Singlegene disorders

Although disorders showing a simple Mendelian pattern of inheritance are rare, they tend to be clinically severe and collectively impose a significant burden.

For autosomal dominant disorders to manifest themselves, only one disease allele is necessary, i.e. both heterozygotes as well as homozygotes (those who carry both disease genes) will be affected. In most instances, where there is one affected parent who is a heterozygote for the disease, approximately 50 per cent of the offspring will show the disorder. Autosomal disorders tend to be severe and manifest themselves in every generation. Huntington's disease and acute intermittent porphyria are examples of autosomal dominant conditions that often present with psychiatric symptoms.

Autosomal recessive conditions such as phenylketonuria require the presence of two disease alleles to show clinical manifestations of the disorder. Thus, they often appear to skip generations. These disorders usually occur in the offspring of two 'carrier' heterozygote parents and are more common where there is a high rate of consanguinity (e.g. marriages between cousins) as these inbred populations will show greater homozygosity at all loci.

The other group of single gene disorders consists of sex-linked conditions such as fragile X syndrome. Normal females have two X chromosomes whereas normal males have one X chromosome and one Y chromosome. Thus, for recessive disorders on the X chromosome, if the mother is a carrier (X*X) and assuming that the father is unaffected (XY), half of her sons will manifest the disorder (X*Y) and half of her daughters will be carriers (X*X). Where the father is affected by an X-linked recessive condition, all the daughters will be carriers. As sons have to inherit their X chromosome from their mother, there will be an absence of father to son transmission. X-linked dominant conditions are extremely rare.

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