2 Fig. 14-1The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?
3 How can we tell the genotype of an individual with the dominant phenotype? The answer is to carry out a testcross:
4 Fig. 14-7 TECHNIQUE RESULTS Dominant phenotype, unknown genotype: Dominant phenotype,unknown genotype:PP or Pp?Recessive phenotype,known genotype:ppPredictionsIf PPIf PporSpermSpermppppPPPpPpPpPpEggsEggsFigure 14.7 The testcrossPpPpPpppppRESULTSorAll offspring purple1/2 offspring purple and1/2 offspring white
9 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Fig. 14-9 Rr Rr Segregation of alleles into eggsSegregation ofalleles into spermSperm1/2R1/2rRR1/2RRrFigure 14.9 Segregation of alleles and fertilization as chance events1/41/4Eggsrr1/2Rrr1/41/4
10 Summary of Mendel’s Principles The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring.In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.The alleles for different genes usually segregate independently of one another.
12 Beyond dominant/recessive FigBeyond dominant/recessiveP GenerationRedWhiteCRCRCWCWincomplete dominance - situation in which one allele is not completely dominant over anotherThere is no white no red: new phenotype pinkGametesCRCWPinkF1 GenerationCRCWGametes1/2CR1/2CWSperm1/2CR1/2CWF2 Generation1/2CRCRCRCRCWEggs1/2CWCRCWCWCW
13 Beyond dominant and recessive alleles Codominance - situation in which both alleles of a gene contribute to the phenotype of the organism
14 Beyond dominant and recessive alleles multiple alleles - three or more alleles of the same gene
15 Fig. 14-11 Allele Carbohydrate IA A IB B i none (a) The three alleles for the ABO blood groupsand their associated carbohydratesRed blood cellappearancePhenotype(blood group)GenotypeIAIA or IA iAIBIB or IB iBFigure Multiple alleles for the ABO blood groupsIAIBABiiO(b) Blood group genotypes and phenotypes
16 Polygenic Inheritance Quantitative characters are those that vary in the population along a continuumQuantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotypeHeight and Skin color in humans is an example of polygenic inheritance
17 FigAaBbCcAaBbCcSperm1/81/81/81/81/81/81/81/81/81/81/81/8Eggs1/81/8Figure A simplified model for polygenic inheritance of skin color1/81/8Phenotypes:1/646/6415/6420/6415/646/641/64Number ofdark-skin alleles:123456
24 (a) Is a widow’s peak a dominant or recessive trait? Fig b1st generation(grandparents)WwwwwwWw2nd generation(parents, aunts,and uncles)WwwwwwWwWwww3rd generation(two sisters)WWwwFigure 14.15a Pedigree analysisorWwWidow’s peakNo widow’s peak(a) Is a widow’s peak a dominant or recessive trait?
25 (b) Is an attached earlobe a dominant or recessive trait? Fig c1st generation(grandparents)FfFfffFf2nd generation(parents, aunts,and uncles)FForFfffffFfFfff3rd generation(two sisters)ffFForFfFigure 14.15b Pedigree analysisAttached earlobeFree earlobe(b) Is an attached earlobe a dominant or recessive trait?
39 Heterozygous 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
40 Relationship among genes Description Example Epistasis Fig. 14-UN3Relationship amonggenesDescriptionExampleEpistasisOne gene affectsthe expression ofanotherBbCcBbCcBCbCBcbcBCbCBcbc9: 3: 4PolygenicinheritanceA single phenotypiccharacter isaffected bytwo or more genesAaBbCcAaBbCc