Exercise 1: Calculating Gene Frequencies

Exercise 1: Calculating Gene Frequencies

Using two decks of cards, let black cards represent dominant alleles and red cards represent recessive alleles.

If you consider the two decks of cards as the gene pool for your population, then there are 104 “genes” in your population. 52 are black and 52 are red. Fill in the chart below to answer the questions.
What are the gene frequencies for this population?
If this population where in Hardy-Weinberg equilibrium, what genotype frequencies would you predict?

Deal out all of your cards in sets of two, face up. These will represent the individuals in your population (each has two genes).
How many individuals are there?
How many are there for each genotype?

Predicted Gene frequencies Genotype frequencies
N # of dom p # of rec q # of AA p2 # of Aa 2pq # of aa q2
Pop 1 52
Observed Gene frequencies Genotype frequencies
N # of dom p # of rec q # of AA p2 # of Aa 2pq # of aa q2
Pop 1 52
Is your population at Hardy-Weinberg equilibrium? Explain.

response:

Shuffle the cards and deal out two piles of 52 cards each.

Take one pile and call it population 2. Count the number of dominant (black cards) and recessive (red cards) alleles in population 1. Record how many of each you have and calculate your dominant allele frequency and your recessive allele frequency. Next calculate your expected frequencies for each genotype if population 1 was in Hardy-Weinberg equilibrium. Record your answers in the chart below. Then shuffle the cards in population 1 and deal them out in piles of two to give genotypes for the individuals in your population. Count the number of each genotype and calculate percentages by dividing that number from the total number of individuals. Record your results in the chart.
Pop. 2 Gene frequencies Genotype frequencies
N=26 # of dom p # of rec q # of AA p2 # of Aa 2pq # of aa q2
expected
observed

 

Is your population in Hardy-Weinberg equilibrium?

response:
Reshuffle all the cards back together (both decks). Again deal out two piles of 52 cards each and select one pile to use as population 3. So far in this exercise you have been calculating gene frequencies by counting genes and dividing by the total number of genes in the population. In natural populations we cannot count genes, so we must determine gene frequencies by looking at phenotypes. In this example we will start by looking at phenotypes. Choose the pile of cards you wish to use and deal out all 52 cards into stacks of two to represent your individual genotypes. Now remember black cards are dominant, and you only need one black card to show the dominant phenotype. Count the number of dominant and recessive phenotypes in your population and record the information in the chart. Based on these numbers, calculate expected values for p and q. (The key is to remember you cannot calculate p by knowing the dominant phenotype because you do not technically know how many are homozygous dominant and how many are heterozygous, but you can calculate q since only the homozygous recessive are represented by the recessive phenotype.) Once you have q, you can calculate the rest of the expected frequencies. Fill in your chart for population 3.

Pop. 3
Gene frequencies Genotype frequencies
N=26 # of dom p # of rec q # of AA p2 # of Aa 2pq # of aa q2
expected
observed
Problems.

If a population is in Hardy-Weinberg equilibrium with a gene frequency of 0.2 for the dominant allele, what would you expect the gene frequency to be for the recessive allele in the next generation?

response:
The presence of crooked baby fingers is controlled by a dominant allele C. People with straight fingers are homozygous for the allele c. In a population of 1000 individuals, 190 had crooked fingers and 810 had straight fingers. Assuming that the population does not deviate from the Hardy-Weinberg equilibrium, what are the allele, genotype, and phenotype frequencies in this population?

response:
Exercise 2: Gene Flow

Using two decks of cards, let black cards represent dominant alleles and red cards represent recessive alleles.

Create two populations of 20 individuals (40 cards each), population 4 with 35 black (dominant) alleles and 5 red (recessive) alleles and population 5 with 10 dominant and 30 recessive. Calculate the gene frequencies for each population and record them in the chart. Keep the piles separate and shuffle each pile separately. Simulate migration between your two populations by pulling 8 cards (4 individuals) from each population and moving them over to the other population. (Be sure to draw your cards from each population before you add the new cards in). Recalculate your allele frequencies and record them after you have done this. Reshuffle each population separately and repeat the process and record your allele frequencies. Do this for at least five times.

Migration Gene frequencies pop 4 Gene frequencies pop 5
# of dom p # of rec q # of dom p # of rec q
start
Migration 1
Migration 2
Migration 3
Migration 4
Migration 5
Explain what is occurring in the gene frequencies of your two populations.

response:

Exercise 3: Genetic Drift

Using two decks of cards, let black cards represent dominant alleles and red cards represent recessive alleles.

There are 104 cards (genes) in your population which represent 52 individuals in your population. 52 of the cards are black (dominant) and 52 are red (recessive). Therefore for this population the gene frequencies for the dominant alleles (p) and recessive alleles (q) are both 0.5. You will now simulate a population reduction that causes the loss of all but 10 individuals by drawing 20 cards at random. The rest of the population obviously died some bloody and horrible death and left these ten individuals to continue on. Calculate the gene frequencies of your new population and record them in the chart. Then shuffle the cards back into the deck and repeat. Record your results for catastrophic events occurring in 20 different populations. Each time allow only 5 (10 cards) to 15 (30 cards) individuals to survive. Record the number of survivors and the gene frequencies of the survivors.

Genetic drift Gene frequencies
N = # of dom p # of rec q
Original pop 52 26 0.5 26 0.5
After die-off 1
After die-off 2
After die-off 3
After die-off 4
After die-off 5
After die-off 6
After die-off 7
After die-off 8
After die-off 9
After die-off 10
After die-off 11
After die-off 12
After die-off 13
After die-off 14
After die-off 15
After die-off 16
After die-off 17
After die-off 18
After die-off 19
After die-off 20
How many times in the 20 catastrophes did the gene frequency change?

response:
Did the gene frequencies tend to change more or less when there were a smaller number of survivors?

response:

Simulate a small population of 8 individuals leaving the large population to begin a new one of their own by dealing genotypes (two card piles for 8 individuals). This time you know 1 individual is homozygous recessive, 3 are heterozygous, and 4 are homozygous dominant. Calculate the gene frequencies for the new population and give the number of each genotype in the chart below.

Now let’s assume that there are 4 women and 4 men, and they pair up as 4 couples for reproduction. The first couple is both homozygous dominant, and they have 6 offspring, so count 12 dominant (black cards) and put them in a pile. The next couple is a homozygous dominant and a heterozygote, and they too have six offspring, but only one is a heterozygote, and the rest by chance are homozygous dominant, so add 11 dominant (black cards) and 1 recessive red card to your offspring pile. The next couple is a homozygous dominant and a heterozygote, and they have 4 offspring, and true to the Punnet Square ratio, 2 are homozygous dominant and 2 are heterozygote, so add six dominant alleles and 2 recessive to your offspring pile. The last couple is homozygous recessive and heterozygous, and the female is prone to headaches, so they only have two offspring and both end up heterozygous, so add 2 black and 2 red to the offspring pile. Now calculate the gene frequencies for the offspring.
Genetic drift Gene frequencies
N = # of dom p # of rec q
Original pop 8
Offspring
The population grew from small to large, but what happened to the gene frequencies?

response:

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