Presentation on theme: "Genesis 25:24-26 24 And when her days to be delivered were fulfilled, behold, there were twins in her womb. 25 And the first came out red, all over like."— Presentation transcript:
1 Genesis 25:24-2624 And when her days to be delivered were fulfilled, behold, there were twins in her womb.25 And the first came out red, all over like an hairy garment; and they called his name Esau.26 And after that came his brother out, and his hand took hold on Esau's heel; and his name was called Jacob . . .
2 Quantitative Genetics Timothy G. Standish, Ph. D.
3 How Could Noah Have Done It? The diversity of appearance in humans and other animals is immenseHow could Adam and Eve or Noah and his family have held in their genomes genes for all that we see today?At least one explanation, that the dark-skinned races descended from Cain who was marked with dark pigment (the mark of Cain mentioned in Gen. 4:15) or Ham as a result of the curse mentioned in Gen. 9:22-27Quantitative or polygenic inheritance offers much more satisfying answer to this quandary
4 DefinitionsTraits examined so far have resulted in discontinuous phenotypic traitsTall or dwarfRound or wrinkledRed, pink or whiteQuantitative inheritance deals with genetic control of phenotypic traits that vary on a continuous basis:HeightWeightSkin colorMany quantitative traits are also influenced by the environment
5 Nature Vs NurtureQuantitative genes’ influence on phenotype are at the crux of the nature/nurture debateSocialism emphasizes the environmentFascism emphasizes geneticsUnderstanding quantitative genetics helps us to understand the degree to which genetics and the environment impact phenotypeAside from political considerations, quantitative genetics helps us to understand the potential for selection to impact productivity in crops and livestock
6 Additive AllelesAdditive alleles are alleles that change the phenotype in an additive wayExample - The more copies of tall alleles a person has, the greater their potential for growing tallAdditive alleles behave something like alleles that result in incomplete dominanceMore CR alleles results in redder flowersCRCWCRCRCWCWF2 Generation2:11:CRCRCRCWCWCWCRCW
7 Additive AllelesIf more than one gene with two alleles that behave as incompletely dominant alleles are involved, variability occurs over more of a continuumIf two genes with two alleles are involved, X phenotypes can resultAdditivealleles43211/166/16 = 3/84/16 = 1/4F21/4 AA1/2 Aa1/4 aa1/4 BB -- 1/16 AABB1/2 Bb -- 2/16 AABb1/4 bb /16 AAbb1/4 BB -- 2/16 AaBB1/2 Bb -- 4/16 AaBb1/4 bb /16 Aabb1/4 BB -- 1/16 aaBB1/2 Bb -- 2/16 aaBb1/4 bb /16 aabb
8 Additive AllelesGraphed as a frequency diagram, these results look like this:
9 Estimating Gene Numbers The more genes involved in producing a trait, the more gradations will be observed in that traitIf two examples of extremes of variation for a trait are crossed and the F2 progeny are examined, the proportion exhibiting the extreme variations can be used to calculate the number of genes involved:4n1= F2 extreme phenotypes in total offspringIf 1/64th of the offspring of an F2 cross of the kind described above are the same as the parents, then641431=N = 3 so there are probably about 3 genes involved
11 Describing Quantitative Traits: The Mean Two statistics are commonly used to describe variation of a quantitative trait in a populationThe Mean - For a trait that forms a bell-shaped curve (normal distribution) when a frequency diagram is plotted, the mean is the most common size, shape, or whatever is being measuredSum of individual valuesXD FrequencyD Trait=nSXiXNumber of individual values
12 Describing Quantitative Traits: Standard Deviation Standard Deviation - Describes the amount of variation from the mean in units of the traitLarge SD indicates great variability68 % of individuals exhibiting the trait will fall within ±1 SD of the mean, 95.5 % ±2, 99.7 % ±3 SD95 % fall within 1.96 SD-1+1Number of individuals in each unit measuredTotal number of individuals in sampleXD FrequencyD Trait68.3%=n(n - 1)nSf(x2) - (Sfx2)Gradations of units of measurements
13 HeritabilityHeritability is a measure of how much quantitative genes influence phenotypeTwo types of heritability can be calculated:Broad-Sense Heritability:H2 - Expresses the proportion of phenotypic variance seen in a sample that is the result of genetic as opposed to environmental influencesNarrow-Sense Heritability:h2 - Assesses the potential of selection to change a specific continuously varying phenotypic trait in a randomly breeding population
14 1 Broad-Sense Heritability Proportion of phenotypic variance resulting from genetic rather than environmental influencesComponents contributing to phenotypic variation (VP) can be summarized as follows:Genetic and Environmental interactionsVP = VE + VG + VGEEnvironmentGeneticsVGE is typically negligible so this formula can be simplified to:VP = VE + VGAs long as this is the case, broad heritability can be expressed as the ratio of environmental to genetic components in phenotypic variation=VPVGH2
15 2 Narrow-Sense Heritability Potential of selection to change a specific continuously varying phenotypic traitNarrow-sense heritability concentrates on VG which can be subdivided as follows:Interactive or epistatic varianceVG = VA + VD + VIAdditiveDominanceVA is typically negligible so this formula can be simplified to:VP = VE + VGAs long as this is the case, narrow-sense heritability can be expressed as the ratio as follows:=VPVAh2