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Human Biology
Course Code
Stephen Wright

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LECTURE 14 March 3rd
Quantitative traits = complex traits
Variation in average differences in genotypes between individuals AND environmental
variation that gives phenotypic variation
Environmental variation because of food, climate, illnesses, etc.
Maristic traits class of discrete traits
Discrete traits either or traits; single-gene traits
Whole-number range of values possibilities are not infinite
Ex. number of progeny
Additive inheritance and incomplete dominance
Additive gene action in heterozygote, exactly intermediate of the two
oThis occurs with additive effects
Incomplete dominance is example of additive gene action
oHeterozygote has trait value or state that is intermediate quantitatively
between the two homozygous classes but NOT EXACTLY
oThis occurs because of environmental variation
Mutations in few genes cause familial hypercolesteremia
Still single-gene disease/disorder
Because any one of the many genes is mutated can result in this
Blurring of lines
oGenotypically heterozygote but phenotypically homozygous can result
because of environmental influence
TODAYs lecture quantitative trait analysis
Phenotype = E and genotype
How much is due to genetic variation?
Broad sense extend of phenotypic variation that is attributed to genetic variation, all
types of genetic variation
Can be quantified by Hsquared (representing broad sense)
Vp = Vg + Ve
Values range between 0 and 1
oThere is no env effect if 1 all variation caused by genetic effects

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oMinimum is 0 env effect is 100%
BSH tells us
oHigh values suggest that the genotype is important in determining
whether this quantitative trait arises due to the segregation of particular
or specific alleles
oImportant in attributing whether this quantitive trait (hair colour,
Cant take Hsquared and predict what the progeny will look like
based on the phenotype of the parents
Hsquared for some humans
Height pretty hight
oGenetic component
Heart rate at the middle gen and env variation equal
What does BSH tell us?
High phenotype is likely to be attributed to its genotype within that family
oCannot extrapolate to other families, to other populations
oValues are specific to the family or the population
Why is BSH not predictive?
Predicting genotype and phenotype of offspring is important, especially to plant
and animal breeders using artificial selection to enhance traits
oThey dont use BSH
Look at the equation
oGenetic variation types include additive effects, dominance effects,
epistatic effects
I stands for interacting
Epistatis occurs when two genes or proteins are INTERACTING,
giving you another phenotype OR one locus masks effect of other
Genetic variation can be divided into 3 types ***
To answer, examine V due to dominance effects
oCannot predict progeny accurately
oPlant breeders want to optimize specific traits
oAs for epistatic variation,
Recessive epistatis R1 masking R2 allele
Every generation, new dominance and epistatic relationships are
formed each generation
So they are not predictive
Passing on genotype, not alleles these new combinations occur
generation to generation
oLeft with additive variation
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