Introduction to population genetics & Hardy-Weinberg AP BIO 2015

Goals for the day Be able to explain & apply the Hardy- Weinberg Equilibrium

Question of the day 4/08 What phenotypes/genotypes do you expect to be over-represented in a cross if genes are tightly linked?

Population Genetics

Hardy-Weinberg Equilibrium Both gene frequency and genotype frequency will remain unchanged (in equilibrium) unless outside forces change those frequencies.

Hardy-Weinberg Equilibrium For the H-W Equilibrium to be operational, the following restrictions(Assumptions) must be in place: Mating must be completely RANDOM. There can be NO mutation. There can be NO immigration or emigration. The population must be “large” (in fact, infinitely large). There can be NO selection operating on the population.

Hardy-Weinberg Equilibrium Let’s think: what are things that could break these assumptions? Come up with 1 thing or 1 scenario that would break each assumption and write each on a space below.

Breaking Assumptions: #1 Mating must be completely RANDOM.

Breaking Assumptions: #2 There can be NO mutation.

Breaking Assumptions: #3 There can be NO immigration or emigration.

Breaking Assumptions: #4 The population must be “large” (infinitely)

Breaking Assumptions: #5 There can be NO selection operating

Hardy-Weinberg Equilibrium What the H-W allows us to do is determine the gene frequencies and genotype frequencies within a population at the PRESENT TIME (NOW/ CURRENT) - It may not allow us to accurately predict future since many of these assumptions will not be true. If our predictions are incorrect though we can think through this list of assumptions and consider what is happening in the population.

Hardy-Weinberg Equilibrium Mathematically, we can represent this as follows: We will let “p” represent the frequency of the _____________ allele in a population Then “q” represent the frequency of the ___________ allele. Then the ALLELES in the entire population can be represented by the expression: _________________

Therefore: --the homozygous dominant individuals in a population can be represented by “______”; --the homozygous recessive individuals in a population can be represented by “______”; --the heterozygous individuals in a population can be represented by “______” Then the GENOTYPES in the entire population can be represented by the expression: ____________________

Hardy-Weinberg Equilibrium We will always write the percentages as decimals. Where 1 = 100% So if the dominant allele represents 70% of the population we will write it as p=

Example, assume that each allele ( _p_ & _q_ ) has a frequency of 0.5 (50%) in a population. If this is the case, then: p = _____ (frequency of the dominant allele) q = _____ (frequency of the recessive allele) p 2 = _____ (frequency of homozygous dominants) 2pq = _____ (frequency of heterozygotes) q 2 = _____ (frequency of homozygous recessives)

Get in the Habit of showing your work! Try to work out the other terms before looking at the answers on the next slide.

Example, assume that each allele ( __ & __ ) has a frequency of 0.5 (50%) in a population. If this is the case, then: p = _____ (dominant allele) q = _____ (recessive allele) p 2 = _____ (homoz dominants) 2pq = _____ (heterozygotes) q 2 = _____ (homoz recessives)

Example, assume that 30% of the population has the recessive phenotype. If this is the case, then: p = _____ (frequency of the dominant allele) q = _____ (frequency of the recessive allele) p 2 = _____ (frequency of homozygous dominants) 2pq = _____ (frequency of heterozygotes) q 2 = _____ (frequency of homozygous recessives)

Example, assume that 30% of the population has the recessive phenotype. If this is the case, then: Which piece of information is given to you?

Example, assume that 30% of the population has the recessive phenotype. If this is the case, then: Which piece of information is given to you? Recessive phenotype only caused by homozygous recessive genotype (q 2 ) is given

Example, assume that 30% of the population has the recessive phenotype. If this is the case, then: p = _____ (frequency of the dominant allele) q = _____ (frequency of the recessive allele) p 2 = _____ (frequency of homozygous dominants) 2pq = _____ (frequency of heterozygotes) q 2 = _____ (frequency of homozygous recessives)