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Lecture 6

BIOLOGY 1M03 Lecture Notes - Lecture 6: Mns Antigen System, Genotype Frequency, Allele Frequency

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Jon Stone

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Chapter 25
The Gene Pool Concept
o Hardy Weinberg
Had an approach to analyze the consequences of matings among all the individuals in
a population
Imagined that all gametes produced in each generation go into a single group,
Gene Pool, then mix at random to produce offspring
Example: Sea Stars, and clams release gametes into the water, where they
mix at random with other individuals and produce a zygote
To determine which genotypes would be present in the next generation and at what
frequency, Hardy Weinberg simply had to calculate what happened, when two
gametes were plucked at random out of the gene pool, many times and each of these
gamete pairs was then combined to form offspring
Deriving Hardy Weinberg Principle
o Began analyzing simple situation possible- that just two genes exist in a population, called
A1 and A2
Use "p" to denote A1; and "q" to denote A2
Since there are only 2 alleles, must add up to 1 (p + q =1)
Step 1: "p" and "q" can have any value b/w 0-1. Suppose Initial
frequency of "p" is 0.7, and "q" is 0.3
Step 2: In gene pool, 70% of gametes carry A1 gene and 30% carry
A2 genes
Step 3: Gamete mix at random to produce offspring. Each time
gamete is involved in forming offspring 70% chance A1 and 30% A2
Step 4: Because only 2 alleles present 3 possible genotypes (A1A1,
Frequencies will be (p2, 2pq, and q2) respectively
Genotype frequencies must add to 1 (p2+2pq+q2=1)
In example would be 0.49, 0.42, 0.09 respectively
Step 5: Easiest way to calculate allele frequencies. Imagine they
form gametes that go into a gene pool. All of gametes from A1A1 +
Half of the A1A2 genotypes carry the allele (A1) (49% +
1/2(42%)=70%); all gametes from A2A2 + Half of the A1A2
genotypes carry the allele (A2) (9% + 1/2(42%)= 30%)
No allele frequency change occurred, called Hardy Weinberg principle
Makes Two Claims:
If frequencies A1 and A2 are given by "p" and "q", then
genotypes A1A1, A1A2, A2A2 will be given by p2, 2pq, and q2
respectively, for generations
When alleles are transferred through meiosis and random
combination of gametes, frequencies don't change over time.
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For evolution to occur, some of the evolutionary factors must
come into play
For population to conform with Hardy Weinberg Principle,
none of the evolutionary mechanisms can be acting on a
No natural selection
No genetic drift, or random allele frequency changes
No gene flow
No mutation
Random mating
Serves as a null hypothesis, for biologist to test whether natural
selection is acting on a particular gene, or whether the other
evolutionary mechanisms are at work
Case Study: Are MN Blood Types in humans in Hardy
Weinberg Proportions
One of first genes that genetics could analyze in natural
population was MN blood group. Most have two alleles
(M and N) at genes
Because the gene codes for a protein on surface of
blood cells, research could determine genotype based
on using antibodies.
Heterozygotes are co-dominant have both M and N
versions on surface
Using allelic, genotypic frequencies and hardy weinberg
calculations and statistics. Were able to determine that
locus follows Hardy Weinberg assumptions.
4 evolutionary mechanisms did not affect M and N
alleles in population, mating was random with respect
to the gene
Case Study: Are HLA genes in Humans in Hardy Weinberg
Studying 2 genes crucial for the functioning of the
immune system
Genes that were analyzed code for proteins that
help immune system cells recognize and destroy
invading bacteria.
Different alleles exis as both the HLA-A and HLA-
B, each recognizes specific proteins from slightly
different disease causing organisms. (HLA alleles
are codominant)
Heterozygotes have higher fitness advantage.
Immune to more types
Using allelic, genotypic frequencies and hardy
weinberg calculations and statistics. Were able to
determine that locus violates Hardy Weinberg
assumptions (mutation, migration and drift are
negligible in area(2 competing explanations)
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