If an abnormal recessive gene is paired with another abnormal one on the opposite chromosome, it will have an effect, but if the opposite gene is normal,the abnormality will not become manifest. Recessive disease in clinical practice usually results from the mating of heterozygous carriers. If the abnormal gene is represented by "a", the disease will appear in the individual with genetic configuration "aa" (homozygote) and not with the configuration "aA" (heterozygote). When two heterozygotes mate, the likely offspring can be considered as in the diagram (Figure 23.1). If a patient has recessively inherited disease, his or her parents are likely to be normal but there might be brothers or sisters with the disease. It is important to enquire whether the parents are blood relatives because this greatly increases the likelihood of transmission. If an individual with recessive disease marries someone with the same recessive disease, all the offspring will be affected. If one spouse is a carrier and the other has the disease, there is a risk that 50% of the offspring would be carriers and 50% would be affected. When a carrier marries a normal individual, 50% of the offspring are carriers. These expected findings could be calculated quite easily using the type of diagram shown in Figure 23.1. Common diseases inherited in this manner include sickle-cell disease and cystic fibrosis.
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