2011 Slide Set VII Spr11

2011 Slide Set VII Spr11 - UNIT II: SLIDE SET VIII I....

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UNIT II: SLIDE SET VIII I. Extensions of Mendel’s Rules (continued) E. Sex Linked Inheritance 1. Sex chromosomes 2. Systems of sex determination a. XY system b. X0 systems c. ZW systems d. Haplo-diploid systems e. Non-chromosomal sex determination 3. X-linked inheritance 4. Y-linked inheritance 5. Effects of sex on autosomal traits
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Exceptions to Mendel’s rules (cont’d): 4. Individuals have 2 versions (alleles) for each gene (i.e., alleles occur on homologous pairs of chr’somes) Exception: genes on the X or Y chromosomes (= “ sex-linked ” genes: genes “linked” to or found on one of the sex chromosomes)
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Y chromosome (replicated) in humans and many other species, the composition of the sex chromosomes varies between males and females. females XX , males XY these X and Y chromosomes can have some same genes but the X and Y act like homologues during meiosis because of “pseudoautosomal” regions (regions in common) Fig. 15.5 (7 th : p. 282) X chromosome (replicated)
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Human copyright © 1994 David Adler http://www.pathology.washington.edu/research/cytopages/idiograms/human/ http://www.pathology.washington.edu/galleries/Cytogallery/main.php?file=human%20karyotypes RECALL: human male and female karyotypes male female no “Y”
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humans: XX (female) x XY (male) male gametes: 1/2 X 1/2 Y female 1/1 X gametes (only X is possible) 1/2 XX 1/2 XY Therefore: in humans, gender of offspring is “determined” by male gametes (sperm) Punnett square explains the 1:1 ratio of males to females observed in the human population
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A closer look at the human Y-chromosome: SRY locus/gene = s ex-determining r egion on Y * SRY gene is “activated”/expressed
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2011 Slide Set VII Spr11 - UNIT II: SLIDE SET VIII I....

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