BI321Sp08 lecture 04

BI321Sp08 lecture 04 - Lecture 4: Sex! Sex linkage, sex...

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Lecture 4: Sex! Sex linkage, sex determination, begin pedigree analysis Text readings: Read Chapter 1, pp 61-76 Chapter 2, pp 61 – 75; do problems 29, 31, 37, 38, 39, 40, 42, 43, 44, 47, 48, 49, 50, 52, 54, 57 Chapter 3, pp 89 – 97; 102 - 109 Chapter 16, pp 555 - 572 Fruitflies sex chromosomes • Male fruitflies have an X and a Y chromosome • They are heterogametic : producing two types of gametes with respect to sex chromosomes • Female fruitflies have two X chromosomes • They are homogametic : producing only one kind of gamete with respect to sex chromosomes
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Morgan found a spontaneous mutant with white eyes • Wild type alleles are the most common in a population (does this necessarily mean the “best” allele?) • Mutant alleles are less common in a population (and usually detrimental) Mendelian interpretation of rare white eye phenotype R/R r/r x P R/r x R/r F1 F2 only This allele is not segregating in a classical Mendelian fashion. Why?
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XX x XY F1 X Y X X gametes gametes XX XY XX XY What could account for a sex-specific phenotype? R/r x R/Y F1 R Y R r gametes gametes R/R R/Y R/r r/Y Perhaps an allele on the X chromosome ? Look! Mendelian notation!
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w + /w x w + /Y F1 w + Y w + w gametes gametes w + /w + w + /Y w + /w w/Y And now with proper Drosophila notation: Heterozygous female Hemizygous male w/w x w + /Y w + Y w w gametes gametes w + /w w + /w w/Y Reciprocal crosses yield different results: w/Y w + /w + x w/Y w Y w + /w w + /w w + /Y w + /Y w + w +
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Humans have X and Y chromosomes XX x XY F1 X Y X X gametes gametes XX XY XX XY Organisms in which chromosomes play a role in sex determination • Humans (XX, XY) • Flies (XX, XY) • Worms (XX, XO) • Birds (ZZ, ZW) • Hymenopterans (haploid, diploid)
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that’s complicated Sex determination in mammals: SRY Y chromosome is critical PAR (pseudoautosomal region) PAR SRY (Sex-determining region of the Y) MRY (Male-specific region of the Y)
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What is SRY? Sex determination in C.elegans XX X:Autosome ratio = 1 XO X:Autosome ratio = 0.5
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Sex determination in flies Sex determination in flies and humans • In humans, maleness depends on the Y chromosome.
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Sex determination in flies and humans • In humans, maleness depends on the Y chromosome. • In flies, it does not. BUT XO males are sterile, probably because their
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BI321Sp08 lecture 04 - Lecture 4: Sex! Sex linkage, sex...

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