2Outline Incomplete Dominance, Codominance, and Multiple Allelism Interaction of genesPedigree StudiesGenetics and Ethics
3Extending Mendel’s Rules Incomplete dominanceheterozygotes have an intermediate phenotypeCodominanceHeterozygotes displays the phenotype of both allelesmultiple allelismMultiple distinct genes versions (i.e., alleles) are present in the populationpolymorphismMultiple distinct phenotypes are present in a population
4Multiple Alleles and Polymorphism IAIBiABnone(a) The three alleles for the ABO blood groupsand their associated carbohydratesAlleleCarbohydrateGenotypeRed blood cellappearancePhenotype(blood group)IAIA or IA iIBIB or IB iIAIBABiiO(b) Blood group genotypes and phenotypesMultiple Alleles and PolymorphismABO blood group in humans are determined by three alleles : IA, IB, and i.
5PleiotropyA gene that influences many traits rather than just one is pleiotropic.Marfan Syndrome (FBN1): defective fibrillinlimbs, spinal chord, heartCystic fibrosis (CFTR): defective salt transportlungs, pancreas, sebacious glands, etc.Lung(s)pancreashealthyCF
6Antagonistic pleiotropy Some effects are good; some are badSickle cell anemia (hemoglobin B)Codominant traitHBB/HBB; HBB/hbb; hbb/hbbHBB/hbbMild sickle cell diseaseMalaria protectionHealthy???Unhealthy
7Heterozygous phenotype same as that of homo- zygous dominant PP Pp Fig. 14-UN2Degree of dominanceDescriptionExampleComplete dominanceof one alleleHeterozygous phenotypesame as that of homo-zygous dominantPPPpIncomplete dominanceof either alleleHeterozygous phenotypeintermediate betweenthe two homozygousphenotypesCRCRCRCWCWCWCodominanceHeterozygotes: Bothphenotypes expressedIAIBMultiple allelesIn the whole population,some genes have morethan two allelesABO blood group allelesIA , IB , iPleiotropyOne gene is able toaffect multiplephenotypic charactersSickle-cell disease
8FigEpistasisA gene at one locus alters the phenotypic expression of a gene at a second locusCoat color in micepigment color (B for black; b for brown)Pigment deposit (C for color; c for no color)BbCcBbCcSperm1/41/41/41/4BCbCBcbcEggs1/4BCBBCCBbCCBBCcBbCc1/4bCBbCCbbCCBbCcbbCc1/4BcFigure An example of epistasisBBCcBbCcBBccBbcc1/4bcBbCcbbCcBbccbbcc9: 3: 4
9Discrete vs. Quantitative Traits Discrete traits.seed color in peas—no intermediate phenotypesQuantitative traitsTraits that fall into a continuumFrequenciesform a bell-shaped curve (normal distribution) for a population.A phenotype distribution that forms a bell-shaped curve.Normal distribution—bell-shaped curve
10Quantitative Traits Result from the Action of Many Genes Wheat kernel color is a quantitative trait.Hypothesis to explain inheritance of kernel colorParentalgenerationF1F2161520aa bb cc(pure-line white)AA BB CC(pure-line red)Aa Bb Cc(medium red)Self-fertilization
11Polygenic Inheritance EggsSpermPhenotypes:Number ofdark-skin alleles:1234561/646/6415/6420/641/8AaBbCcPolygenic InheritanceTraits that vary in the population along a continuumAdditive effect of 2+ genes on a single phenotypeSkin color in humans is an example of polygenic inheritance
12Relationship amonggenesDescriptionExampleEpistasisOne gene affectsthe expression ofanotherBbCcBbCcBCbCBcbcBCbCBcbc9: 3: 4PolygenicinheritanceA single phenotypiccharacter isaffected bytwo or more genesAaBbCcAaBbCc
13Applying Mendel’s Rules to Humans Humans terrible genetic models– Generation time is too long– Parents produce relatively few offspring– Breeding experiments are frowned uponHuman disorders follow 5 patterns1) Autosomal dominant 2) Autosomal recessive3) X-linked recessive 4) X-linked dominant5) Y-linkedPedigrees (family trees)analyze the human crosses that already exist.
14Human Pedigree Reports KeyMaleAffectedmaleMatingOffspring, inbirth order(first-born on left)FemaleAffectedfemale
15Is a widow’s peak a dominant or recessive trait? Fig b1st generation(grandparents)WwwwwwWw2nd generation(parents, aunts,and uncles)WwwwwwWwWwww3rd generation(two sisters)WWwworWwWidow’s peakNo widow’s peakIs a widow’s peak a dominant or recessive trait?
16Autosomal Recessive Traits If a phenotype is due to an autosomal recessive alleletrait = homozygousparents (w/o trait) = heterozygous carriers.Carriers carry the allele and transmit it even though they do not exhibit the phenotype.Carrier maleCarrier femaleAffectedmalefemaleIIIIIIIVEach row represents a generationCarriers (heterozygotes) are indicated with half-filled symbols
17Autosomal or Sex-Linked trait? Equally often in males and femaleslikely to be autosomal.Males more likely to have the traitusually X-linked.Hemophilia is an example of an X-linked trait resulting from a recessive allele.Queen VictoriaPrince AlbertFemale carrier of hemophilia alleleIIIIIIIVAffected male
18Frequency of Dominant Alleles Not necessarily more common (NOT always “WT”)one baby out of 400 in the United States is born with extra fingers or toesDominant allele; uncommon occurrenceIn this example, the recessive allele is far more prevalent than the population’s dominant allele
19What are the Societal Implications of this Knowledge?
20Fetal Testing Tests to determine in utero if a child has a disorder. 14th to 16th week of pregnancyBlood or amniocentesisFetal tests can reveal a serious disorderTrisomy 21, 18, etc.Some testing after birtheg Type I diabetes
21Eugenics Science of “improving the genetic stock” of humans Old TestamentPlato’s Republic (description of the ideal society )Francis Galton“National Eugenics Laboratory”Experimental studies of heredityTwinsKarl PearsonThe higher birth rate of the poorSupplant by "higher" races
22US Propaganda and Policy The Immigration Act of 1924quota for different nationalitiesperceived tendencies towards crime etc.Forced Sterilization
23Eugenics and the Third Reich Nazi GermanyThe Aryan Nation and the HolocaustHuman racesIll-founded conceptPopulations with overlapping gene pools.No major difference in the genome sequence