Transcript Genetics: A Conceptual Approach 3/e

Benjamin A. Pierce
Genetics: A Conceptual Approach
THIRD EDITION
CHAPTER 6
Pedigree Analysis, Applications,
and Genetic Testing
©
2009 W.
H.©Freeman
andand
Company
Copyright
2008
W. H. Freeman
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 Arabic
numerals (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 for
each 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 family
pedigree (for example, by marriage) are considered homozygous
normal
-
result: pedigrees can frequently rule out, but not necessarily prove,
a certain pattern of inheritance
Autosomal recessive
I
II
III
IV
-
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 generation
are affected
Autosomal dominant
I
II
III
IV
-
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
X-linked recessive
I
II
III
IV
-
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
X-linked dominant
I
II
III
IV
-
-
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
Y-linked
I
II
III
IV
-
only males are affected
the trait is passed from an affected father to all of his sons
Mitochondrial inheritance
I
II
III
IV
-
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