Lecture 17 : Population Genetics Flashcards
What is Population Genetics?
Hows does it relate to Mendel?
Mendel’s rules tell us how the
GENES PRESENT in the PARENTS are REDISTRIBUTED among the OFFSPRING.
Population genetics DESCRIBES how the GENES PRESENT in a POPULATION of PARENTS are REDISTRIBUTED AMONG A POPULATION of OFFSPRING.
Define Population: characteristics (4)
- A subdivision of a species
- COMMUNITY of individuals where MATES are usually FOUND.
- Share a COMMON SET of GENES: the GENE POOL
- Populations EVOLVE through CHANGES in their GENE POOL.
Importance of Genetic Variation? (2)
- Basis of all evolution.
- AFFECTS how the Population RESPOND to ENVIRONMENTAL CHANGES (NATURAL SELECTION) or SELECTIVE BREEDING (ARTIFICIAL SELECTION.)
How much genetic variation exists in natural populations? or how does its how?
How is it caused?
- Most populations show extensive PHENOTYPIC VARIATION.
- Phenotypic Variation = Genetic + Environmental Variation.
An example of Evidence of Genetic Variation BETWEEN Populations?
- Common Garden Experiments:
- shows that phenotypic differences among populations persist even when grown in the same environmental conditions.
eg. Achillea lanulosa growth in different regions of Aus.
eg. Body size of melanogaster flies along the east coast of Aus.
Examples of Evidence of Genetic Variation WITHIN populations? Explain them?
- INBREEDING: recessive alleles that were hidden become expressed.
Example: rare genetic disorders in humans: achromatopsia, Cystinosis, Albinism. - ARTIFICIAL SELECTION; the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations
- MOLECULAR STUDIES: A laboratory method that uses a sample of tissue, blood, or other body fluid to check for certain genes, proteins, or other molecules that may be a sign of a disease or condition, such as cancer.
Example:
+ Blood cell antigens (A, B, O blood groups in humans)
+ Electrophoresis (a process where proteins or DNA are run through a GEL using an electric current.
PV92 locus
protein electrophoresis
Humans approx 25, 000 protein-coding genes. Proportion of loci is 7% that suggests 1750 genes heterozygous in each indidvual.
The DNA region, or locus, that you’ll copy is called PV92; it’s part of an intron on chromosome 16. Everybody has the PV92 locus, but there are two common alleles: with or without an Alu transposon inserted
How do we describe the Genetic Structure of populations?
(2)
- Genotypic frequencies
- describes the distribution of genotypes in a population - Allelic or gene frequencies
- describes a population in terms of allele frequencies.
How to Calculate genotypic frequencies?
To calculate
1. Add up the number of individuals possessing a genotype
- then divide by the total number of individuals in the population
eg. For a locus with 3 genotypes AA, Aa and aa, the frequency (f) of each genotype is:
- f(AA) = number of AA individuals/ N
- f(Aa) = number of Aa individuals/ N
- f(aa) = number of aa individuals/ N
The SUM OF ALL THE GENOTYPIC FREQUENICES ALLWAYS EQUALS 1.
Allelic or Gene Frequencies? Which one and why? Explain how?
- Because there are FEWER ALLELES THAN GENOTYPES
GENETIC VARIATION can be DESCRIBED IN FWERWE TERMS USISLING ALLELE FREQUENCIES.
- Calculated from
(1) the numbers
or (2) the frequencies of the genotypes.
How to calculate allele frequencies?
to calculate FROM THE NUMBER OF GENOTYPES:
1. COUNT the number of COPIES of an ALLELE
2. DIVIDE by the TOTAL number of ALLELES in the POPULATION.
EG.
for a locus with 2 Alleles (A and a)
p = f(A) - (2 n(AA) + n(Aa) / 2N
q = f(a) - (2 n(aa) + n(Aa) / 2N
Where n(AA), n(Aa) and n(aa) represent the number of AA, Aa and aa individuals respectively and N represents the total number of individuals.
How to Calculate Allele frequencies? (3)
- Divide by 2N because each DIPLOID INDIVIDUAL has 2 alleles at a locus
- In HAPLOID organisms genotype frequencies = allele frequencies
- The SUM of Allele frequencies always equals to 1.
q = 1 - p
How to calculate allele frequencies from genotypic frequencies?
to calculate from GENOTYPIC FREQUENCIES,
1. ADD the FREQUENCY of the HOMOZYGOTE for EACH ALLELE and Half the frequency of the heterozygote
eg. for a locus with 2 alleles (A and a)
p = f (A) = f(AA) + 1/2f(Aa)
q = f (a) = f(aa) + 1/2f(Aa)
Calculating allele frequencies for loci with multiple alleles
The sample principles apply - look at examples
Calculating allele frequencies (X -linked loci)
The same principles apply but REMEMBER that a FEMALE POSSES 2 X CHROMOSOMES and therefore has 2 X-LINKED GENES,
whereas MALES have ONLY ONE X CHROMOSOME and have one X-linked allele.
to calculate the number of genotypes
p = f (XA) = (2 nXAXA + nXAXa+ nXAY) / (2 nFEMALES + nMALES)
q = f (Xa) = (2 nXaXa + nXAXa + nXaY) / (2 nFEMALES + nMALES)
to calculate using genotype frequencies
p = f(XA) = F(XA XA ) + 1/2 f(XA Xa) + f (XAY)
q = f(Xa) = F(Xa Xa ) + 1/2 f(XA Xa) + f (XaY)
look ad worked examples
