chapter 15 chromosomal basis of inheritance. mendel’s inheritance with independent assortment
TRANSCRIPT
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Chapter 15
Chromosomal Basis of Inheritance
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Mendel’s inheritance with independent assortment
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Mendel had factors, we now know these factors were
specific genes at specific loci, (locations) on specific
chromosomes. This is the chromosome theory of
inheritance.
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Morgan’s experiments with Fruit Flies
A.Morgan used fruit flies to trace a gene to a specific chromosome
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1. Drosophila Melangaster – wildtype phenotype = most common and mutant phenotypes = caused by mutations of the wildtype gene.2. Sex-linked- using the gene for white eyes Morgan found that if a female had the trait and mated to a male with wildtype, only her sons would be white-eyed.
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Sex Linkage
This meant the gene must be on the X and not the Y.
A daughter would receive the wildtype X from the father which would override the faulty X from mom.
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Linked genes – genes close together on the same chromosome
• Morgan found that flies have linked genes for body color and wing size.
• Gray and normal wings were wildtype, black and vestigal wings were mutant types.
• Gray flies with vestigal wings and black flies with normal wings are much harder to breed than the ones in b.
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Independent assortmentGenes which are not linked sort
independent of one another.When they come together in a zygote it is called recombination.
a. A frequency of 50% or more recombination means the genes are not linked
b. Linked genes do not sort independently, they travel together, making recombination percentages low.
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Recombination information can be used to map the genes on a
chromosome.
• The measurements are called centimorgans
• If the recombination frequency between cn and b is 9%, cn and vg 9.5%, b and vg 17% the map would look like this;
• 17 centimorgans
b cn vg
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• If the genes are far apart they behave as if they were on different chromosomes
• This slide shows recombination during meiosis.
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Recombination calculations
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Sex chromosomes
A. In humans XX is female and XY is male
1. The SRY gene has been shown to trigger the development into a male fetus at about 2 months old.
2. SRY probably regulates other genes
3. Some XX male and XY females exist with mutated SRY genes
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Sex-linked
• Genes on the X chromosome will only be present in one copy in males, as they only have one X and the Y does not contain the same genes.
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Hemophilia
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Males are said to be hemizygous for these traits
a. Colorblindness is an example of a non- lethal sex-linked trait- females can acquire it from their father (colorblind) and mother (carrier).
b. Duchene muscular dystrophy – weakening of the muscles,1/3500 males- usually die by 20s
c. Hemophilia- absence of certain clotting factors.
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X inactivation- ch.17
• In females one X becomes inactivated and condenses into a Barr body, which one is inactive is random, and will be different in different cells.
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Errors
A.Nondisjunction- when meiosis is not accurate and some cells are missing a chromosome (anueploidy) and one has too many chromosomes(polyploidy)
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Turner’s
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Metafemale
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Down’s Syndrome-1/700 births trisomy 21
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Klinefelter – XXY, XXXY
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XYY
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Other nondisjunctions
• Metafemale - XXX2. Turner – X0
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Other errorsA. Deletions – Chromosome fragment is lost
1. Cri-du-chat- chromosome 5B.Duplication – chromosome fragment attached to homologueC.Inversion – reverse orientation of fragment on chromosomeD.Translocation – fragment on a nonhomologue
1. Implications of cancer - CML
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Imprinting
• Genomic imprinting – One active copy either maternal or paternal.
• Fragile X – caused in part by maternal imprinting.
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Extranuclear genes
A. Small circular DNA in mitochondria and chloroplasts
B. Maternal inheritance is the rule