders - Sex-linked recessive inheritance
The sex chromosome Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacteria)., although mainly responsible for determining genderIn Biology, often used to describe someone's assigned sex at birth. A person's biological sex and gender identity are not always the same., carry some geneThe basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. that code for a number of body functions.
These genes are described as sex-linked.
The other 22 pairs of chromosome Genetic structures that usually occur in functional pairs in the nucleus of cells (except gametes and bacteria). have the same genes in the same position on both chromosomes.
The difference in size between the X chromosome and the shorter Y chromosome means some genes appear only on the X chromosome.
The effect of sex-linked genes is more obvious in males because the X chromosome can contain genes the Y does not.
A recessive An allele that will only show in the phenotype if there is no dominant allele present (i.e. when two recessive alleles are present). allele on the X chromosome will show in the phenotypeThe visible characteristics of an organism which occur as a result of its genes. as there is no dominant allele present to override it.
A female will have another allele on her other X chromosome which, if dominant, would mask the effect of a recessive allele.
Haemophilia
- Sufferers are unable to clotA clump of platelets and blood cells that forms when a blood vessel is damaged. their blood resulting in excessive bleeding, even from small cuts or bruises.
- It is a sex-linked, inherited condition caused by a recessive An allele that will only show in the phenotype if there is no dominant allele present (i.e. when two recessive alleles are present). allele on the X chromosome.
- Sufferers are almost exclusively males because they only need one recessive allele.
- However, if his X chromosome carries the dominant normal allele, he will be normal.
- Females with two normal alleles will also be normal.
- Only in the rare case of a female having two recessive alleles will they show the condition.
- Those women with one normal and one recessive allele are called carrierSomeone who does not suffer from a condition but carries the allele and can pass it to his or her offspring.. Although they themselves are normal, they can pass the recessive allele to half of their children.