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Chapter 3

CAS BI 206 Chapter Notes - Chapter 3: Penetrance, Allele Frequency, Epistasis

Course Code
CAS BI 206
Chip Celenza

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Chapter 3: Extensions to Mendel’s Laws
Extensions to Single-Gene Inheritance
Multifactorial traits affected by 2+ genes
Dominance is not always complete (ie, offspring will not always be A or a, they can be a mix
Incomplete dominance offspring
phenotype does not resemble
either true-breeding parent
Neither allele is
recessive/dominant to
each other display
intermediate of two
Have exact phenotypic &
genotypic ratios (1:2:1)
Each allele different
version of protein each functional one works in tandem with other
Codominance both traits of true-breeding parents show up equally in heterozygote phenotype
Alleles aren’t always dominant/recessive D/R relationship is on an allele-allele basis
Dominance series listing of alleles from most dominant recessive (made of > and =)
Mutations new alleles
Allele frequency % of certain allele occurrence
Wild-type allele most common allele in population (W+)
Mutant allele rare allele in population
Monomorphic gene with only one common wild-type allele
Polymorphic gene with more than one common wild-type allele
Pleiotropy single allele determining multiple distinct characteristics (protein has cascade of
May be dominant/recessive/incompletely dominant/etc for different characteristics
Recessive lethal gene gene may be dominant for one trait but is recessive for lethality (3:1 ratio
from Aa parents)
Extensions to Multifactorial Inheritance
Genotypic classes phenotypes defined in terms of absence or presence of dominant alleles of 2
Complete dominance both D (A-B-); One (A-bb); other (aaB-); neither (aabb)
Incomplete dominance/codominance give rise to more than 4 phenotypes
Complementary gene action aaBB x AAbb same phenotype (at least one dominant allele of
each required for phenotype)
Epistasis the effects of one gene’s alleles (epistatic) hides the effects of another gene’s alleles
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