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Variation

Variation. Continuous phenotypic variation. The most common variation acted on by natural selection. Phenotype of individuals determined by measurement. Changes and patterns of variation determined statistically. A contrast to Mendelian genetics. Quantitative phenotypic

lawrence-rich
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Variation

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  1. Variation

  2. Continuous phenotypic variation • The most common variation acted on by natural selection. • Phenotype of individuals determined by measurement. • Changes and patterns of variation determined statistically.

  3. A contrast to Mendelian genetics Quantitative phenotypic characters: common in nature Range of variation Note shape Distribution of the heights of 1,000 Harvard College students aged 18 to 25 (Adapted from Castle.)

  4. White spotting in Dutch rabbits: a quantitative character

  5. N = 825

  6. Possible genetic basis for some continuous characters Contributing and noncontributing alleles No dominance Each contributing allele makes a small contribution to the phenotypic expression Note shape The results of crosses between two strains of wheat differing in three gene pairs that determine grain color

  7. The mathematical model of continuous character inheritiance • Two kinds of alleles • contributing: a+, b+, c+, etc. • non-contributing: a, b, c, etc. • Genes a, b, c, etc. in different chromosomes. • Locations in chromosomes called Quantitative Trait Loci (QTLs) • Binomial expansion emulates independent assortment of the alleles.

  8. Percent survival of D. melanogaster flies with 16 different combinations of chromosomes exposed to a uniform dose of DDT

  9. Length of corolla

  10. Determining heritability: h2 = VG/(VG + VE) One method:

  11. BUT! No recombination Recombination

  12. Arrangement of phenotypic variation in natural populations Some recognize subspecies Recognizable subspecies would have to be allopatric.

  13. Problem: incongruence of characters

  14. Human race concept. • There is no satisfactory biological definition of a human race! • Misconception: there are character states unique to particular groups of humans • The characters traditionally used are quantitative characters with continuous variation.

  15. “Racial” characters are quantitative characters continuous characters e.g. skin color Phenotypic expression in and among populations generally fits a normal distribution

  16. A common “racial” characteristic is skin color.

  17. Melanosomes

  18. All individuals have approximately the same number of melanocytes. • Therefore, what is the basis of skin color differences? • Can “races” be recognized based on skin color?

  19. These groups easy to identify because of non-overlapping variation. Gaps 685 nm

  20. Skin color in 22 human populations Samples of males Mean +/- one standard deviation

  21. Global Patterns of Human Variation • Can be examined genetically • Can be examined phenotypically • Are phenotypic differences concordant with genetic differences?

  22. Eight classes of genetic Similarity enzyme & blood group loci Arrayed by increasing difference Distribution of eight classes of skin pigmentation intensity Concordance or discordance?

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