Selection, lethal alleles, heterozygote superiority

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Soil + Crop Sciences
SOCR 330
Sarah Ward

28 November HYPP in Quarter Horses Hyperkalemic periodic paralysis Sudden uncontrollable muscle twitching or paralysis Due to abnormal sodium transport in muscles Autosomal dominant mutation Predominant in quarter horse bloodlines due to line breeding Affected animals almost all descended from one sire “Impressive” (1968 – 1995) As of 2006 over 100,000 registered quarter horses, paints and appaloosas worldwide have Impressive somewhere in pedigree Selection Differential reproductive success among genotypes Principal force altering allele frequencies in large populations Three types of selection Stabilizing Directional Disruptive Different Types of Selection Stabilizing selection: intermediate phenotypes favored Selection operates against very high and very low birth weights in humans Directional selection: one extreme phenotype favored Disruptive selection: both extreme phenotypes favored Fitness and Selection Fitness measures contribution of individual genotype to the next generation Genotype in a population with greatest reproductive success assigned a relative fitness of RF = 1 Less successful genotypes assigned proportionally lower RF values Selection coefficient S = (1 – RF) S measures intensity of selection against a genotype Example of Different Relative Fitness Values Deer population contains 3 genotypes AA (fastest) RF = 1.0 S = 0.0 Aa (medium) RF = 0.8 S = 0.2 aa (slow) RF = 0.6 S = 0.4 Calculating Changes in All
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