Lecture 28 Sex Linkage Errors&Exceptions Notes

Lecture 28 Sex Linkage Errors&Exceptions Notes -...

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Lecture 28 Sex-linked genes; Errors in chromosomal dynamics Campbell Chapter 15: 6 th Ed. pp. 272; 276-282 7 th Ed. pp. 276, 282-285; 287. 8 th Ed. pp 288-292, 297-302 PART I: Sex linkage Chromosomes in homologous pairs are called autosomes, while the sex chromosomes are not in homologous pairs. In humans and many other species (including Drosophila), females have 2 X chromosomes, while males have one X and one Y chromosome in each somatic cell. (Other species may differ, as shown in Fig. 15.8/15.9 in Campbell. For instance, grasshoppers have an X-O system, where males have only 1 X chromosome, and no other sex chromosome. Chickens have a ZW system; that is, females have 2 different sex chromosomes, Z and W, while males have two Z chromosomes.) The Y chromosome contains few genes, and we won’t talk about it any more. However, many important genes are on the X chromosome. Genes on the X chromosome are sex- linked . We say that males are hemizygous for these genes, meaning that they have only one copy. During meiosis and gamete formation, the sex chromosomes segregate. That means that each egg gets one X chromosome, while each sperm can get either an X or a Y chromosome. Since all eggs contain an X chromosome, fertilization of an egg with a sperm containing an X chromosome gives a female (XX), while fertilization of an egg with a sperm containing a Y chromosome gives a male (XY). Genes on the X chromosome are transmitted according to Mendel’s laws, but with a twist; phenotypes that reflect sex-linked traits are transmitted with different frequencies to male and female offspring, reflecting the fact that females are XX and males XY. In his early studies of fruit fly genetics, Morgan studied a gene for eye color. Wild type flies have red eyes, while mutants have white eyes. Red is dominant, and white recessive. Morgan did a similar experiment to the one Mendel did with peas. He first made true-breeding stocks of flies with either red or white eyes and used these flies as his P generation. He crossed a red-eyed female with a white-eyed male. As expected, all the offspring in this F1 generation had red eyes. He then crossed these individuals of the F1 generation with each other. As expected from Mendelian genetics, the progeny in the resulting F2 generation had a 3:1 ratio of red:white eye color. However, all the females had red eyes, while half of the males had red eyes and half white eyes. It turns out that the eye color gene is sex-linked; that is, it's on the X chromosome. You can follow through the genotypes of the P, F1 and F2 generations to explain the observed results. The eye color gene is called w for the mutant allele and w+ for the wild type allele. P generation
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This note was uploaded on 09/04/2009 for the course SBU 101 taught by Professor Debag during the Spring '09 term at SUNY Stony Brook.

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Lecture 28 Sex Linkage Errors&Exceptions Notes -...

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