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Bio 321 F13 lecture 3 v2

Xwy hemizygotes to generate white

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Unformatted text preview: ll be white eyed females Ÿ༉25% of offspring will be red eyed males Ÿ༉25% of offspring will be white eyed males females: 50% red eyed Ÿ༉X+Xw heterozygotes 50% white eyed Ÿ༉XwXw homozygotes males 50% red eyed Ÿ༉X+Y hemizygotes 50% white eyed Ÿ༉XwY hemizygotes rela0ve size of X compared to rest of genome varies widely •  D. melanogaster and related species –  1 of 5 major chromosome arms =>~20 of genes are X- linked •  ~3000- 3500 of the ~17000 genes •  Homo sapiens –  X is one of 23 chromosomes •  one of the smaller ones •  ~1400- 1700 of the ~21000 genes •  ~7% of the genes what about the Y chromosome •  D. melanogaster –  Y is very small and mostly repe00ve, non- coding DNA –  12 genes known (as of 2008) –  Y is not necessary for viability (X0 males develop normally soma0cally) –  Y required for fer0lity (XO males are infer0le) •  H. sapiens –  231- 344 genes (es0mates vary) –  X0 humans Turner syndrome (Ullrich- Turner syndrome) •  females with short stature and many other morphological and developmental abnormali0es –  infer0le •  incidence: 1:2000 – 1:5000 flies vs. humans •  both have XY genotypic sex determina0on •  Different mechanisms –  Drosophila uses X:A ra0o •  females 1:1 (2 X chromosomes, 2 copies of each autosome) •  males 1:2 (1 X chromosome, 2 copies of each autosome) •  Y chromosome genes irrelevant to soma0c development –  humans – Y chromosome genes play many vital roles in development •  SRY (sex- determining region on Y) gene and others the heterogame0c sex and new muta0ons •  most new muta0ons, if recessive, and on autosomes, are lost –  even advantageous ones –  ini0al frequency = 1/2N (N = pop size) –  rare alleles almost completely in heterozygotes •  recessive phenotype not expressed –  dri3 determines their fate •  but if on the X are expressed in males –  most muta0ons are deleterious => stronger purifying selec0on on X in males –  faster evolu0on of X linked genes...
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