The Chromosomal Basis of Inheritance 24 October, 2003 Text Chapter 15

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The Chromosomal Basis of Inheritance24 October,
2003Text Chapter 15
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Genes and Chromosomes
The behavior of chromosomes in meiosis and
fertilization explains Mendels rules of
inheritance.
Genes on different chromosomes assort
independently.
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Morgans Mutant
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Morgans experiments showed that some genes are
inherited along with the X chromosome.
Genes that are on the same chromosome are linked.
Those on the X chromosome are X-linked.
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Crossing over allows linked genes to appear to
assort independently.
Remember, the purpose of crossing over is to
generate diversity in offspring arising from
sexual reproduction.
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Crossing over results in recombinant gametes.
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Recombinant Offspring
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Recombination Distance
Genes that are far apart are more likely to have
a crossing-over event occur between them than
genes that are closer together on the chromosome.
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Mapping
Knowing the recombination frequencies between
several linked genes allows the researcher to
construct a genetic map. Note that the maximum
genetic distance discernable in a cross is 50
(indistinguishable from independent assortment.)
Larger genetic distances are determined by adding.
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Sex Determination
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Sex-linked genes
(carried on the X
chromosome) show unique patterns of inheritance.
Since the Y chromosome has few genes, recessive
sex-linked alleles are not masked in males.
A carrier mother will pass the allele to 1/2 of
her offspring. Sons will be affected, daughters
will be carriers.
A father with an x-linked trait will transmit the
allele to all daughters, but no sons.
Carrier female x affected male produces affected
females
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X Inactivation
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Nondisjunction
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Meiotic nondisjunction occurs when homologues or
sister chromatids fail to separate in anaphase.
This error leads to gametes that have too many or
too few chromosomes.
If these gametes are involved in fertilization,
the resulting offspring will have an abnormal
number of chromosomes (aneuploidy). They may
have one (monosomy) or three (triploidy) copies
of a chromosome.
This changes the gene dosage of the genes on the
affected chromosome
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Alterations of Chromosome Structure
Gene dosage can also be changed by structural
alterations within a chromosome, including
deletion, duplication, translocation, and
inversion.
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Down Syndrome
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Genomic Imprinting
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