Lecture 21 Flashcards
vContrast polygenic traits and monogenic traits.
Polygenic traits: are dependent on contributions from multiple genes
Monogenic traits: traits dependen on the contribution of a single gene(eg mendels pea plants)
Is it possible for a monogenic trait to be quantitative? Explain your reasoning.
Quantitative traits: are measurable traits represented by a number and are typically polygeneic traits
however monogenic traits can be quantitative
e.g: Mendel with the tall and short flowers
while the trait is dependent on one gene the trait can still exist on a continuum of length
Is it possible for a quantitative trait to give rise to discreet variation? Explain your reasoning.
Quantative traits usually do not give rise to discreet variation however if there is a threshold expression of a gene that gives rise to a phenotype there will be discrete variation/
Define multi-factorial trait
There is an environmental influence as well as a genetic infleunce from multiple genes ( e.g height)
What is the mid-parent value for a trait? Why is the mid-parent value very often an underestimate
of offspring height in humans?
the mid-parent value is the average of the mother and father’s traits.
it is often an underestimate of offspring height in humans as it does not consider the environmental effect of dietary quality on height.
Distinguish between major and minor effect genes and how these ideas relate to the production of
different eye colors in humans.
major genes: genotype
two major genes with a large effect on eye color where the genotype at each gene determine if your eye color in the blue/green category or the brown eye category
minor genes: have small effect and in the case of eye color they alter the ey color very slightly
If a trait is determined by 5 additive genes, each with two alternative alleles, how many phenotypic categories would be expected in the absence of an environmental effect on phenotype.
11 phenotypic categories ( 0,1,2,3,4,5,6,7,8,9,10) gene dosages for the alternative alleles
2n+1 gene dosages(given two alleles per genen in population)
Suppose height were determined by three genes and there was no environmental effect, and each
gene had “tall” alleles and “short” alleles.
You observe and individual with a phenotype indicating the
presence of exactly 4 “tall” alleles. Can you infer their genotype? Is the same true if they had the
phenotype indicative of 6 “tall” alleles?
you would not be able to infer much about the genotype as there is no 1:1 correspondence between: phenotype and genotype.
However, if you had 6 tall alleles you would be able to tell that all genes are homozygous dominant.
Why is variation in F1s very low when parental lines that differ in a quantitative trait are crossed?
Edward East: took two pure breeding lines of tobacco plant with a short corolla and long corolla parental ( petals)
- corolla length is a quantitative trait
- there is also an environmental variation
crossing the two parents results in the f1s being an intermediate for the trait ( heterozygous) and mostly genetically uniform as they are directly receiving all alleletypes from each parent( with each parent being homozygous at almost all loci)
Why does variance increase when the F1s are crossed to one another
when you cross f1s you result in f2s with increased phenotypic variance due to the introduction of independent assortment and the increase in genes.
If the variance does not increase substantially in the F2s, is that indicative of many or few genes
underlying a trait? Explain using the ftle-Wright equation
n= D^2/8(s,2-s,1)
n=lower bound of the genes influencing. a trait
D+ difference in phenotype between parental
s1=variance in f1s
s2: variance in f2s
if f2 variance is not substantially larger than f1 variance the denominator would be smaller making the value n the or number of genes larger
Define genetic liability.
the innate likelihood of developing a disease based on a genotype
certain alleles increase genetic liability by passing a threshold for diseases/vs non disease–> contribute to. continuous variation of genetic liability
if you cross two heterozygotes each has three genes that contribute to a disease
you will have offspring with 0-6 alleles possibly contributing to the disease however in terms of genetic liability there can be a threshold where offspring need 5 or more liability alleles across the 3 loci to be affected
Give an example of when VG = 0. Give an example when VE = 0. Why are these cases useful?
vp=vg+ve
In a clonal or inbred population, the inviduals are genetically uniform therefore VG=0 and all of the variance is driven by environment (never in a natural population), this allows us to measure the variance caused by environmental conditions
In a tightly control lab experiment ( Ve is zero) allowing vg to be measured.
Which component of VG is narrow sense heritability concerned with?
Broad sense Heritability = Vg/vp
narrow sense = Va/vp =h2
narrow sense heritability= va/vp–> important for determining how responsive things are to selection and allows us to select for given traits within a population that are desirable
If a trait has low heritability, does that mean the trait is not determined by genes? Explain.
Heritability = The proportion of
phenotypic variation attributable to
genetic variation
High heritability of a trait= a lot of the phenotypic variance is driven by differences in genotype
low heritability: phenotypic variances has more influence from environmental variation as opposed to genetic variation
It does NOT mean it’s NOT determined by genes there is just more environmental factors influencing the phenotypic variation.
e.g: deviations from having two eyes indifferent species is due to environment
