You are a genetic counselor, and several members of the family...

Question

You are a genetic counselor, and several members of the family whose pedigree for an inherited disorder is depicted in Genetic Analysis 17.2 consult with you about the probability that their progeny may be afflicted. What advice would you give individuals III-1, III-2, III-4, III-6, III-8, and III-9?

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next question. It says, identify the mode of inheritance of an unknown trait in the following pedigree. And I'm given here a pedigree showing three generations of a family with a disorder that seems to have affected individuals in each generation. And when I look over a pedigree to determine mode of inheritance, I always like to start with two questions. And the first one being, are there skipped generations and the second one being, do I see a sex difference in the number of individuals affected? And that can help me tease out whether I'm dealing with a dominant trait or a recessive trait, whether dealing with an autism away trait or a sex linked trait. So in this case, with the first question, I do not have any skip generations. Every affected individual has at least one affected parent. So that would usually usually lead me to say that this must be a dominant trait or expect it to be a dominant trait since the answer to our there skip generations is no. However, when I take a look down at my answer, choices that I'm given this multiple choice question, I have a autism, a recessive inheritance, be excellent recessive inheritance. See, maternal inheritance or D Y linked inheritance, autism all dominant is not a choice. So I'm gonna have to look a little deeper and assume the story is a little more complicated than, and autism will dominant trait. So I'm just going to cross out where I jotted dominant trait down after my question, knowing that's not an option here. So I will cross out choices, autism alone, recessive inheritance. And I'm gonna go ahead and rule that out. Um As I would expect to see some skip generations if that were the case. So I'll rule that one out. And now we have left X linked recessive inheritance, maternal inheritance and Y linked inheritance. Well, I can also go ahead and rule out choice dy linked inheritance. Why linked inheritance, something carried on the Y chromosome? I should see zero affected daughters and every affected son should have an affected father since the father will always pass the Y chromosome onto the son. And when I look at my pedigree here, I see four affected females in the pedigree. So already I know that's not Y linked as well as the fact that at least two affected males or three affected males, I should say have an affected mother. So for those, both those reasons, choice D is going to be ruled out. It is not Y linked inheritance. So we're left with excellent recessive inheritance. And maternal inheritance as our modes of inheritance to look at. Well, in the case of excellent recessive inheritance, I really would not expect to see many affected females. Excellent recessive traits are found very rarely in females since they have to inherit two copies of the treat. Um one from each parent, whereas affected males only need to inherit one copy. So the fact that I have four affected females in this pedigree um would lead me to eliminate choice. B so I've got left choice. See, maternal inheritance. And let's just look and see whether that makes sense. Maternal inheritance, inheritance would mean that an affected person can only have inherited the gene from their mother that although males and females can inherit it, males cannot pass it on, only females can. And as we look at our pedigree, we see in generation one, um the affected parent is the mother and we see all three of the offspring and generation two are affected. Um That's also going to strike us as unusual. When we look at this pedigree, we see both in generation to all three Children affected. We look in generation three um of the three different families listed there, one of the families has all three Children affected. So that should also keep us in to the fact that this is an unusual pattern of inheritance, seeing it inherited by every member of a family, all of the offspring. But let's look back again with maternal inheritance in mind. Uh Generation two has an infected mother and all three offspring affected. Now, the three Children in generation 22 are males, those male Children. When we look in generation three, none of their Children are infected. So we don't see the two males in generation to passing the trade on The one female in Generation two who is affected, we look at her offspring and all of them are affected. Should we see that every affected person has an affected mother and that males affected males are not passing the trade to any of their offspring. So that indicates that this is correct choice. See maternal inheritance does describe the inheritance pattern. We see here. See you in the next video.

Key Concepts

Pedigree Analysis

Inheritance Patterns

Genetic Counseling

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