Lecture 6

Cummings 3: pp 66-71; 4: pp 75-85

PEDIGREE ANALYSIS: AUTOSOMAL

II. Pedigree analysis: procedures.

III. Autosomal recessive traits.

Terms

A. The purpose of genetic analysis is to establish the pattern of transmission of inherited traits.
1. Mendelian ratios do not apply in individual human families because of the small size.

2. Controlled matings cannot be made as is possible in experimental genetics.

3. Pooling of families is possible; however, there are biasses that must be recognized and for which corrections must be made.

4. Nevertheless, humans are the most interesting species to study genetically.

B. There are two principal methods of genetic analysis in humans.
1. In segregation analysis, the distribution of phenotypes in pooled sibships is compared to genotypic ratios predicted by Mendelian rules. It is particularly useful for autosomal recessive traits. The problems are noted above, but there are statistical techniques to correct for biasses if the data are collected correctly. We will not deal with segregation analysis in this course.

2. Pedigree analysis consists in collecting information on affected and nonaffected persons in a family, preparing a pedigree chart, and looking for exceptions to standard tranmission patterns. It is particularly useful for autosomal dominant, sex-linked, and maternally-inherited traits, since multiple generations are affected in these patterns of transmission.

A. Collect information on families and construct pedigree charts according to conventions given in text (p. 70).

B. Test against possible hypotheses:

1. Autosomal dominant

2. Autosomal recessive

3. X-linked dominant

4. X-linked recessive

5. Y-linked

6. Pseudoautosomal dominant or recessive

7. Maternal (mitochondrial)

C. Discard all hypotheses that do not fit the pedigree.
1. If only one remains undiscarded, accept it.

2. If two or more remain, which is the more likely? However, do not discard a hypothesis unless it has been ruled out.

D. Once a mode of inheritance is established, it can be used to predict genetic risk for other persons with the same trait.
1. Pregnancy outcomes

2. Late onset disorders

3. Risk of recurrence of a disorder.

A. Autosomal recessive inheritance applies to those phenotypes that require two similar alleles for expression.
1. For rare or uncommon disorders, the typical pedigree involves mating of two heterozygotes, who produce one or more affected (homozygous) offspring. Affected persons are found only in the one sibship.

2. When two affected persons mate, only affected offspring can result.

3. As in experimental crosses, risk of an affected child from a mating of two heterozygotes is 1/4.

4. Both males and females are equally at risk, since the trait is autosomal.

5. For very rare traits, a high proportion of the homozygotes result from consanguineous matings.

B. In the case of recessive disorders, the wild-type allele is dominant.

C. Examples of autosomal recessive inheritance:

1. Albinism.

2. Cystic fibrosis.

3. Sickle-cell anemia.

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