Below is a partial pedigree of hemophilia in the British Royal Fa...

Question

Below is a partial pedigree of hemophilia in the British Royal Family descended from Queen Victoria, who is believed to be the original 'carrier' in this pedigree. Analyze the pedigree and indicate which females are also certain to be carriers. What is the probability that Princess Irene is a carrier?

Partial pedigree chart showing hemophilia in the British Royal Family, tracing descent from Queen Victoria.

Accepted Answer

Hi everyone. Let's take a look at this practice problem. Together in the following pedigree, identify what percent of the offspring of the second generation will be carriers of the trait here we're given our pedigree. Now pedigrees can feel overwhelming at first just because there's a few steps in understanding them and then in trying to figure out the answer to your problem. But I'm going to lay out the steps and explain them and then we'll approach this problem together and work through it. So when looking at a pedigree there are four main steps especially for this type of problem. First you want to I. D. The affected individuals. Then you want to determine the type of inheritance. Using that you'll determine the gina types of your parents and whatever offspring you're interested in. And lastly you will identify the carriers. And so let's begin with step one. We want to identify all the affected individuals. Now recall that in pedigrees, males are represented by squares and females by circles and we can see that all affected males. All affected individuals are males. And so that's these three individuals right here. This tells us our trait must be X linked since only males are affected. So we've done step one already. We'll cross that off. Now we need to determine the type of inheritance. Well we already know that it is X linked but now we need to determine whether the trait is dominant or recessive. Now because this second generation does not have any individual that is affected. We know our trait must be recessive. If our trait was dominant offspring in the second generation would be affected. So again our trait inheritance is X linked recessive. So we've already done two steps. We're halfway there now. For the third step we want to determine the gina types. I'm gonna go ahead and use R. To represent our trait. Capital R. Will be for the dominant, treat little, R. Will be for the recessive trait. And that represents our effected traits. Since our trait is recessive. So since our inheritance is X linked recessive in the first generation, our parent male must carry the recessive gene since males only have one X. Chromosome. So his genotype will be X. Little R. Y. And the female must be homesickness for the dominant A lil. So her genotype will be X. Big R. X. Big R. And we know that because if she was dominant there would be offspring in the second generation that would be affected. So now we know our parent types and we can determine the offspring gina types. And we have six offspring that we're concerned with. We have three males and three females. So the males have blue dots under them and the females have green dots under them. I'm going to cross out the other individuals just so that we know not to worry about them. Okay let's take our males now are males would all inherit the white chromosome from their father. And the only type of X chromosome that they can inherit from the mother is X. Big are. So this is the genotype for our males. Now for females, the females in the second generation can only inherit the dominant. A little X. Big are from the mother and from the father they have to inherit the recessive. So X. Little R. So X. Big R. X. Little R. Is the genotype for all the females and the second generation. So now we've already done step three. Lastly we'll do step four, identify the carriers. So all the females in the second generation would be carriers. None of the Mills are carriers because none of them have that recessive allele. So there are three females In the second generation, over six offspring total Which gives us half which is 50%. So 50 of the offspring. And the second generation will be carriers of the trait. Alright, everyone I know this was a bit of a longer problem. Pedigrees can become really kind of second nature once you know how to approach them. So hopefully this video helps you and I'll see you for the next practice problem.

Key Concepts

X-Linked Inheritance

Pedigree Analysis

Probability in Genetics

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