Euploid variations, involving entire sets of chromosomes

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Euploid variations, involving entire sets of
chromosomes
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Aneuploid variations, involving individual
chromosomes within a diploid set
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Change in chomosome numberTranslocation

• Via centromere fusion

Fertilization would Involve these two chromosomes
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Mutations Chromosomal rearrangementsInversions
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Frequencies of different 3rd chromosome
inversions in D. pseudoobscura
(Adapted from Dobzhansky 1947.)
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Six proposed major steps in the evolution of
human chromosome no. 3
(Modified from Hino et al., 1993)
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2n numbers
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Measuring genetic variation

• 1. DNA sequencing
• 2. Electrophoresis (allozymes)
• E.g. phosphoglucomutase)
• 18 killifishes (Fundulus zebrinus)

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• 2. Electrophoresis
• DNA fragmented by specific restriction enzymes
• Restriction fragment length polymorphisms
• C-C chemokine receptor 32 (Coreceptor used by
  HIV)
• CCR5 gene and the ?32 mutation

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Calculating allele frequencies

• 1. Biparental species
• E.g. delta 32
• Britain 283 individuals
• 223 /
• 57 /?32
• 3 ?32 /?32
• 566 alleles
• Freq. 446 57 / 566 0.89
• Freq. ?32 6 57 / 566 0.11
• ?32 /?32 confers immunity to HIV

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• 2. Uniparental species
• Aspidoscelis neotesselata (triploid tesselated
  whiptail lizard)

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• 34 loci assessed for protein phenotypes
• Only one locus (mannose-6-phosphate isomerase)
  revealed variation
• Sample of 17 individuals
• ABC (N 9) BBC (N 5) ACC (N 3)
• Sample of 51 alleles
• Frequency of allele A 9 3 / 51 0.24
• Frequency of allele B 9 10 / 51 0.37
• Frequency of allele C 9 5 6 / 51 0.39

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A. neotesselata genetic variation at MPI locus
Florence La Junta Lime
Postformational mutation
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Amount of genetic variation in populations

• Can be assessed from levels of heterozygosity
• Biparental species
• A. sexlineata 7
• Uniparental species
• A. tesselata 56
• A. neosselata 74

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A contrast to Mendelian genetics
Quantitative phenotypic characters common in
nature Range of variation
Distribution of the heights of 1,000 Harvard
College students aged 18 to 25
(Adapted from Castle.)
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White spotting in Dutch rabbits
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Possible genetic basis for some
continuous Characters Contributing and
noncontributing alleles No dominance Each
contributing allele makes a small contribution
to the phenotypic expression But
The results of crosses between two strains of
wheat differing in three gene pairs that
determine grain color