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Genetics
A single gene disorder (also called Mendelian disorder) is one that is determined by a single genetic locus and the specific allele on one or both members of a chromosome pair. Single gene defects are rare, with a frequency of less than 1 in 200 births. But since there are about 6,000 known single gene disorders, their combined impact is significant. The incidence of serious single gene disorders is estimated to be about 1 in 200 births. Single-gene disorders are characterized by the pattern of transmission in families -- this is called a pedigree. The term "kindred" includes the relatives outside of the immediate nuclear family. The affected individual that initially comes to light (or is of immediate interest) is called the proband. The brothers and sisters of the proband are called siblings. Text Continues Below
There are only 5 basic patterns of single gene inheritance: The observed effect of an abnormal gene (the appearance of a disorder) is called the abnormal phenotype. A phenotype expressed in the same way (in both homozygotes and heterozygotes) is dominant. A phenotype expressed only in homozygotes (or, for X-linked traits, expressed in males but not females) is recessive. Heterozygotes for a recessive gene are called carriers. They usually don't express the phenotype clinically, but it can frequently be identified by sensitive laboratory methods. In AUTOSOMAL DOMINANT INHERITANCE, the abnormality or abnormalities usually appear in every generation. Every affected child has an affected parent and each child of an affected parent has a 50% chance of inheriting the disease. Normal members of the family do not transmit the disease. Males and females are equally likely to have the disease and to transmit the disease. Male-to-male transmission can occur (unlike with X-linked inheritance), and males can have unaffected daughters (unlike with X-linked dominant inheritance).
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