Post on 26-Dec-2015
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
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