Two affected individuals have married one another.įamilial hypercholesterolaemia provides an example of the first situation the heterozygote frequency may be as high as 1 in 500, so one might expect chance marriages between such individuals to occur with a frequency of 1 in 250,000.The gene is common and relatively mild or late-onset in its effects.Homozygosity requires both parents to have transmitted the mutated gene this is most unlikely to happen unless one of the following is the case:
Homozygosity in autosomal dominant disease:Īlmost all patients seen with autosomal dominant conditions will be heterozygotes, having inherited their disorder from only one side of the family or representing new mutations. The uncovering of the molecular basis of some of this variability is proving to be one of the most interesting fields of human genetics, as well as helping to resolve the practical problems encountered in genetic counselling.
Problems arise from the variability of gene expression that is seen in many dominantly inherited disorders and which, until recently, has not been understood to any significant extent. The risk for offspring and more distant descendants of unaffected family members is not increased over the general population risk, provided that the individual really is unaffected. 50%, one in two, or with an odds ratio of regardless of sex and regardless of whether the disease is fully developed or sub- or pre-clinical. In its fully developed form, the pattern of autosomal dominant inheritance is characteristic and allows precise risks to be given The risk to offspring of affected members will be one half (i.e. In Huntington’s disease, however, the homozygote appears to be a little different from the heterozygote, so this is one of the few cases known to fit both definitions. familial hypercholesterolaemia) or lethal (e.g. The homozygous state is either unknown or excessively rare in dominantly inherited disorders, but when it does occur it is usually much more severe than the normal heterozygous form (e.g. An autosomal dominant disorder or trait can be defined either by the observed pattern of transmission between generations in families or as one that is largely or completely expressed in the heterozygote. Although, in theory, autosomal dominant inheritance is the simplest model for genetic counselling, in practice it provides some of the most difficult problems, with traps for the unwary that require special mention.
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