Chromosomes & Heredity Chromosome Theory of Heredity Genes are located on the Chromosomes Each gene occupies a specific place A gene may exist in several forms or alleles Each chromosome has just one allele for each of its genes

SUMMARY OF SUTTON’S WORK 1. A pair of factors determines each characteristic in an organism. 2. A gamete contains one factor from each pair. 3. Factors assort independently during meiosis. 4. Factors pair during fertilization to restore the original number of factors. 5. Individual factors remain unchanged from one generation to the next.

GENE LINKAGE Linked genes do not undergo independent assortment Effects of Gene Linkage Morgan’s GgWw X ggww didn’t produce expected results See fig. 10-4: Gene for body color & wing size are linked Linkage Groups Packages of genes that tend to be inherited together These “packages” are homologous chromosomes CROSSING OVER Recombinants: Individuals with new combinations of genes GENE MAPPING Sturtevant reasoned the frequency of crossing over occurs in proportion to the distance of the genes on the chromosome

CROSSING OVER During Prophase One Synapsis: Process of drawing the homologous chromosomes together by putting all four chromatids into parallel alignment. Nonsister chromatids break at one or more sites and exchange corresponding segments; and exchange of alleles. When alleles are different, crossing-over may occur and cause unlike alleles to be exchanged, which breaks up old combinations of alleles creating a new combination of alleles.

SEX LINKAGE

SEX LINKAGE Sex Chromosomes autosomes: Same chromosomes for Males & Females XX = FemaleXY = Male Sex Determination: 50% chance of male or female Genes on Sex Chromosomes

Genes on Sex Chromosomes Sex-linked: A gene located on one of the sex chromosomes See fig See fig