Topic 7—B: Populations and Evolution- 1. The Hardy-Weinberg principle Flashcards
Definition of species
A group of similar organisms that can reproduce to give fertile offspring
Definition of population
A group of organisms of the same species living in a particular area at a particular time
- so they have the potential to interbreed
- species can exist as one ore more populations
Definition of gene pool
The complete range of alleles present in a population
Definition of allele frequency
How often an allele occurs in a population
- usually given as a percentage of the total population
What is the hardy Weinberg principle?
- its a mathematical model that predicts the frequencies of alleles in a population wont change from one generation to the next
What does the hardy Weinberg equilibrium state?
Frequency of dominant and recessive alleles in a population will remain constant from generation to generation provided certain conditions exists
What conditions need to exist for this prediction to be true?
- no mutations are occurring
- large population
- no natural selection
- no migration is taking place
- mating is random
What can the hardy weinbergs equations be used for?
- estimate the frequency of particular alleles, genotype and phenotypes within populations
- can be used to test whether or not the hardy Weinberg principle applies to particular alleles in particular populations i.e. to test whether selection or any factors are influencing allele frequencies
Hardy Weinberg equations
- there are 2 equations
- one is used for working out allele frequency
- other one is usually used when you’re dealing with genotype (and phenotype) frequencies
- both were designed to be used in situations where a gene has 2 alleles
Allele frequency
- total frequency of all possible alleles for a characteristic in a certain population is 1.0 (100%)
- so the frequencies of the individual alleles (dominant and recessive) mist add up to 1
Equation: p+q=1
P= the frequency of one allele (usually the dominant one)
Q= the frequency of the other allele (usually the recessive ones)
Genotype frequency
- the total frequency of all possible genotypes for one characteristic in a certain population is 1.0
- so the frequencies of the individual genotypes must add up to 1
- but there are 3 genotypes: homozygous recessive, homozygous dominant, heterozygous
Equation:
P^+2pq+q^=1
P^2=frequency of homozygous dominant
2pq= frequency of heterozygous genotype
Q^2= frequency of homozygous recessive
- ## these genotype frequencies can be used to work out phenotype frequencies if you know how genotypes relate to phenotypes
Predicting allele frequency (example)
A species of plant have either red or white flowers
Allele R (red) is dominant and allele r (white) is recessive
If the frequency of R is 0.4 in population W what is the frequency of r?
P+q=1
R+r=1
0.4+r=1
R=1-0.4=0.6
Frequency of the r allele in population W= 0.6
Allele frequencies if you’re given information about genotype (or phenotype) frequencies
There are 2 alleles for flower colour (R and r)
Three possible genotypes- RR, Rr, rr
If the frequency of genotype RR is 0.56 in population x what is the allele frequency of r?
RR= P^2
R=p
P^2=0.56
P= square root of 0.56= 0.75
R= 0.75
P+q=1
R+r=1
0.75+r=1
r=1-0.75=0.25
Predicting genotype frequency
- there are 2 alleles for flower colour ( R and r)
Three possible genotypes- RR, Rr,rr
In population Y the frequency of genotype RR=0.34
Frequency of genotype Rr=0.27
Find the frequency of rr in population Y
P^2+2pq+q^2=1
P^2= homozygous dominant genotype RR
2pq= heterozygous genotype Rr
Q^2=homozygous recessive rr
P^2+2pq+q^2=1
RR+Rr+rr=1
0.34+0.27+rr=1
rr= 1- 0.34- 0.27= 0.39
rr=0.39
Predicting phenotype frequency
If R is dominant and r is recessive then a plant with a red flower phenotype could have the genotype RR or the genotype Rr
Plants with the genotype rr will have a white flower phenotype
In population z the frequency of the genotype Rr is 0.23 and the frequency of the genotype rr is 0.42
Find the frequency of the red flower phenotype in population z (RR and Rr)
P^2+2pq+q^2
P^2+0.23+0.42=1
1-0.42+0.23
1-0.65=0.35 (RR)
0.35+0.23=0.58
Predicting the percentage of a population that has a certain genotype
The frequency of cystic fibrosis (genotype ff) in the UK is currently approximately 1 birth in 2500
Use this information to estimate the percentage of people in the UL that are cystic fibrosis carriers (Ff)
To do this you need to find the frequency of the heterozygous genotype Ff i.e. 2pq using both equations
- calculating q
Q^2= 1/2500=0.0004 q= square root of 0.00004=0.02
Calculating p
P+q=1
P+0.2=1
1-0.2=0.98
Calculating 2pq
2xpxq
2x0.98x0.02=0.039
0.039x100=3.9%
When does the hardy Weinberg principle not apply?
- as the frequency of the allele has changed between generations
