Download Some Basic Genetics

Some Basic Genetics
Charles Goodnight
Evolution
Mendelian Genetics
Gregor Mendel
Mendelian Genetics
Mendelian Genetics
Mendelian cross
Genes!
Mendel’s law of Segregation
Two Loci
Epistasis
Polygenic Inheritance
Connecticut Agricultural College
Undergraduates, 1914
Three Alleles
Simple “Monohybrid” Cross
Doing it Mathematically
The Point:
A Mendelian Cross is a special case of a
population of one “male” and one
“female”.
There is no reason why several males
and several females cannot be used in a
Punnett Square.
Example: Two males Two
Females
General Formula
Sad fact: I had to change the allele
names so we could use p and q as
frequencies!
An Example
This works for any allele
frequencies
p q
(p2
2*p*q
q 2 ) = ( p2
0.1, 0.9
(0.12
2*0.1*0.9
0.92) = (0.01
0.18
0.81) = 1
0.3, 0.7
(0.32
2*0.3*0.7
0.72) = (0.09
0.42
0.49) = 1
0.467, 0.533
(0.4672
2*0.467*0.533
0.284) = 1
2*p*q
0.5332) = (0.218
q2 ) = 1
0.498
0.7, 0.3
(0.72
2*0.7*0.3
0.32) = (0.49
0.42
0.09) = 1
(0.92
2*0.9*0.1
0.12) = (0.81
0.18
0.01) = 1
0.9, 0.1
Side note on calculating gene
frequencies
Calculate gene frequencies as homozygotes +
heterozygotes/2
AA+Aa/2
sqrt(AA)
p q
(AA
0.5, 0.5
(0.25
= 0.5
0.5
0.25)
0.25 + 0.5/2 = 0.5 sqrt(0.25)
(0.3
0.55
0.4
0.3)
0.3 + 0.4/2 = 0.5
Aa
aa)
0.5, 0.5
sqrt(0.3) =
True HWC equilibrium is a
multigenerational
Phenomenon
When HWC is true:
Gene Frequencies don’t change
Genotype frequencies don’t change
Phenotype frequencies don’t change
Mayr: Evolution: Change in gene frequencies
Hardy Weinberg Castle Equilibrium: Gene frequencies don’t
change
HWC equilibrium = NO GENETIC EVOLUTION.
Hardy-Weinberg-Castle
Equilibrium
Forces of Evolution
Selection
Migration
Mutation
Genetic Drift
Assumptions of HardyWeinberg-Castle Equilibrium
NO Selection
NO Migration
NO Mutation
NO Genetic Drift
Random Mating
NO genetic Evolution!
Nonrandom mating
AA
2
p
Random
“assortative” p
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Aa
2pq
0
aa
q2
q
Gene frequency = homozygotes + 1/2 heterozygot
Random
Freq(A) = p2 +2pq/2 = p
“assortative” Freq(A) = p +0/2 = p
Hardy Weinberg Proportions
Mixing Sexes with different
gene frequencies for an X
linked gene
Females
AA
X
?
Males
ay
Mixing Sexes with different
gene frequencies
Linkage also affects HWC
equilibrium
One locus HWC is restored by one generations of random
mating
Two locus HWC is restored over time, rate depends on
degree of linkage.
Genotypes
Gamete types: AB, Ab, aB, Ab
AB
Ab
ab
Ab Etc.
AB
ab
AB
aB
D = “Linkage Disequilibrium”
D = freq{AB}freq{ab}-freq{Ab} freq{aB}