lecture12F06 - Lecture 12: Genetics: Genetics & SexLinkage;...

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Lecture 12: Genetics: Genetics & Sex- Linkage; Chromosome variation The significance of Morgan’s discovery of the white-eyed male fly. sex chromosomes, sex-linkage In mammals, sex is determined chromosomally by an XY system X-Y chromosomes, autosomes, SRY T he pattern of inheritance produced by sex-linked recessive traits. basic pattern, hemizygous. Sex-linked traits and genetic disorders in humans. number of X-linked genes, example genetic diseases/traits Sex can be determined in a variety of ways X-Y system, X-0 system, Z-W system, haplodiploidy, environmental sex determination X-inactivation. Barr body. Chromosomal rearangements and errors nondisjunction, aneuploidy, trisomy 21, inversions and similar rearrangements Imprinting The puzzle of sexual reproduction Reproductive handicap, asexual species, The Red Queen hypothesis Readings: Ch. 15, 15.3 to end; Ch. 23 p. 469 Sexual Reproduction, Fig. 23.16 You need to be able to explain the following to yourself and to me (on exams):
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Fig. 15.3 T. H. Morgan first showed, using a recessive white eye mutant of the common fruit fly Drosophila melanogaster, that a gene had to be located on a specific chromosome. Morgan & the white-eyed male fly. sex-linkage sex chromosomes
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Figure 15.4 In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F 1 and F 2 offspring have? The F 2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes, and half had red eyes. Morgan then bred an F 1 red-eyed female to an F 1 red-eyed male to Produce the F 2 generation. RESULTS P Generation F 1 Generation X F 2 Generation Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F 1 offspring all had red eyes. EXPERIMENT Morgan & the white-eyed male fly. sex-linkage sex chromosomes
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CONCLUSION Since all F 1 offspring had red eyes, the mutant white-eye trait ( w ) must be recessive to the wild-type red-eye trait ( w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P
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This note was uploaded on 04/07/2008 for the course IB 201 taught by Professor Hughes/berlocher during the Spring '07 term at University of Illinois at Urbana–Champaign.

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lecture12F06 - Lecture 12: Genetics: Genetics & SexLinkage;...

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