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8.2
 New alleles arise by mutations—rare, stable,
inherited changes in the genetic material.
 Wild type is the allele present in most of the
population. Other alleles of that gene are mutant
alleles.
 A gene with a wild-type allele that is present less
than 99% of the time is called polymorphic.
 One gene may have more than two alleles.
 Multiple alleles increase the number of possible phenotypes and may show a
hierarchy of dominance in heterozygotes.
 Example: coat color in rabbits
 Incomplete dominance
 Some alleles are neither dominant nor
recessive, and heterozygotes have an
intermediate phenotype.
 Example: flower color in snapdragons
 Codominance
 Two alleles of a gene produce both
phenotypes in the heterozygote.
 Example:
 ABO blood group system has three
alleles of the gene: IA, IB, and IO.
 IA and IB are codominant.
 Blood Types Video (Donuts)
 Blood Donors – Khan
 Blood - TEDEd
Figure 8.11 ABO Blood Groups Are Important in Transfusions
 Epistasis – genes interacting with
one another
 Phenotypic expression of one gene
is influenced by another gene.
 Example: Coat color in Labrador
retrievers
 Allele B (black) is dominant to b (brown)
 Allele E (pigment deposition) is
dominant to e (no pigment deposition—
hair is yellow)
 Gene E determines the phenotypic
expression of gene B.
Figure 8.12 Genes Interact Epistatically
 Hybrid vigor (heterosis): hybrid offspring grow larger, produce more seeds, etc, than
the parental varieties.
 “Vigor” is a complex trait; most complex phenotypes are determined by multiple genes.
 Most are quantitative traits: they must be measured, rather than assessed
qualitatively (e.g., grain yields).
 Genotype and environment often interact to determine phenotype.
 Example: point restriction coat patterns
 These rabbits and cats have a mutant allele for the coat color gene.
 The enzyme encoded by the gene is inactive at temperatures above about 35°C.
 The extremities are cooler than the main body (around 25°C), so the fur on these
regions is dark.
 Two parameters describe the effects of genes and environment on phenotype:
 Penetrance: proportion of individuals with a certain genotype that show the expected
phenotype
 People who inherit a mutant allele of the gene BCRA1 develop breast cancer in their lifetime – but
some people, for reasons unknown, other than genes must be interacting people may not develop
breast cancer, so the BRCA1 is said to be incompletely penetrant
 Expressivity: degree to which genotype is expressed in an individual
 Can be variable in its expressivity as one person may develop both breast and ovarian cancer while
another person may only develop breast cancer
 Many people, but not all, who inherit a mutant allele of the gene BRCA1 develop
breast cancer.
 The mutation is said to be incompletely penetrant.
 A woman with the mutant allele may get both breast and ovarian cancer, but another
woman may only get breast cancer.
 The mutation is said to have variable expressivity.
 Heritability: relative contribution of genetic versus environmental factors to variation
in a character in a population
• Example: human height
 Heritability varies from 0 to 1.
 For human height, heritability varies from about 0.65 to 0.8.
 If heritability is 0.65, 65% of the variation in height is due to genetic factors, 35% is
due to environmental effects.
 Heritability estimates apply only to variation within populations, not individuals.