Genetics: A Conceptual ApproachTHIRD EDITION

Copyright 2008 © W. H. Freeman and Company

CHAPTER 6

Pedigree Analysis, Applications,and Genetic Testing

Benjamin A. Pierce

© 2009 W. H. Freeman and Company

Pedigree Analysis

- pedigree analysis is a scrutiny of records of matings

- pedigrees use standard sets of symbols to depict family trees and lineages

- pedigrees provide concise and accurate records of families

- pedigrees are helpful in following and diagnosing heritable traits (for example, diseases and medical conditions) by describing patterns of inheritance

- pedigrees are useful in mapping (locating and isolating) genes “responsible” for certain traits

Pedigree construction

- use standard set of symbols

- one generation per row (oldest at the top)

- siblings are shown in order of birth (from left to right)

- generations are given Roman numerals (I, II, III, IV, etc)

- individuals within a generation (row) are given Arabicnumerals (1, 2, 3, 4, etc)

Analyzing pedigrees

- trial and error: consider one pattern of inheritance at a time for each mating in the pedigree and try to find evidence against it; repeat foreach pattern of inheritance, for example, autosomal recessive or dominant, X-linked recessive or dominant, etc

- patterns of inheritance follow Mendelian rules; Mendelian ratios are rarely observed

- assumption: for rare traits unaffected people entering into a familypedigree (for example, by marriage) are considered homozygousnormal

- result: pedigrees can frequently rule out, but not necessarily prove,a certain pattern of inheritance

I

II

III

IV

Autosomal recessive

- the trait is found equally in both males and females- affected individuals usually have unaffected parents- the pattern of inheritance is often horizontal with several generations

of unaffected individuals, but then several siblings in one generationare affected

I

II

III

IV

Autosomal dominant

- the trait is found equally in both males and females- every affected individual has at least one affected parent- trait shows vertical pattern of inheritance, that is affected

males and females are observed in each generation

The human pseudoachondroplasia phenotype is determined by a dominant allele D, that interferes with bone growth during development

The age of onset of Huntington disease

I

II

III

IV

X-linked recessive

- more males than females are affected- all the sons of an affected mother will be affected- half the sons of a carrier mother will be affected- all daughters of carrier mothers will be normal, but half will be carriers- affected males do not transmit the trait to their sons- trait often skips a generation

I

II

III

IV

X-linked dominant

- trait observed in both males and females- affected males ALWAYS transmit the trait to their daughters, but to

NONE of their sons- affected females will transmit the trait to both sons and daughters- trait does not skip generation

I

II

III

IV

Y-linked

- only males are affected- the trait is passed from an affected father to all of his sons

I

II

III

IV

Mitochondrial inheritance

- both males and females are affected- the trait is passed from an affected mother to all her progeny- affected males do not transmit the trait to any of their progeny