BIOL 2040 Lecture Notes - Lecture 11: Bighorn Sheep, Genotype Frequency, Genetic Variation
3 May 2018
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Quantitative Genetics II
November 3, 2015
Narrow Sense Heritability
HN = VA/Vp or slope of O-P regression
• Determines how much evolution (change in mean) will occur due to selection
• Vantage Point 1 : Slope of Offspring-Parent Regression
Codominance = additivity
• Phenotype is sum of allelic effects
• Offspring look like parents
• O-P regression has slope of around 1
Dominance
• O-P regression has a slope less than 1
• Offspring do not look exactly like their parents
• Therefore, dominance reduces narrow-sense heritability
Additive Genetic Variance and HN
• Whenever homozygotes differ, there is additive genetic variation (VA)
• Whenever heterozygotes are not exactly halfway between homozygotes (and VE=0), there is
dominance genetic variance
• VA + VD = VG
•
• HN is the proportion of variability than can be passes on from parent to offspring
VA example: bird wingspand
• A (dominant) &B (additive) alleles
• Homozygous phenotypes have different values
• Mean = 18.5 inches
• Variance in phenotypes = 1.333 in2
• Therefore, Vp = 1.33
• VE = 0
• VG = 1.33
• All the phenotypic variance is due to differences in genotype
• So HB = VG/VP = 1 (broad-sense heritability)
• HOWEVER, some of difference among genotypes is due ADDITIVE EFFECTS of alleles, and some to
DOMINANCE EFFECTS
• For example:
• differences among aabb (16cm), aaBb (17) and aaBB (18) are ONLY due to additive effects at B locus
• differences among AABB (20cm), AaBB (20) and aaBB (18) are due to BOTH additive effects AND
dominance effects at A locus
• (By the way, for additive effects at A locus: diff betw aa & AA is 2 cm, which is 2 units of A allele, so
average effect of a unit of A is 1cm)
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• THEREFORE, some genetic variance is additive and some is dominance: VG = VA + VD and HN = VA / (VA
+ VD + VE) < 1
In General:
• VG & VA are affected by genotype (& allele) frequencies
• VG, VA, VE apply to ONE population at ONE time
• If change environment and/or genotype frequencies, and HN changes
• Strong selection reduces additive genetic variance
Examples: Narrow Sense Heritability
• Dairy Cattle (Holstein)
o Continual direction selection erodes (reduces) genetic variation (especially additive genetic
variation)
o Thus, directional selection decreases VA and HN
• Big Horn Sheep
o Lifetime weaning = 0.86
o Body Mass = 0.02
Measuring HB and HN in real population:
• If you know genotype:
o Can directly measure HB = VG/VP
o Can directly estimate HN
• Generally:
1) HN : O-P regression in nature
• 2) HN & HB : use a breeding design
▪ usu. each male mated to several females
▪ measure parents and offspring
▪ use statistical approach to estimate variance components
Modes of Selection
• Directional Selection
o Directional selection may also be negative
o Favoring one side
o After selection, there should be a shift in distribution
• Stabilizing Selection
o Favors middle values rather than extreme values
o After selection, distribution is smaller (more narrow)
o The average remains constant; reduces variability
• Disruptive – middle values have lowest fitness
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Hn = va/vp or slope of o-p regression: determines how much evolution (change in mean) will occur due to selection, vantage point 1 : slope of offspring-parent regression. Codominance = additivity: phenotype is sum of allelic effects, offspring look like parents, o-p regression has slope of around 1. Dominance: o-p regression has a slope less than 1, offspring do not look exactly like their parents, therefore, dominance reduces narrow-sense heritability. Additive genetic variance and hn: whenever homozygotes differ, there is additive genetic variation (va, whenever heterozygotes are not exactly halfway between homozygotes (and ve=0), there is dominance genetic variance, va + vd = vg. So hb = vg/vp = 1 (broad-sense heritability) Vg & va are affected by genotype (& allele) frequencies. Vg, va, ve apply to one population at one time. If change environment and/or genotype frequencies, and hn changes.
