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Published bySuzanna Marsh Modified over 7 years ago
The Chromosomal Basis for Inheritance
Thomas Hunt Morgan Early 1900s Columbia University (New York) Studied genetics of Drosophila melangaster (the common fruit fly)
Why Drosophila? Only four pairs of chromosomes A single mating produces hundreds of offspring A new generation can be produced every 2 weeks
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.)
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
Practice A white-eyed female fruit fly is mated with a wild- type male. What are the expected phenotype and genotype ratios?
X Inactivation Barr bodies Inactivation is varied, leading to a mosaic of traits e.g. Tortoiseshell cats, sweat glands in human females
Note that various systems of sex determination exist. X-Y X-O Z-W Haplo-diploid
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
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.)
Normal vs vestigial wings
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?
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.
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
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%.
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?
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%.
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