Unit 8: Genetic Variation in Populations Flashcards
Define
Evolution
Changes in the genetics and/or features of a population over multiple generations
Two types of characters that can be measured in a population
- Discrete (single gene traits)
- Quantitative (polygenic traits)
What is the cause of all genetic variability?
Mutations
How can sexual reproduction affect genetic variability?
“Reshuffles alleles”
Enables new combinations of alleles that may lead to novel phenotypes
Causes:
1. Crossing over
2. Independent assortment of chromosomes
3. Random fertilization
How can bacteria exchange DNA?
- Conjugation
- Transformation
- Transduction
These are called types of lateral or horizontal gene transfer, since they share genes laterally (within a generation)
Define
Population
A group of organisms of the same species that shares a gene pool
Define
Gene pool
Combination of all copeis of every type of allele at every locus of all members of a population
Define
Fixed allele
A gene for which only one version (allele) exists
Variability = 0 for fixed genes
How can you calculate allele frequency?
Add all alleles (two per homozygote and one per heterozygote) in a population, then divide by the total number of alleles in the population (two per individual in diploid organisms)
For co-dominant or incomplete traits, you can distinguish homozygous dominant and heterozygous individuals. You can directly calculate allele frequencies in these traits.
For traits where homozygous dominant and heterozygous are indistinguishable, you would use Hardy-Weinberg equilibrium to estimate allele frequencies
Consider a population where:
n = 100 (population size)
p = 0.4
q = 0.6
p² = 0.16
2pq = 0.48
q² = 0.36
What does p mean?
40% of all of the alleles in a population are the dominant allele. So for 100 individuals, there are a total of 200 alleles; 40% (80/200) of those alleles are dominant, distributed amongst homozygous dominant and heterozygous individuals.
Consider a population where:
n = 100 (population size)
p = 0.4
q = 0.6
p² = 0.16
2pq = 0.48
q² = 0.36
What does q mean?
60% of all of the alleles in a population are the recessive allele. So for 100 individuals, there are a total of 200 alleles; 60% (120/200) of those alleles are dominant, distributed amongst homozygous recessive and heterozygous individuals.
Consider a population where:
n = 100 (population size)
p = 0.4
q = 0.6
p² = 0.16
2pq = 0.48
q² = 0.36
What does p² mean?
16% of the population (16 individuals) are homozygous dominant
For Hardy-Weinberg, this means the number of individuals that are expected to get two copies of the p allele, at random, are equal to the chance of getting p (0.40), twice (ergo 0.40 x 0.40).
Consider a population where:
n = 100 (population size)
p = 0.4
q = 0.6
p² = 0.16
2pq = 0.48
q² = 0.36
What does 2pq mean?
48% of the population (48 individuals) are heterozygous
For Hardy-Weinberg, this means the number of individuals that are expected to get p at random (0.4) then q at random (0.6) is 0.24 AND the number expected to get q at random (0.6) then p at random (0.4) is 0.24… so together that makes 48%
Consider a population where:
n = 100 (population size)
p = 0.4
q = 0.6
p² = 0.16
2pq = 0.48
q² = 0.36
What does q² mean?
36% of the population (36 individuals) are homozygous recessive
For Hardy-Weinberg, this means the number of individuals that are expected to get two copies of the q allele, at random, are equal to the chance of getting q (0.60), twice (ergo 0.60 x 0.60).
Ways of measuring variability in a population
Nucleotide variability
Gene variability
Describe
Nucleotide variability
SNP (pronounced “snip”) - Single nucleotide polymorphisms; any nucleotide location in a population’s genome that can vary
Populations with higher numbers of SNPs are generally more genetically diverse
*Remember that nucleotide variability doesn’t directly indicate unique phenotypes!!
Describe
Gene variability
Presence of different alleles
Often uses a measure of heterozygosity
What is
Heterozygosity
The number of genes in a population that have more than one possible allele
If a population has 1,000 genes and 50 of them have multiple versions, then the population’s heterozygosity is 50/1000, or 5%
Describe
Discrete Characters
Traits controlled by one (or very few) genes, that have distinct phenotypes
Often called “either/or” traits
Ex: presence/absence of Achondroplasia, Sickle cell, or other genetic conditions
Measured using allele frequencies
Describe
Quantitive Characters
Traits controlled by many genes and often have environmental influences
Traits exist on a continuum
Ex: eye color (which may be various shades of brown, green, blue…); height; etc
Usually measured / shown as a histogram
What mutations contribute to genetic variation?
The ones in gametes
How are “new genes” made?
Duplication of existing gene, which allows changes to accumulate in one
Ex: gene for hemoglobin in vertebrates originated as a duplication of a myoglobin gene; having two copies allowed for favorable mutations to accumulate in the hemoglobin gene
Neutral variation
Variability that is not selected upon by natural selection - it doesn’t hurt or help individuals with it
ex: attached / detached earlobes in humans
5 conditions for Hardy-Weinberg Equilibrium
- No mutations
- No natural selection
- No sexual selection
- No immigration / emigration
- Infinitely large population
How does violation of a Hardy-Weinberg’s condition lead to evolution?
- Mutations introduce new alleles
- Natural selection and
- Sexual selection… both lead to an increase allele frequencies of certain alleles
- Immigration introduces new alleles to a population while emigration removes some alleles from a population
- Allele frequencies of small populations will fluctuate due to random events
