Lecture 5 Answers - Lecture 5: Study Questions and Answers...

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Lecture 5 : Study Questions and Answers 1. Describe the XX-XO system of sex determination. In this system, which is the heterogametic sex and which is the homogametic sex? In the XX-XO sex determination system, females have two copies of the sex determining chromosome, whereas males have only one copy. Males must be considered heterogametic because they produce two different types of gametes with respect to the sex chromosome: either containing an X or not containing an X. 2. How does sex determination in the XX-XY system differ from sex determination in the ZZ-ZW system? In the XX-XY system, males are heterogametic and produce gametes with either an X chromosome or a Y chromosome. In the ZZ-ZW system, females are heterogametic and produce gametes with either a Z or a W chromosome. 3. Give the typical sex chromosomes found in the cells of people with Turner syndrome and Klinefelter syndrome. Turner syndrome: XO Klinefelter syndrome: XXY (rarely XXXY or XXYY) 4. What characteristics are exhibited by an X-linked trait? Males show the phenotypes of all X-linked traits, regardless of whether the X-linked allele is normally recessive or dominant. Males inherit X-linked traits from their mothers, pass X-linked traits to their daughters, and through their daughters to their daughters’ descendents, but not to their sons or their sons’ descendents. 5. What are Barr bodies? Barr bodies are darkly staining bodies in the nuclei of female mammalian cells. By inactivating all X chromosomes beyond one, female cells achieve dosage compensation for X-linked genes. 6. In certain salamanders, the sex of a genetic female can be altered, making her into a functional male; these salamanders are called sex-reversed males. When a sex- reversed male is mated with a normal female, approximately 2/3 of the offspring are female and 1/3 are male. How is sex determined in these salamanders? Explain the results of this cross. The 2:1 ratio of females to males could be explained if a quarter of the progeny are embryonic lethals. The sex-reversed male has the same chromosome complement as normal females. If females are homogametic (XX), then matings between sex- reversed males (XX) and normal females (XX) must result in all (XX) female progeny, with no obvious reason for embryonic lethality. However, if females are heterogametic (ZW), then ZW (sex reversed male) crossed to ZW (normal female) results in 1/4 ZZ (male), ½ ZW (female), and 1/4 WW (embryonic lethal). The net result is a 2:1 ratio of females to males.
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7. In Drosophila, yellow body is due to an X-linked gene that is recessive to the gene for gray body. a. A homozygous gray female is crossed with a yellow male. The F1 are intercrossed to produce F2. Give the genotypes and phenotypes, along with the expected proportions, of the F1 and F2 progeny. We will use X+ as the symbol for the dominant gray body color, and Xy for the
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This note was uploaded on 05/07/2011 for the course BIOLOGY 352 taught by Professor Townsend during the Spring '08 term at San Diego State.

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Lecture 5 Answers - Lecture 5: Study Questions and Answers...

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