4F1 Generation All F1 plants produce yellow-round seeds (YyRr). Figure 15.2bAll F1 plants produce yellow-round seeds (YyRr).F1 GenerationRRyyrrYYLAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.MeiosisRrrRMetaphase IYyYy11RrrRAnaphase IYyYyFigure 15.2 The chromosomal basis of Mendel’s laws.RrrRMetaphase II22YyYyYyYYyYyyGametesRRrrrrRR1/4YR1/4yr1/4Yr1/4yR
5LAW OF INDEPENDENT ASSORTMENT Figure 15.2cLAW OF SEGREGATIONLAW OF INDEPENDENT ASSORTMENTF2 GenerationAn F1 F1 cross-fertilization3Fertilization recombines the R and r alleles at random.3Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.9: 3: 3: 1Figure 15.2 The chromosomal basis of Mendel’s laws.
9P Generation F1 Generation F2 Generation Figure 15.4bCONCLUSIONP GenerationwwXXXYwwSpermEggsF1 GenerationwwwwwSpermFigure 15.4 Inquiry: In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have?EggswwF2 Generationwwwwww
14Recessive Sex-Linked Trait Female must be homozygous in order to express the traitFewer females with sex-linked disordersMales are hemizygousa condition where only one copy of a gene is present in a diploid organismMore males than females have x-linked disordersExamplesColor blindness, hemophilia, Duchenne muscular dystrophy
15Sex Linkage Problem Color Blindness - recessive A man who is color blind is married to a female that carries the color blindness allele. What is the probability that a son would be color blind? Daughter? A daughter that is a carrier?
16Sex Linkage Problem Hemophilia- recessive A man with hemophilia marries a woman with normal clotting factors but is heterozygous. What is the probability that they will have a … daughter with hemophilia? What is the probability that they will have a… son with hemophilia? What is the probability that their son… will have hemophilia?
18Cell division and X chromosome inactivation Figure 15.8X chromosomesAllele for orange furEarly embryo:Allele for black furCell division and X chromosome inactivationTwo cell populations in adult cat:Active XInactive XActive XBlack furOrange furFigure 15.8 X inactivation and the tortoiseshell cat.
21Wild type (gray-normal) FigureEXPERIMENTP Generation (homozygous)Double mutant (black body, vestigial wings)Wild type (gray body, normal wings)b b vg vgb b vg vgF1 dihybrid (wild type)Double mutantTESTCROSSb b vg vgb b vg vgTestcross offspringEggsb vgb vgb vgb vgWild type (gray-normal)Black- vestigialGray- vestigialBlack- normalb vgFigure 15.9 Inquiry: How does linkage between two genes affect inheritance of characters?Spermb b vg vgb b vg vgb b vg vgb b vg vgPREDICTED RATIOSIf genes are located on different chromosomes:1:1:1:1If genes are located on the same chromosome and parental alleles are always inherited together:1:1::RESULTS965:944:206:185
25965 Wild type (gray-normal) 391 recombinants 2,300 total offspring Figure 15.10bRecombinant chromosomesbvgb vgb vgb vgEggsTestcross offspring965 Wild type (gray-normal)944 Black- vestigial206 Gray- vestigial185 Black- normalb vgb vgb vgb vgb vgb vgb vgb vgb vgFigure Chromosomal basis for recombination of linked genes.SpermParental-type offspringRecombinant offspringRecombination frequency391 recombinants 2,300 total offspring 17%
26Recombinant Frequency Problem Total offspring – 1566Parental types – 1472Recombinants – 94Frequency = (# of recombinants/Total) * 100= %=(distance between the two allelesaka map units)
27Recombinant Frequency Problem A wild type fruit fly (heterozygous for gray body color and red eyes) was mated with a black fruit fly with purple eyes. What is the recombinant frequency for these genes?Wild type – 721Black purple – 751Gray purple – 49Black red - 45
30Errors and Exceptions Alterations of chromosome number Increase or decrease in the number of chromosomes from the normal diploid numberOccurs during anaphase of meiosisAlterations of chromosome structureActual chromosome is altered
31Alterations of Chromosome Number NondisjunctionCertain homologous chromosomes fail to separate during meiosisMay occur in anaphase I or anaphase IIResults in aneuploidy
32Nondisjunction of homo- logous chromosomes in meiosis I (a) FigureMeiosis INondisjunctionMeiosis IINon- disjunctionGametesFigure Meiotic nondisjunction.n 1n 1n 1n 1n 1n 1nnNumber of chromosomesNondisjunction of homo- logous chromosomes in meiosis I(a)Nondisjunction of sister chromatids in meiosis II(b)
36Alterations of Chromosome Structure DeletionChromosomes lose a fragment of DNACri du chat syndrome (#5)A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhoodInversionFragments break off then reattach in reverse orientation
37Alterations of Chromosome Structure TranslocationFragments reattach to nonhomologous chromosomesChronic mylegenous leukemia (22 switches with 9)
39Mother v. Father Inheritance For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traitsGenomic imprintingInduces intrinsic changes in chromosomes inherited from males and femalesSilencing of certain genes that are “stamped” with an imprint during gamete productionPrader-Willi Syndrome (paternal)Angelman Syndrome (maternal)Fragile X Syndromeconsequence of maternal genomic imprinting
40Mutant Igf2 allele inherited from mother Figure 15.17bMutant Igf2 allele inherited from motherMutant Igf2 allele inherited from fatherNormal-sized mouse (wild type)Dwarf mouse (mutant)Normal Igf2 allele is expressed.Mutant Igf2 allele is expressed.Figure Genomic imprinting of the mouse Igf2 gene.Mutant Igf2 allele is not expressed.Normal Igf2 allele is not expressed.(b) Heterozygotes
42Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasmMitochondria, chloroplasts, and other plant plastids carry small circular DNA moleculesExtranuclear genes are inherited MATERNALLY because the zygote’s cytoplasm comes from the eggThe first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant