Copyright © 2009 Pearson Education, Inc. Chapter 4 Extensions of Mendelian Genetics Copyright © 2009 Pearson Education, Inc.
Mutations are the source of variation Wild-type vs. Dominant Loss-of-function mutation Leads to reduction or loss of gene product Gain-of-function mutation Enhances the function of the wild-type Neutral mutation No detectable changes in function
Copyright © 2009 Pearson Education, Inc. Symbols Mendel used upper/lowercase Most common other system is used by fruit fly geneticists Use the initial letter of the mutant phenotype Lowercase if recessive Wild-type gets same letters with a “+”
Copyright © 2009 Pearson Education, Inc. Figure 4.1 Incomplete Dominance
Copyright © 2009 Pearson Education, Inc. Codominance Two alleles are responsible for producing two distinct, detectable gene products Both alleles show up in the phenotype MN blood group in humans L M L M = type M L M L N = type MN L N L N = type N
Copyright © 2009 Pearson Education, Inc. GenotypePhenotype LMLMLMLM M LMLNLMLN MN LNLNLNLN N Codominance
Copyright © 2009 Pearson Education, Inc. Multiple Alleles The number of alleles in a POPULATION doesn’t have to be just 2 Each individual still only has 2 ABO blood group in humans Three alleles I A Produces A antigen I B Produces B antigen I O no antigen
Copyright © 2009 Pearson Education, Inc. GenotypeAntigenPhenotype IAIAIAIA AA IAIOIAIO AA IBIBBB IBIOBB IAIBA & BAB IOIONoneO ABO Blood Groups
Copyright © 2009 Pearson Education, Inc. Table 4.1
Copyright © 2009 Pearson Education, Inc. Lethal Alleles Recessive lethal mutations Homozygous recessives do not survive May have a mutant phenotype in the heterozygote Dominant lethal mutations Homozygous dominant and heterozygous do not survive Affected individual must survive until adulthood
Copyright © 2009 Pearson Education, Inc. Recessive Lethal Mutations Usually a result of loss-of-function mutation One copy of a functional gene is usually enough to permit survival Heterozygotes are usually OK but may have a distinct phenotype Example: Yellow Mice
Copyright © 2009 Pearson Education, Inc. GenotypePhenotype AAAgouti (w.t.) AAYYellow AYAYLethal Yellow Mice
Copyright © 2009 Pearson Education, Inc. Figure 4.4
Copyright © 2009 Pearson Education, Inc. Dominant Lethal Mutations One copy of the allele results in death The one working allele may not be enough to compensate Mutant gene may override the functioning wild-type product Example: Huntington Disease Typical onset around age 40
Copyright © 2009 Pearson Education, Inc. Figure 4.5 Gene pairs with two modes of inheritance
Copyright © 2009 Pearson Education, Inc. Phenotypes are often affected by more than one gene Gene interaction Several genes interact to influence a phenotype
Copyright © 2009 Pearson Education, Inc. Recessive Epistasis Recessive allele at one locus masks the expression of another set of alleles at a second locus GenotypePhenotype AAagouti Aaagouti aablack GenotypePhenotype BBcolor Bbcolor bbalbino
Copyright © 2009 Pearson Education, Inc. F2 RatioGenotypePhenotype Phenotypic Ratio 9/16A-B-agouti 9/16 agouti 3/16A-bbalbino 3/16aaB-black4/16 albino 1/16aabbalbino3/16 black AaBb x AaBb = Modified phenotypic ratio = 9:4:3 (still adds up to 16) Precursor molecule (colorless)Black pigmentAgouti pattern Gene BGene A
Copyright © 2009 Pearson Education, Inc. Dominant Epistasis Dominant allele at one locus masks the expression of the alleles at a second locus GenotypePhenotype AAWhite AaWhite aaWhatever locus B is GenotypePhenotype BBYellow BbYellow bbGreen
Copyright © 2009 Pearson Education, Inc. F2 RatioGenotypePhenotype Phenotypic Ratio 9/16A-B-white 12/16 white 3/16A-bbwhite 3/16aaB-yellow3/16 yellow 1/16aabbgreen1/16 green AaBb x AaBb = Modified phenotypic ratio = 12:3:1 (still adds up to 16)
Copyright © 2009 Pearson Education, Inc. Complementary Gene Interaction Dominant allele at two different loci is necessary to produce a phenotype F2 RatioGenotypePhenotype Phenotypic Ratio 9/16A-B-purple 9/16 purple 3/16A-bbwhite 3/16aaB-white 7/16 white 1/16aabbwhite Modified phenotypic ratio = 9:7 (still adds up to 16) Precursor molecule (colorless)Intermediatepurple Gene BGene A (colorless)
Copyright © 2009 Pearson Education, Inc. Summer Squash
Copyright © 2009 Pearson Education, Inc. Novel phenotypes Gene interaction results in new phenotypes in the F 2 generation F2 RatioGenotypePhenotype Phenotypic Ratio 9/16A-B-disc9/16 disc 3/16A-bbsphere 6/16 sphere 3/16aaB-sphere 1/16aabblong1/16 long Modified phenotypic ratio = 9:6:1 (still adds up to 16)
Copyright © 2009 Pearson Education, Inc.
Figure 4.11 Complementation Analysis
Copyright © 2009 Pearson Education, Inc. Pleiotropy A single gene has multiple phenotypic effects Two examples Marfan Syndrome Porphyria Variegata
Copyright © 2009 Pearson Education, Inc. Figure 4.12 X-Linkage
Copyright © 2009 Pearson Education, Inc. Figure 4.13
Copyright © 2009 Pearson Education, Inc. Table 4.3
Copyright © 2009 Pearson Education, Inc.
GenotypePhenotype FEMALEMALE HHHen-feathered HhHen-feathered hhHen-featheredCock-feathered Sex-Limited
Copyright © 2009 Pearson Education, Inc. GenotypePhenotype FEMALEMALE BBBald (less pronounced)Bald BbNot baldBald bbNot bald Sex-Influenced
Copyright © 2009 Pearson Education, Inc. Penetrance Percentage of individuals that show at least some degree of expression of a mutant genotype Expressivity Range of expression of the mutant phenotype
Copyright © 2009 Pearson Education, Inc. Conditional Mutation – Temperature Effect
Copyright © 2009 Pearson Education, Inc. Nutritional Effects Auxotroph Microorganism that has a mutation that prevents the synthesis of a nutrient In humans, ingestion of certain dietary substances can harm individuals with abnormal genetic makeups Phenylketonuria Galactosemia Lactose Intolerance