Genetics and Allele Frequencies - Online Tutor, Practice Problems & Exam Prep

The modern synthesis combines Darwinian evolution and Mendelian genetics, focusing on how alleles are inherited in populations. Key concepts include phenotype, genotype, alleles, and the gene pool, which represents all alleles in a population. Allele frequency, calculated as the proportion of a specific allele to the total alleles, is crucial for understanding evolution. For two alleles, p and q, the relationship p + q = 1 holds. Changes in allele frequency indicate evolutionary processes, emphasizing the importance of genetic variation in populations.

Mendelian and Population Genetics

We said that the modern synthesis was the combination of Darwinian evolution and Mendelian genetics. So, we need to remember our Mendelian genetics. Alright. Now, this is something you should be familiar with from learning previously in the course, but we're going to review some key concepts here. Let's just remember that Mendelian genetics tracks how alleles are inherited.

We're going to say here in single matings, and importantly, that's in diploid organisms or organisms with 2 sets of chromosomes. Right? You make your Punnett square, and you can predict what the offspring may look like. Now, with that, we have some vocabulary that goes along with it.

We have a phenotype. A phenotype is an organism's traits, and that importantly could come from genetics or from the environment, but we're specifically concerned about those traits that are caused by genetics. In our example here, we have brown and white rabbits. So being brown or white, that's the phenotype we're interested in. Now we have a gene.

A gene is just a section of DNA that codes for a trait. You can also define it as a section of DNA that codes for a protein, but in classical Mendelian genetics, we say it's a section of DNA that codes for a trait, and that's typically what we'll be using going forward. In our example, we have some chromosomes on these rabbits here, and you can see that we are interested in this a gene. This a gene is what's causing the different phenotypes. Now, we have an allele.

An allele has those different versions of a specific gene. Again, in our example, we have the a gene, but we have the big A and the little a alleles. The genotype is going to be an organism's specific set of alleles. Right? This brown rabbit here is, little a, little a.

That's its genotype. Now, that is a homozygous genotype because it has two copies of the same allele. Now, this white rabbit, right, because of simple dominance rules, it could be either, big A big A, that would be another homozygous genotype, or it could be big A little a, and we call that the heterozygous genotype. And that means having 2 different alleles. Alright.

So remember, Mendelian genetics is thinking about single matings. But going forward, we're still going to be using these same terms as we move to talk about population genetics. Population genetics instead tracks how alleles are inherited. We're going to say here in the whole population. Alright?

We're not concerned about single matings and what is going to come out. We're wondering how these alleles are passed on in all the organisms. Now, to think about this, we're going to introduce this idea of the gene pool. The gene pool is just all the alleles in the population. We're going to say here, regardless of the organisms they are in.

So, we have our white and brown rabbits here. What we want to think of those alleles, we're taking them out of those rabbits and, you know, it says here in our cartoon, all the alleles in the pool. Right? They're all jumping in the same pool. We're sort of just not thinking about what organisms they're in right now.

We're just thinking totally what the alleles in the population are. Now, from that, we can get our allele frequency. And that's just the proportion of the gene pool made up of one allele. So, if when I take all those alleles out of the organisms and I sort of put them in one bag, well, if 60% of them are big A and 40% of them are little a, those would be our allele frequencies.

Now, remember our change in allele frequency, that was our measure of evolution. So going forward, we want to think of this gene pool. We want to think about all the alleles in the population at once and focus less exactly, a lot of times, on exactly which organisms they are in. Alright. It takes this population viewpoint to do that, and that really is one of the things from the modern synthesis that was brought into modern biology.

We'll practice it going forward. I'll see you there.

Genetics and Allele Frequencies Example 1

This example tells us that the number of alleles in 2 different gene pools are shown below, and we need to answer the following. So we have to answer these questions down here, but let's just look at our gene pools first. We have gene pool 1 with 400 big A alleles and 600 little a alleles, Gene pool 2 with 5,000,000 of each big A and little a alleles. Alright. So let's just remember this concept of a gene pool.

Usually, you're thinking about the alleles in an organism, but now we're trying to imagine taking all the alleles out of organisms, not worrying about what the individual organisms are like. We're just going to take all the different alleles in the population and put them in one big bag or one pool together. Alright. So knowing that, let's answer the questions.

Which population has a higher frequency of the little a allele? Well, we can see very clearly gene pool 2 has more little a alleles, but we're worried about the frequency here, how common it is. So just some real simple math. In gene pool 1, we have 600 little a alleles, and well 600 plus 400, we have 1,000 total alleles. Right?

So the allele frequency in gene pool 1 is going to be 0.6 little a alleles. In gene pool 2, well, you can see right away it's even. So it's 5,000,000 of those little a alleles divided by 10,000,000 total alleles, which is going to give you 0.5. Alright. So 0.6 or 60 percent of the alleles in gene pool 1 are the little a allele, while 0.5 or 50 percent of the alleles in gene pool 2 are little a alleles, which means that gene pool 1, it has the higher allele frequency.

Remember changes in allele frequency, that's gonna be our measure of evolution going forward. Alright. Now it says, based on these allele frequencies, can you tell which population has a higher frequency of heterozygotes? Why or why not? What do you think?

Well, we're gonna learn how to make that prediction and to predict which one you would think would have more heterozygotes, but right now we're just looking at alleles from a population. We don't know how those alleles are actually distributed in individuals. So here I'm gonna say no. We do not know how alleles are distributed among genotypes. Alright, and that's gonna be a lot of what we're going to do going forward.

Learning how we make those predictions, make predictions using genotypes and allele frequencies to understand how alleles sort of move through populations. Alright. More to come. I'll see you there.

When all the alleles in a population are considered together, independently of the individuals they come from, we call that the ___________.

