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ch15 - Chapter 15 The Chromosomal Basis of Inheritance...

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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 Morgan’s 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: + Morgan’s 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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ch15 - Chapter 15 The Chromosomal Basis of Inheritance...

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