Chapter 3: Patterns of Inheritance

PRINCIPLES OF MENDELIAN INHERITANCE

Mendelian disorders result from a mutation at a single genetic locus. A locus may be present on an autosome or on a sex chromosomes, and it may manifest in a dominant or a recessive mode. There are over 10,000 traits believed to be inherited in a mendelian fashion, but only a few of the more common disorders of interest to the obstetrician-gynecologist have been highlighted in this chapter to illustrate patterns of inheritance.

The patterns of inheritance for the various mendelian traits are illustrated by the idealized pedigrees in Figure 3-1. An autosomal recessive trait (disease) is expressed only when the mutant gene is present in a double dose (homozygous state). Both parents are heterozygous and possess one copy of the mutant gene and one copy of the normal or functional gene. Autosomal recessive traits are characterized as follows:

1. There is rarely a positive family history outside the affected sibship.

2. Males and females are equally likely to be affected.

3. Heterozygous parents are usually unaffected and have a 25% chance of producing an offspring affected with the disease.

Autosomal dominant traits manifest in the heterozygous state (single-gene dose) and are characterized by the following:

1. They can be transmitted from generation to generation.

2. The probability that a person carrying the gene will pass it on to his or her offspring is 50%.

3. Males and females are equally likely to be affected.

Males (XY) and females (XX) differ in the number of X chromosomes they possess. As a result, the inheritance pattern of mutations carried on the X-chromosome will differ from the inheritance pattern of mutations on autosomes. A recessive trait controlled by a gene on the X chromosome will be expressed in all males carrying the allele. Affected males are said to be hemizygous. Females will be affected if they are homozygous or if they inactivate most of the X chromosomes carrying the normal allele. The following are characteristics of X-linked inheritance:

1. There is no male-to-male transmission.

2. All daughters of an affected male receive the mutant gene and are therefore carriers.

3. One-half of the sons, and one-half of the daughters of a heterozygous female receive the mutant gene.

The distinction between X-linked dominant and X-linked recessive is unclear, but in general, X-linked recessive refers to a trait that is not clinically expressed in the heterozygous female, and X-linked dominant to a trait that is expressed in the heterozygous female.