2 Chromosome Theory of Inheritance Mitosis and meiosis were first described in the late 1800sThe chromosome theory of inheritance states:Mendelian genes have specific loci (positions) on chromosomesChromosomes undergo segregation and independent assortmentThe behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortmentChromosome Theory of Inheritance
3 Figure 15.2 The chromosomal basis of Mendel’s laws P GenerationYellow-roundseeds (YYRR)Green-wrinkledseeds ( yyrr)YyrYRRryMeiosisFertilizationRYyrGametesAll F1 plants produceyellow-round seeds (YyRr)F1 GenerationRRyyrrYYLAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.MeiosisLAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.RrrRMetaphase IYyYy11RrrRAnaphase IYyYyFigure 15.2 The chromosomal basis of Mendel’s lawsRrMetaphase IIrR22YyYyyYYyYYyyGametesRRrrrrRR1/4YR1/4yr1/4Yr1/4yRF2 GenerationAn F1 F1 cross-fertilization339: 3: 3: 1
4 The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologistMorgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factorsThomas Hunt Morgan
5 Why Drosophila melanogaster? Several characteristics make fruit flies a convenient organism for genetic studies:They breed at a high rateA generation can be bred every two weeksThey have only four pairs of chromosomesMorgan noted wild type, or normal, phenotypes that were common in the fly populationsTraits alternative to the wild type are called mutant phenotypesWhy Drosophila melanogaster?
7 Morgan’s finding supported the chromosome theory of inheritance In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)The F1 generation all had red eyesThe F2 generation showed the 3:1 red:white eye ratio, but only males had white eyesMorgan determined that the white-eyed mutant allele must be located on the X chromosomeMorgan’s finding supported the chromosome theory of inheritance
8 Sex Determination Varies by species X-Y: male gamete determines sex of offspring (humans)X-O: number of sex chromosomes determines sex; females have 2; males have 1; males produce two types of gametes & determine sex (many insects)Z-W: female gamete determines sex of offspring (birds, butterflies, fish)Haplo-diploid: female is a fertilized gamete; male unfertilized (plants, bees)
9 For a recessive sex-linked trait to be expressed Sex LinkageFor a recessive sex-linked trait to be expressedA female needs two copies of the alleleA male needs only one copy of the alleleSex-linked recessive disorders are much more common in males than in females
10 Linked GenesMorgan did other experiments with fruit flies to see how linkage affects inheritance of two charactersMorgan crossed flies that differed in traits of body color and wing sizeMorgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)He noted that these genes do not assort independently, and reasoned that they were on the same chromosome
11 Recombination with Linked Genes Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypesMorgan proposed that some process must sometimes break the physical connection between genes on the same chromosomeThat mechanism was the crossing over of homologous chromosomesRecombination frequencies are used to map the location of genes on a chromosome
12 Exceptions to the Chromosome Theory of Inheritance There are two normal exceptions to Mendelian geneticsOne exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleusExceptions to the Chromosome Theory of Inheritance
13 For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traitsSuch variation in phenotype is called genomic imprintingGenomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete productionIt appears that imprinting is the result of the methylation (addition of –CH3) of DNAGenomic imprinting is thought to affect only a small fraction of mammalian genesMost imprinted genes are critical for embryonic developmentGenomic Imprinting
15 Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are genes 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 plantInheritance of Organelle Genes
16 Inheritance of Organelle Genes Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systemsFor example, mitochondrial myopathy and Leber’s hereditary optic neuropathyInheritance of Organelle Genes
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