genesis 25:24-26

©2000 Timothy G. Standish

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 . . .


Timothy G. Standish, Ph. D.

Quantitative Quantitative GeneticsGenetics


How Could Noah Have Done It?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


DefinitionsDefinitions 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


Nature Vs NurtureNature 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


Additive AllelesAdditive Alleles

CRCWCRCR CWCW

F2 GenerationF2 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 CR alleles results in redder flowers

CRCR

CRCW

CRCW

CWCW

CR CW

CR

CW


Additive AllelesAdditive 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

432321210

F2

1/4 AA

1/2 Aa

1/4 aa

1/4 BB -- 1/16 AABB1/2 Bb -- 2/16 AABb1/4 bb -- 1/16 AAbb1/4 BB -- 2/16 AaBB1/2 Bb -- 4/16 AaBb1/4 bb -- 2/16 Aabb1/4 BB -- 1/16 aaBB1/2 Bb -- 2/16 aaBb1/4 bb -- 1/16 aabb

1/16

6/16 = 3/8

1/16

4/16 = 1/4

4/16 = 1/4


Additive AllelesAdditive Alleles Graphed as a frequency diagram, these results

look like this:


Estimating Gene NumbersEstimating 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:

4n1 = F2 extreme phenotypes in total offspring

641

43

1= N = 3 so there are probably about 3 genes involved


Economic ImplicationsEconomic ImplicationsEnvironment or genetics?


Describing Quantitative Traits:Describing Quantitative Traits:The MeanThe 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

=nXi

Sum of individual values

Number of individual values

X

X

Frequency

Trait


-1 +1

68.3%=

n(n - 1)nf(x2) - (fx2)

Describing Quantitative Traits:Describing Quantitative Traits:Standard DeviationStandard 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


HeritabilityHeritability Heritability is a measure of how much quantitative genes influence

phenotype Two types of heritability can be calculated:

1 Broad-Sense Heritability: H2 - 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: h2 - Assesses the potential of selection to change a specific

continuously varying phenotypic trait in a randomly breeding population


1 Broad-Sense Heritability1 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

VP = VE + VG

Genetics

Genetic and Environmental interactions

Environment

Proportion of phenotypic variance resulting from genetic rather than environmental influences

Components contributing to phenotypic variation (VP) can be summarized as follows:

VP = VE + VG + VGE

VGE is typically negligible so this formula can be simplified to:

=VP

VGH2


2 Narrow-Sense Heritability2 Narrow-Sense Heritability

As long as this is the case, narrow-sense heritability can be expressed as the ratio as follows:

VP = VE + VG

Dominance

Interactive or epistatic variance

Additive

Potential of selection to change a specific continuously varying phenotypic trait

Narrow-sense heritability concentrates on VG which can be subdivided as follows:

VG = VA + VD + VI

VA is typically negligible so this formula can be simplified to:

=VP

VAh2

genesis 25:24-26

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