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Chromosomes & Heredity

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.

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Chromosomes & Heredity

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  1. 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

  2. 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.

  3. 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

  4. 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.

  5. SEX LINKAGE

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

  7. Genes on Sex Chromosomes • Sex-linked: A gene located on one of the sex chromosomes • See fig. 10-10 • See fig. 10-11

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