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Chromosomes & Heredity
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Chromosome Theory of Heredity
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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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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
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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
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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
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SEX LINKAGE
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Sex Chromosomes
autosomes: Same chromosomes for Males & Females
XX = Female
XY = Male
Sex Determination: 50% chance of male or female
Genes on Sex Chromosomes
Genes on Sex Chromosomes
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Sex-linked: A gene located on one of the sex chromosomes
See fig. 10-10
See fig. 10-11