In order to be able to track the evolution of an allele in a population of mosquitoes, we first need to be able to calculate its allele frequency. Nothing easier than that! You simply divide the number of copies of one allele, let's say kdr_wt , by the total number of copies of all the kdr alleles in a population of mosquitoes. (which equals the sum of the number of copies of kdr_wt plus the number of copies of kdr_mut).
The allele frequency is characteristic for every allele (which doesn't mean that they can't change over time) and has a value between zero and one.Exercise: Calculate Allele Frequency.
- Q. What is the allele frequency of kdr_mut in the first generation of figure 1?
- A. 0.5
- Q. Given that there are only two alleles for the kdr gene and you know the frequency of one, can you calculate the allele frequency of the second allele?
- A. When there are only 2 alleles for a gene in a population, A and B , then the frequency of A is always equal to 1 minus the frequency of B and vice versa.
As mentioned before, allele frequencies do vary over time! To understand how allele frequencies can change from one generation to the next we must consider what happens during gamete formation. During the formation of sperm and eggs, cells with 2 copies of every gene undergo meiosis leaving the daughter cells with only a single copy of each gene. The process that determines which copy of each gene a daughter cell gets is completely random. It's as if nature flips a coin for each gene to decide which daughter cell it ends up in. As we shall see soon this random process has very important evolutionary consequences.
Exercise: Fill in the genotypes of the second generation of mosquitoes in figure 1.
You'll notice that the genotypes of the second generation of mosquitoes in figure 1 have been left blank. We're going to carry out a simulation to determine the genotypes of the second generation and discover what impact the random processes during meiosis have on allele frequencies from one generation to the next.
In order to do the simulation we need to flip a coin to mimic the random nature of meiosis. For each mosquito in the second generation flip 2 coins. The first coin determines which of the father's alleles is transmitted to the offspring (heads: the left allele; tails: the right allele) and the second coin determines which of the mother's alleles the mosquito will have.
- Q. Is the allele frequency of kdr_mut in the second generation different from that of the first generation?
- Q. Is the allele frequency you calculated different from that of you colleagues? Why do you think this is?
- Q. What are the maximum and minimum frequencies of kdr_mut in the second generation.