the chromosomal basis for inheritance. thomas hunt morgan early 1900s columbia university (new york)...
TRANSCRIPT
![Page 1: The Chromosomal Basis for Inheritance. Thomas Hunt Morgan Early 1900s Columbia University (New York) Studied genetics of Drosophila melangaster (the common](https://reader031.vdocuments.mx/reader031/viewer/2022032722/56649cda5503460f949a429e/html5/thumbnails/1.jpg)
The Chromosomal Basis for Inheritance
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Thomas Hunt Morgan
Early 1900s
Columbia University (New York)
Studied genetics of Drosophila melangaster (the common fruit fly)
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Why Drosophila?
Only four pairs of chromosomes
A single mating produces hundreds of offspring
A new generation can be produced every 2 weeks
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Morgan's studies revealed...
Genes are located on specific chromosomes at specific loci (locations).
There are many genes on a single chromosome.
Genes can be linked to various degrees (i.e. they can be inherited together if they are located on the same chromosome.)
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Sex-linkage
Morgan discovered that Drosophila eye color was linked to the X chromosome, supporting the chromosome theory of inheritance.
X linkage and Y linkage exist
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Practice
A white-eyed female fruit fly is mated with a wild-type male. What are the expected phenotype and genotype ratios?
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X Inactivation
Barr bodies
Inactivation is varied, leading to a mosaic of traits
e.g. Tortoiseshell cats, sweat glands in human females
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Note that various systems of sex determination exist.
X-Y
X-O
Z-W
Haplo-diploid
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Morgan's research supports Mendel's Laws
Homologous chromosomes account for Mendel's Law of Segregation.
Non-homologous chromosomes account for Mendel's Law of Independent Assortment
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Linked Genes
Genes located near each other on the same chromosome tend to be inherited together in genetic crosses.
Refers to two or more genes on a single chromosome. (This is not the same as sex-linkage.)
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Normal vs vestigial wings
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Practice
Let b+ = gray body and b = black body. Let vg+ = normal wings and vg = vestigial wings
What is the expected phenotype ratios if a fly heterozygous for both traits is crossed with one that is homozygous recessive for both traits?
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Genetic Recombination
Parental types
Non-parental types = recombinant types = recombinants
If chromosomes independently assort, 50% recombination frequency is expected.
Linkage is suspected when recombination is below 50%.
Due to crossing-over.
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Linkage/genetic maps
Based on the idea that the farther apart two genes lie on a chromosome, the more likely a cross over event will occur between them.
1 map unit = 1% recombination frequency
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For example...Three genes (b, cn, and vg) are found on a single chromosome.
Recombination frequencies are b-cn 9%, cn-vg 9.5%, and b-vg 17%.
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Practice
Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency between A and B is 28%, B and C is 5%, and A and C is 12%. What is the linear order of these genes?
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Practice
Determine the sequence of genes along a chromosome based on the following recombination frequencies: A-C 28%, A-B 8%, A-D 25%, B-C 20%, B-D 33%.