Cutter_Lecture_04 - (2 1 Phenotypicvariation...

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Genetic and Phenotypic Variation (2) 1
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Genetic variation for viability Phenotypic variation Differences among individuals in a trait The trait can be “fitness” or a “fitness related” trait Survival or viability Reproductive output Genetic variation Arbitrary locus with no known phenotypic effect E.g. anonymous microsatellite locus or allozyme variants Locus with known phenotypic effect E.g. Mc1r locus affecting melanin production Unknown locus/loci with quantifiable phenotypic effects How much genetic variation for viability is present in nature? Bring wild organisms into the lab to test under standard conditions 2
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Detecting recessive lethals Drosophila pseudoobscura (fruitfly) Mate a randomly selected male from nature to a laboratory “marked” female 1 copy of chrom2 is: Cy L + Other copy of chrom2 is: + + Pm Cy = curly wing, L = lobed eye, Pm = plum eye Do 2 nd chroms from wild males have any recessive lethal alleles? + Pm + Cy + L 3
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Detecting recessive lethals Expect ¼ of F3’s to be +/+ If 0%, recessive lethal present If <¼, deleterious recessive (backcross) (sib mating) 4
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Viability of het VS. hom flies 10% of the copies of chrom2 contain recessive lethal alleles When made homozygous, flies die ~50% of other chromosomes caused lower viability when homozygous Heterozygous chrom2 flies had higher average viability than flies homozygous for chrom2 Masking of deleterious alleles through dominance Most deleterious alleles are rare, so effects of a fully homozygous chromosome reflects the combined effects at many loci 5
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Humans? Each human genome contains 3 5 recessive lethal alleles Alleles that cause death when homozygous Consanguineous mating results in higher incidence of homozygosity Inbreeding depression = increased inviability/sterility of inbred progeny relative to outbred 6
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How much of the genome is heterozygous?
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