When a gene bearing a defect or disease gives rise to the disease even though the other one of the pair is normal, it is said to be dominant. An affected heterozygote can, therefore, have 50% of affected children when married to a normal spouse. Of course, if both spouses carry the abnormal dominant gene, all the offspring will be affected. Dominant inheritance can only be shown with certainty if three successive generations show the disease and if about 5% of individuals are affected. Also, one sex should not be affected more than the other (Figure 23.2). Examples of this type of inheritance are hereditary retinoblastoma and Marfan's disease.
Figure 23.2. Dominant inheritance.!
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