Chapter 15 Mendelian Inheritance has its Physical Basis in the Behavior of Chromosomes Chromosome Theory of Inheritance Mendelian genes have specific loci along chromosomes that undergo segregation and independent assortment Behavior of homologous chromosomes during meiosis can account for segregation of alleles at each genetic locus to different gametes Fig 15 2 Behavior of nonhomologous chromosomes can account for the assortment of the alleles for two or more genes located on different chromosomes Morgan s Experimental Evidence Scientific Inquiry Chromosomes are location of Mendel s heritable factors Morgan s Choice of Experimental Organism Drosophila melanogaster fruit fly Breed quickly and produce large numbers of offspring Only 4 pairs of chromosomes 3 autosomes 1 sex Wild Type phenotype for a character most commonly observed in natural populations Mutant phenotypes Due to alleles assumed to have originated as changes or mutations in the wild type allele White eyes symbolized by w Red eyes symbolized by w for wild type trait HD Huntington s Disease Correlating Behavior of a Gene s Alleles with Behavior of a Chromosome Pair Mated white eye male with red eye female all F1 red eye offspring dominant wild type Bred F1 together producing 3 1 phenotypic ratio White trait showed only in males red white All females were red Eye color linked to sex Females have XX males have XY Sex linked Genes Exhibit Unique Patterns of Inheritance The Chromosomal Basis of Sex 2 varieties of sex chromosomes X Y Y is smaller than X XX Female XY Male Each egg cell has X chromosome Male can have X or Y SRY sex determining region of Y Presence absence determines whether there will be testes ovaries Sex linked gene a gene located on either sex chromosome X linked genes present in males and females Sex linked Genes Exhibit Unique Patterns of Inheritance cont Inheritance of X Linked Genes Fathers pass X linked alleles only to daughters mothers pass to both If X linked trait is due to recessive allele female will express only if homozygous for that allele Hemizygous any male with recessive allele will express the trait Duchenne muscular dystrophy 1 3500 males weakening of muscles and loss of coordination X linked Hemophilia X linked absence of 1 proteins required for blood clotting X Inactivation in Female Mammals Barr body condensed compact object of the inactive X in female cells Lies along inside of nuclear envelope Most of the genes of it are not expressed Reactivated in ovaries Occur randomly and independently in each embryonic cell After X chromosome is inactivated in particular cell all mitotic descendants of that cell have same inactive X Heterozygous female will have half of cells expressing one allele and others express alternate allele XIST X inactive specific transcript active only on chromosome that will become Barr body Linked Genes Tend to be Inherited Together Because They are Located Near Each Other on the Same Chromosome Linked Genes genes located near each other on the same chromosome inherited together in genetic crosses 2 genes Deviates from Mendel s Law of Independent Assortment How Linkage Affects Inheritance Black and small gray and big inherited together more often than crossing Genetic Recombination production of offspring with combinations of traits that differ from those found in either parent Genetic Recombination and Linkage Recombination of Unlinked Genes Independent Assortment of Chromosomes