• A. Genotype.
• B. Gene pool.
• C. Phenotype.
• D. Mendelian genetics.

Which of the following terms is correctly matched to its definition?

• A. Allele: an organism’s traits.
• B. Phenotype: an organism’s specific set of alleles.
• C. Heterozygote: having two copies of the same allele.
• D. Genotype: an organism’s specific set of alleles.

Allele Frequencies

We defined evolution as a change in allele frequency. So now we've got to learn to calculate allele frequency. I understand some people get a little intimidated when doing math in biology class, but don't worry. This is straightforward, and we'll take you through it step by step. We're going to start out just by saying that to calculate allele frequency, we're just going to divide the number of a specific allele.

Just the one that you're interested in. You're going to divide how many of those there are by the number of total alleles. Right? That's how you get a frequency: divide what you're interested in by the total. We're always going to be using a standard system of 1 gene and 2 alleles.

That means we're always going to have 2 allele frequencies, a frequency for each allele, and we're going to identify those using the variables \( p \) and \( q \). \( p \) is just going to be the frequency of the first allele, and in our examples, we'll always indicate that using a big A, and \( q \) is going to be the frequency of the second allele. In our examples, we'll always indicate that using a little a. But just know, as you do other problems, you're going to see other genes with other symbols for the alleles. There will still be 2 alleles.

The frequency of one will be \( p \), the frequency of the other will be \( q \). I just want also to note, I called it first and second. I didn't call it the dominant and recessive allele. That's just because not all genes have a dominant and recessive relationship. If they do, you can use those terms.

Alright. Now because there's only 2 alleles, when we add \( p \) and \( q \) together, it's always going to equal 1. Right? Because together, that's 100% of the alleles in the gene pool. Alright.

So now let's figure out how we actually calculate our allele frequency. We're going to divide the number of the allele we're interested in by the total, but it's a little more complicated than that because we don't just have a gene pool. Remember, all these alleles are actually in individuals. So, counting the alleles is a little more complicated. Alright.

So we're going to use this sample population where we have 100, big A, big A homozygotes, 250 heterozygotes, and 150 little a, little a homozygotes. Now, the first thing we want to do, as stated here in our steps, is count the total big A and little a alleles. To count the number of big A alleles, we're going to do 2 times the number of AA homozygotes in the population, and we need to multiply that by 2 because each homozygote has 2 of these big A alleles. And to that, we will add the number of heterozygotes because each heterozygote only has 1 big A allele. So we'll have \( 2 \times 100 + 250 \).

So here we have 2 times 100 which is 200, plus 250. That's 450. So there are 450 total big A alleles in this population. We can do basically the same thing with little a alleles.

We're going to take the number of heterozygotes because there's only one little a allele in the heterozygote. We don't need to multiply that by anything. And then we're going to do 2 times the number of little a, little a homozygotes, because each one of these again has 2 little a alleles. So we'll write 250 and then look at the homozygotes. We're going to do 2 times 150. So 2 times 150 is 300 plus 250. That is 550.

So 550 and 450, I can see right away if I add those together, I'm going to get 1,000 total alleles. But, if you're like me, I make a lot of math mistakes. So when I can, I try to do things 2 ways to check myself. We're going to have this other way to calculate the total number of alleles in the population. We're just going to do 2 times the number of individuals in the population, because each individual has 2 alleles.

So total alleles are going to be 2 times, and then we're just going to add together our different numbers for each genotype here. So we have 2 times \(100 + 250 + 150\). Alright. 100 plus 250, that's 350. Plus 150, that's 500. So 2 times 500 gives me 1,000 total alleles. The math worked out, checks out. We're good.

Okay. So now to calculate allele frequency, we're just going to take a specific allele and divide it by the total alleles. So for \( p \), that's going to be the number of big A alleles. So we had 450 of those. And we're going to divide that by the total, which is 1,000. So \( \frac{450}{1,000} = 0.45 \). For \( q \), we're going to do the same for the little a alleles. So little a alleles numbered 550, divided by the total alleles, which is 1,000. So \( \frac{550}{1,000} = 0.55 \). We got our allele frequencies. \( p = 0.45 \), \( q = 0.55 \).

In other words, 45% of the alleles in this population are the big A allele. 55% of the alleles in this population are the little a allele. Alright. Before we go on, we just want to note that if there is only 1 allele in the population, in that case, \( p \) or \( q \) is just going to equal 1. We'll give a little vocab word here. That means that that allele is fixed, or sometimes we'll say it's reached fixation. Now when we say fixed here, we don't mean like it was broken and now it's fixed. We mean it's sort of fixed in place because evolution requires multiple alleles or requires genetic variation. So if \( p \) or \( q \) equals 1, that means there's only 1 allele in the population, and that frequency can't change. It's fixed in place.

Alright. We'll practice all of this. Come on. I'll see you there.

The number of individuals in a population with each of the following genotypes is listed below. What is the frequency of the small t allele?

TT: 640

Tt: 200

tt: 160

• A. 0.32
• B. 0.16
• C. 0.26
• D. 0.52

Gia and Kojo collect a sample of beetles to calculate the frequency for the W allele. After genotyping the individuals in their sample, they count the following genotypes:

WW: 50

Ww: 170

ww: 120

Gia calculates the frequency by taking: $W=\frac{50+50+170}{680}$

Kojo calculates the frequency by taking: $W=\frac{2\left(50\right)+170}{50+170+120}$

Who calculated the frequency incorrectly, and what was their mistake?

• A. Kojo because he divided by the number of individuals instead of total alleles.
• B. Kojo because he multiplied the number of homozygotes by 2 but not the number of heterozygotes.
• C. Gia because she added instead of multiplying the number of W alleles found in the homozygote.
• D. Gia because she did not multiply the total number of individuals by 2.