10 - Biology 1001 Spring 2008 (B. Fall), Class notes, topic...

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Biology 1001 Spring 2008 (B. Fall), Class notes, topic #10—Genes within individuals III (sex determination; sex linkage; polygenic traits; pleiotropy) Preparation: complete the reading assignments in your text (Freeman, Biological Science, 2 nd ed.): pp. 281- 297 Learning objectives: 1. Appreciate that there are different mechanisms of sex determination in different organisms. 2. Understand the concept of chromosomal sex determination and sex linkage. 3. Appreciate the difference between autosomal and sex-linked genetics crosses. 4. Appreciate that a gene can influence more than one trait (pleiotropy), and that a trait can be influenced by more than one gene (polygenic). 5. Understand that genes interact with their physical and genetic environments, and this interaction may influence the phenotype. Notes: I. Sex determination A. In some animals (e.g., many reptiles), sex is environmentally determined. For example, whether individual crocodiles and many turtles become male or female depends on the temperature at which the eggs are incubated. Many coral reef fish change from one sex to the other as they get older. B. In most animals (e.g., birds, mammals), sex is determined genetically . Those in which sex is determined genetically have sex chromosomes (e.g., X-Y), plus autosomes (all others). Mammals and some others (including fruit flies) have an X-Y sex chromosome system. Diploid human cells, with 46 chromosomes, contain 22 pairs of autosomes, plus one pair of sex chromosomes—either XX (females) or XY (males). 1. Homogametic sex —produces the same kind of gametes (with respect to sex chromosomes); e.g., mammal and fruit fly females, XX, produce gametes all of which contain one X chromosome. 2. Heterogametic sex —produces two kinds of gametes (with respect to the sex chromosomes); e.g., mammal males, XY, produce gametes with either an X or a Y chromosome. Birds, butterflies, moths, and some other animals have an opposite system (ZW): females are heterogametic, males homogametic. 3.
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This note was uploaded on 04/21/2008 for the course BIOL 1001 taught by Professor Fall during the Spring '08 term at Minnesota.

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10 - Biology 1001 Spring 2008 (B. Fall), Class notes, topic...

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