A summary of the lecture on human and quantitative genetics including notes on hardy-weinberg and linkage disequilibrium and how we can identify the diseased gene based on pedigree diagrams.
Gene expression and post-transcriptional modification
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University of Exeter (UoEX)
University of Exeter
Introduction To Genetics (BIO1334)
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There are 3 types of polygenic trait: Quantitative genetics
Metric – continuous scale
- Mean : μ=
∑ xi Heritability of traits described how Broad :
n much variation is genetic H2 = VG / VP
- Population variance: Broad sense – measures the
importance of genetic variation Narrow :
σ=
∑ ²(xi−μ) relative to the total variation in the h 2 = VA / V P
N phenotype. (i.e how much is genetic h2 values should vary
and how much is environmental) between 0 and 1 and can
Meristic – discrete scale Narrow sense – measures the
e.g. bristle number on Drosophila be used to predict change
importance of additive genetic in the population mean
variation relative to the total
Threshold – present or absent (trait only under selection. When h2
variation (i.e. variation due to a
present once you meet the threshold) increases, response to
particular allele).
- Multifactorial selection increases.
- E.g. obesity, cancer, heart disease
All of these are based on the theoretical
Phenotypic variation in a trait can be
partitioned. Phenotype = genotype +
environment.
The variation we see in a population is
related to: VP = VG + VE + VGE Population genetics theory states that the frequencies of alleles
P – phenotypic, G – genetic, E – and genotypes in a population will remain constant unless acted
environmental, GE – G*E interaction. upon by non-Mendelian processes. Equilibrium = no evolution
The phenotype is affected by multiple genes, Evolution: a change in the frequency of an allele within a gene
population often contains multiple alleles. pool.
Genetic variation can be partitioned into Hardy-Weinberg equilibrium:
variation based on alleles or additive effects P + q = 1. P2 + q 2 = 1
[VA], dominance interactions [VD], and
epistatic interactions [VI]. Assumptions:
1) Organisms are diploid, sexual and have discrete generations
Linkage disequilibrium (D): The non-random 2) Allele frequencies are the same in each sex
association between two polymorphic loci. 3) Mendelian segregation occurs
Otherwise defined as “deviations from the 4) Mating occurs at random
expectations of independent assortment 5) Population size is large so no genetic drift occurs
and the HW equilibrium caused by either 6) No gene flow (immigration, emigration)
physical linkage or population demography. 7) No mutation
Under HWE, the frequency of a given 8) No selection
haplotype equals the frequency of each Hardy-Weinberg is therefore a way of testing whether a
allele multiplied together. population has evolved or not.
D= (pAB*pab) – (pAb*paB)
Under HW and no linkage D=0. Allele frequencies
Differ from genotypic frequencies. When calculating allele
frequencies, remember for homozygotes that the allele numbers
will be double the number of individuals for that genotype.
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