This situation occurs because the germline of one parent is affected by a

This situation occurs because the germline of one

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offspring. This situation occurs because the germline of one parent is affected by a mutation but the parent's somatic cells are unaffected. Skipped generations are not seen in classic autosomal dominant pedigrees. Males and females are equally likely to exhibit autosomal dominant diseases and to pass them on to their offspring. A gene that is not always expressed phenotypically is said to have incomplete penetrance. Penetrance may be age-dependent, as in Huntington disease and familial breast cancer. Variable expressivity is a characteristic of many genetic diseases. Most commonly, parents of children with autosomal recessive diseases are bothheterozygous carriers of the disease gene. In this case, the recurrence risk for autosomal recessive diseases is 25%. Males and females are equally likely to be affected by autosomal recessive diseases. Consanguinity is sometimes present in families with autosomal recessive diseases, and it becomes more prevalent with rarer recessive diseases. Carrier detection tests for an increasing number of autosomal recessive diseases are available. The frequency of genetic diseases approximately doubles in the offspring of first-cousin matings. In each normal female somatic cell, one of the two X chromosomes is inactivated early in embryogenesis. X inactivation is random, fixed, and incomplete (i.e., only part of the chromosome is actually inactivated). It may involve methylation. Gender is determined embryonically by the presence ofthe SRY gene on the Y chromosome. Embryos that have a Y chromosome (and thus the SRY gene) become males, whereas those lacking the Y chromosome become females. When the Y chromosome lacks the SRY gene, an XY female can be produced. Similarly, an X chromosome that contains the SRY gene can produce an XX male. X-linked genes are those that are located on the X chromosome. Nearly all known X-linked diseases are caused by X-linked recessive genes. Males are hemizygous for genes on the X chromosome. X-linked recessive diseases are seen much more often in males than in females because males need only one copy of the gene to express the disease. Fathers cannot pass X-linked genes to their sons. Skipped generations are often seen in X-linked recessive disease pedigrees because the gene can be transmitted through carrier females. Recurrence risks for X-linked recessive diseases depend on the carrier and affected status of the mother and father. A sex-limited trait is one that occurs in only one of the sexes. A sex-influenced trait is one that occurs more often in one sex than in the other.

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