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Biology 102: Basic Genetics9 chapters | 121 lessons | 8 flashcard sets

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Lesson Transcript

Instructor:
*Katy Metzler*

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

So, you know what inbreeding is and what effects it can have on a population. In this lesson, we'll walk you through the next step: how to calculate just how inbred an individual is.

By now you've learned that inbreeding is mating between genetically related individuals and that it causes an increased proportion of homozygous individuals in a population. Now that we understand what inbreeding is and what effects it can have, we need to learn how to calculate just how inbred a particular individual is. We do this by looking at the individual's pedigree, which is basically their family tree, and by using a special equation.

Before we look at the equation, let's look at an inbred pedigree. Remember that in pedigrees the females are represented by circles and the males are represented by squares. A line connecting a male and a female symbolizes that they've gotten married, and any people that branch off of that line are their kids.

Okay, at first glance, you might notice that there is something unusual about this family tree. It has a closed loop. Uh oh, that's not supposed to happen. A closed loop in the family tree means that some close relatives have had kids together. Sounds like inbreeding!

Individual A in this pedigree is inbred because she is the child of two parents, B and C, that have relatives in common. Which relatives do they have in common? D and E, who are A's great grandparents. All right. We get that A is inbred, but since we're population geneticists, we really like to use equations, and we want to find out just how inbred A actually is.

To do that, we will use an equation to calculate the **inbreeding coefficient ( F)**. The inbreeding coefficient tells us the probability that an individual has two identical alleles for a given gene. Let's back up a little and remember that

Since we have two copies of each of our chromosomes (except Y in males), we can have two different alleles for each of our genes. There are lots of different alleles in a population, and people often inherit different ones from each parent. This inbred individual, however, would have two exact same alleles for some of her genes because they ultimately came from the same relative in her family tree.

Here's the equation for the inbreeding coefficient: ** F = the sum of [(1/2)^n * (1 + F sub A)] for all inbreeding paths**.

Okay, let's calculate the inbreeding coefficient for individual A. First, we need to draw **inbreeding paths**, which are basically lines connecting A's parents and the relatives they have in common. We said that the couple D and E are the relatives that A's parents, B and C, have in common.

So, first we draw a path starting from A's mother B, then to B's mother, and then to D, who's A's great grandfather. We're not done yet, though: we need to keep drawing the path until it comes back to A's father. Back down through A's grandfather on her father's side, and finally to C, who is A's father.

Now, take a look at that path. How many people are in it, not counting A? 1, 2, 3, 4, 5 people in the path. That is our first *n*! Awesome!

But is D the only relative that A's parents have in common? Nope. E is also a relative they share. So, we need to draw a path from A's mom all the way to E, and then back to A's dad. As you can see, there are 5 people in this path, too. So, our second *n* is also 5.

We're almost ready to do the equation, but there's another variable we didn't figure out yet, *F* sub *A*. That's the inbreeding coefficient for the shared relatives themselves. For our purposes, we don't know whether or not D and E are inbred themselves. So, let's just assume they aren't, which would mean their *F* sub *A* is equal to 0.

Okay, equation time! Remember that *F* equals the sum of this equation for all of the inbreeding paths. We had two paths, so we will do the equation twice and add them up.

(1/2)^*n* in this case is (1/2)^5 because there were 5 people in the first inbreeding path. Multiply that by (1 + 0) because we are assuming that D and E are not inbred. That works out to (1/2)^5, which is 0.03125.

Now, we have to do the equation for the second path. Again, it's (1/2)^5 * (1 + 0). And again, we get 0.03125.

Don't forget the last step: add the results for the two paths together. 0.03125 + 0.03125 = 0.0625. That's the inbreeding coefficient for individual A. We did it!

But wait, what does it actually mean? What this number means is that if you look at one of A's genes, there's a 6.25% chance that she has two of the exact same alleles for that gene, meaning that she's homozygous. For any one of her genes you look at, there's the same 6.25% chance of homozygosity!

If you estimate that humans have around 20,000 genes, this means that she is most likely homozygous for many, many genes. Hopefully, not any recessive ones that cause serious disorders though! Keep in mind that she could be homozygous dominant for those genes, too, and being homozygous dominant is less likely to cause a disease.

In today's lesson, we learned how to calculate the **inbreeding coefficient ( F)**, which is the probability that an individual has two identical alleles for a given gene, by looking at a pedigree and tracing

Once you have completed this lesson you should be able to:

- Use a pedigree to identify the inbreeding paths of an individual's ancestry
- Calculate the inbreeding coefficient for an individual

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Biology 102: Basic Genetics9 chapters | 121 lessons | 8 flashcard sets

- Theories of Evolution: Lamarck vs. Darwin 7:30
- Hardy-Weinberg Equilibrium I: Overview 4:44
- Hardy-Weinberg Equilibrium II: The Equation 11:42
- Hardy-Weinberg Equilibrium III: Evolutionary Agents 12:51
- Inbreeding: Definition and Effects 5:28
- How to Calculate the Coefficient of Inbreeding 6:35
- Natural Selection: Definition, Types & Examples 8:14
- Genetic Fitness: Selection 4:19
- Speciation: Allopatric and Sympatric Speciation 7:49
- Prezygotic Reproductive Barriers & Speciation: Definition & Examples 8:52
- Postzygotic Reproductive Barriers: Definition & Examples 7:02
- Go to Population Genetics and Evolution

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