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Mendel's Laws II Notes

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University of Pittsburgh
Biological Sciences
BIOSC 0160

1/28 Chapter 13: Mendel’s Laws II Mendel and Chromosomes • Chromosomal Theory of Inheritance o Mendelian heritable factors (genes) occur on chromosomes at particular places o Chromosomes undergo Mendel’s Laws of Segregation and Independent Assortment  Segregation: paternal and maternal alleles at a given locus separate during gamete production  Independent Assortment: paternal and maternal alleles at two or more loci separate irrespective of one another during gamete production Morgan Links Gene to Chromosomes • Thomas Hunt Morgan: Columbia University • Drosophila melanogaster o A species of fruit fly o Can produce many offspring o Breed every two weeks o Polytene chromosomes (from cells on the salivary glands)  Allowed Morgan to map chromosomes using banding patterns o Gene naming conventions  Each locus has an italicized letter  A mutant is given a letter alone denotes (b for black)  Normal (wild type) is given a + in addition (b+ for gray) • Eye Color and Sex o Cross: white-eyed male and red-eyed (wild type) female o All F1progeny had red eyes 1/28 o F1progeny was crossed  As expected, there was a 3:1 phenotypic ratio  Surprisingly, only males had white eyes  This result was traced to the possession of a Y chromosome in males (Y lacks a copy of the eye color gene) Linkage • Genes on the same basic chromosome are said to be linked • Drosophila o Dihybrid: body color, wing shape  Gray body (b ) is dominant over black body(b) +  Normal wings (vg ) are dominant over vestigal wings (vg) o Cross + + + +  P: b b vg vg x bbvgvg  F1: b bvg vg o Test Cross (to see if the two genes are linked)  b bvg vg x bbvgvg  If the genes are unlinked, and expected phenotypic ratio is 1:1:1:1 Genotype b bvg vg b bvgvg bbvg vg bbvgvg Phenotyp Gray, Gray, Black, Black, e norma
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