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Unformatted text preview: Chapter 15: The Chromosomal Basis of Inheritance Thomas Hunt Morgan Studied genetics using fruit flies ( Drosophilia melanogaster ): Easily cultured Prolific breeders Short generation time 4 pair chromosomes (3 pair autosomes, 1 pair sex chromosomes) Conventions : Female: XX Male: XY Wildtype : phenotype most common in nature Mutant : alternative to wildtype Morgans genetic symbols: For any character, symbol is based on first mutant discovered If mutant recessive, then lowercase letter If mutant dominant, then capital letter Wildtype: + Morgans Lab 2 big discoveries: Sex-linked genes Genes on same chromosome do not assort independently Sex-Linked Genes First mutant in flies: white eyes Ran a test cross P generation: white male X red female F1: 100% red eyes (red dominant) F2: 100% red females 50% red males. 50% white males Expected 3:1 ratio, but sex distribution odd How possible? Eye color linked to sex of individual, gene on X chromosome X: female 2 copies, males 1 copy Recessive trait, female needs 2 copies to express- not possible in this cross 1...
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