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©2000 Timothy G. Standish Genesis 25: 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...
©2000 Timothy G. Standish Timothy G. Standish, Ph. D. Quantitative Genetics
©2000 Timothy G. Standish How Could Noah Have Done It? The diversity of appearance in humans and other animals is immense How 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. 9:22-27 Quantitative or polygenic inheritance offers much more satisfying answer to this quandary
©2000 Timothy G. Standish Definitions Traits examined so far have resulted in discontinuous phenotypic traits –Tall or dwarf –Round or wrinkled –Red, pink or white Quantitative inheritance deals with genetic control of phenotypic traits that vary on a continuous basis: –Height –Weight –Skin color Many quantitative traits are also influenced by the environment
©2000 Timothy G. Standish Nature Vs Nurture Quantitative genes’ influence on phenotype are at the crux of the nature/nurture debate Socialism emphasizes the environment Fascism emphasizes genetics Understanding quantitative genetics helps us to understand the degree to which genetics and the environment impact phenotype Aside from political considerations, quantitative genetics helps us to understand the potential for selection to impact productivity in crops and livestock
©2000 Timothy G. Standish Additive Alleles CRCWCRCW CRCRCRCR CWCWCWCW F 2 Generation 2:11: Additive alleles are alleles that change the phenotype in an additive way Example - The more copies of tall alleles a person has, the greater their potential for growing tall Additive alleles behave something like alleles that result in incomplete dominance More C R alleles results in redder flowers CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW CRCR CWCW CRCR CWCW
©2000 Timothy G. Standish Additive Alleles If more than one gene with two alleles that behave as incompletely dominant alleles are involved, variability occurs over more of a continuum If two genes with two alleles are involved, X phenotypes can result Additive alleles F 2 1/4 AA 1/2 Aa 1/4 aa 1/4 BB -- 1/16 AABB 1/2 Bb -- 2/16 AABb 1/4 bb -- 1/16 AAbb 1/4 BB -- 2/16 AaBB 1/2 Bb -- 4/16 AaBb 1/4 bb -- 2/16 Aabb 1/4 BB -- 1/16 aaBB 1/2 Bb -- 2/16 aaBb 1/4 bb -- 1/16 aabb 1/16 6/16 = 3/8 1/16 4/16 = 1/4
©2000 Timothy G. Standish Additive Alleles Graphed as a frequency diagram, these results look like this:
©2000 Timothy G. Standish Estimating Gene Numbers If 1/64th of the offspring of an F2 cross of the kind described above are the same as the parents, then The more genes involved in producing a trait, the more gradations will be observed in that trait If 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: 4n4n 1 = F 2 extreme phenotypes in total offspring = N = 3 so there are probably about 3 genes involved
©2000 Timothy G. Standish Economic Implications Environment or genetics?
©2000 Timothy G. Standish Describing Quantitative Traits: The Mean Two statistics are commonly used to describe variation of a quantitative trait in a population 1 The 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 measured = n XiXi Sum of individual values Number of individual values X X Frequency Trait
©2000 Timothy G. Standish % = n(n - 1) n f(x 2 ) - ( fx 2 ) Describing Quantitative Traits: Standard Deviation 2 Standard Deviation - Describes the amount of variation from the mean in units of the trait Large SD indicates great variability 68 % of individuals exhibiting the trait will fall within ±1 SD of the mean, 95.5 % ±2, 99.7 % ±3 SD 95 % fall within 1.96 SD s X Frequency Trait Total number of individuals in sample Number of individuals in each unit measured Gradations of units of measurement
©2000 Timothy G. StandishHeritability Heritability is a measure of how much quantitative genes influence phenotype Two types of heritability can be calculated: 1 Broad-Sense Heritability: H 2 - Expresses the proportion of phenotypic variance seen in a sample that is the result of genetic as opposed to environmental influences 2 Narrow-Sense Heritability: h 2 - Assesses the potential of selection to change a specific continuously varying phenotypic trait in a randomly breeding population
©2000 Timothy G. Standish 1 Broad-Sense Heritability As long as this is the case, broad heritability can be expressed as the ratio of environmental to genetic components in phenotypic variation V P = V E + V G Genetics Genetic and Environmental interactions Environment Proportion of phenotypic variance resulting from genetic rather than environmental influences Components contributing to phenotypic variation (V P ) can be summarized as follows: V P = V E + V G + V GE V GE is typically negligible so this formula can be simplified to: = VPVP VGVG H2H2
©2000 Timothy G. Standish 2 Narrow-Sense Heritability As long as this is the case, narrow- sense heritability can be expressed as the ratio as follows: V P = V E + V G Dominance Interactive or epistatic variance Additive Potential of selection to change a specific continuously varying phenotypic trait Narrow-sense heritability concentrates on V G which can be subdivided as follows: V G = V A + V D + V I V A is typically negligible so this formula can be simplified to: = VPVP VAVA h2h2
©2000 Timothy G. Standish
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