Genetics

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Genetics

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.

There are only 5 basic patterns of single gene inheritance:

• Autosomal dominant
• Autosomal recessive
• X-linked dominant
• X-linked recessive
• Maternal (mitochondrial) 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).

In AUTOSOMAL RECESSIVE INHERITANCE, the parents of an affected individual may not express the disease. On average, the chance of an affected child's brothers or sisters having the disease are 1 in 4. Males and females are equally likely to be affected. For a child to have symptoms of an autosomal recessive disorder, the child must receive the defective gene from BOTH parents.

Because most recessive disorders are rare, a child is at increased risk of a recessive disease if the parents are related. Related individuals are more likely to have inherited the same rare gene from a common ancestor.