Chapter 3. Genetic Disorders & Sex Chromosome Abnormalities

Mendelian Laws of Inheritance

Types of Inheritance

Autosomal Dominant

In autosomal dominant inheritance, it is assumed that a mutation has occurred in 1 gene of an allelic pair and that the presence of this new gene produces enough of the changed protein to give a different phenotypic effect. Environment must also be considered because the effect may vary under different environmental conditions. The following are characteristic of autosomal dominant inheritance:

1. The trait appears with equal frequency in both sexes.

2. For inheritance to take place, at least 1 parent must have the trait unless a new mutation has just occurred.

3. When a homozygous individual is mated to a normal individual, all offspring will carry the trait. When a heterozygous individual is mated to a normal individual, 50% of the offspring will show the trait.

4. If the trait is rare, most persons demonstrating it will be heterozygous (Table 3–1).

Autosomal Recessive

The mutant gene will not be capable of producing a new characteristic in the heterozygous state in this circumstance under customary environmental conditions—ie, with 50% of the genetic material producing the new protein, the phenotypic effect will not be different from that of the normal trait. When the environment is manipulated, the recessive trait occasionally becomes dominant. The characteristics of this form of inheritance are as follows:

1. The characteristic will occur with equal frequency in both sexes.

2. For the characteristic to be present, both parents must be carriers of the recessive trait.

3. If both parents are homozygous for the recessive trait, all offspring will have it.

4. If both parents are heterozygous for the recessive trait, 25% of the offspring will have it.

5. In pedigrees showing frequent occurrence of individuals with rare recessive characteristics, consanguinity is often present (Table 3–2).