Ii

OlièTl

The person from whom the pedigree is initiated is called the proband.

Children in each family are listed left to right in birth order.

Twins are represented by diagonal lines extending from a common point.

Within each generation, family members are identified by Arabic numerals

Deceased family members are indicated with a slash.

OlièTl

The person from whom the pedigree is initiated is called the proband.

Filled symbols represent family members with Waardenburg syndrome.

.and open symbols represent unaffected members.

Children in each family are listed left to right in birth order.

Twins are represented by diagonal lines extending from a common point.

I 6.3 Waardenburg syndrome is an autosomal dominant disease characterized by deafness, fair skin, visual problems, and a white forelock. (Photograph courtesy of Guy Rowland).

extending from a common point (IV-14 and IV-15; non-identical twins).

When a particular characteristic or disease is observed in a person, a geneticist studies the family of this affected person and draws a pedigree. The person from whom the pedigree is initiated is called the proband and is usually designated by an arrow (IV-I in Figure 6.3a).

The limited number of offspring in most human families means that it is usually impossible to discern clear Mendelian ratios in a single pedigree. Pedigree analysis requires a certain amount of genetic sleuthing, based on recognizing patterns associated with different modes of inheritance. For example, autosomal dominant traits should appear with equal frequency in both sexes and should not skip generations, provided that the trait is fully penetrant (see p. 000 in Chapter 3) and not sex influenced (see p. 000 in Chapter 5).

Certain patterns may exclude the possibility of a particular mode of inheritance. For instance, a son inherits his X chromosome from his mother. If we observe that a trait is passed from father to son, we can exclude the possibility of X-linked inheritance. In the following sections, the traits discussed are assumed to be fully penetrant and rare.

Autosomal Recessive Traits

Autosomal recessive traits normally appear with equal frequency in both sexes (unless penetrance differs in males and females), and appear only when a person inherits two alleles for the trait, one from each parent. If the trait is uncommon, most parents carrying the allele are heterozygous and unaffected; consequently, the trait appears to skip generations (< Figure 6.4). Frequently, a recessive allele may be passed for a number of generations without the trait appearing in a pedigree. Whenever both parents are heterozygous, approximately /4 of the offspring are expected to express the trait, but this ratio will not be obvious unless the family is large. In the rare event that both parents are affected by an autosomal recessive trait, all the offspring will be affected.

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