The following pedigree is for the X-linked-recessive trait for...

Question

The following pedigree is for the X-linked-recessive trait for color blindness. Using XN for the normal allele and Xn for the color blindness allele, fill in the top half of the boxes/circles with the genotype. Also, fill in the bottom half of the boxes/circles with the phenotype (Normal vision or color blind). If it is impossible to know for certain a specific allele in the genotype, then place a '?' to represent the allele that is in question.

Accepted Answer

Okay, everyone. We have another big pedigree question. The following pedigree is for the x linked recessive trait for color blindness. Using x capital m for the normal allele and x lowercase n for the color blindness allele, fill in the top half of the boxes or circles with the genotype. Also, fill in the bottom half of the boxes or circles with the phenotype, normal vision or color blind. If it is impossible to know for certain the specific allele in the genotype, then place a question mark to represent the allele that is in question. And they tell us that x with a capital n, And they tell us that x with a capital n is normal, and they say that x with a lowercase n is color blind, which I'm going to just write because the boxes aren't very big for me, so there's not a lot of room to write. Okay. So now I'm not gonna lie to you. These are going to be more challenging than the ones we've done before. Because they're x linked, there's gonna be different probabilities for males and females. And usually with these types of pedigrees, you can't just start at the top and work your way down. You're going to have to find an individual that you know their genotype and work backwards from. So where I'm going to start is not at the top. I'm going to start here. This is a female and she is color blind, which means that she not only has one of her x's x's with the color blind allele, but both of them. So her genotype has to be x little n x little n for her to be colorblind. Okay. So now let's work back from that. We can see that her brother right here is normal. So he has to have x, capital n, y. Okay? So he is normal. We do not know about their other brother. So let's just ignore him for right now. We don't know about him. Okay. So we know these 2 siblings. So that means that the color blind daughter had to get a color blind allele from her mother and her father, which means that her father right here has a color blind allele, which means he is color blind. Okay? C b for color blind. Okay. So now we also know that the mother's phenotype is normal vision. And she had she gave her normal allele to her son. So she has one normal allele, but because she had a color blind daughter, that means she must have donated her color blind allele, so she is a heterozygous individual. Okay. So now, let's go and let's go back even farther. So let's do the mother's family. Okay? So we're gonna do this mother's family. So she is normal, but she's a heterozygous individual. We're told that her brother is normal, so he must be x capital n y. And we're told that her sister is normal, and we were given her genotype. Whoops. Okay. So he is normal. Her sister's normal. Her father is normal. So he has x, capital n, y. So what is her mother? Okay. So first off, the one of the daughters here and the son have a normal phenotype. So remember the son gets his x chromosome from his mother, and it was normal. So she has to have at least one normal allele. But because she has a daughter who is a heterozygous individual for that recessive colorblind allele, that means that the mother must also be heterozygous. So that means that this daughter let me point to her. This daughter, right here, got the normal allele from the dad, and the recessive colorblind allele from the mom. Okay? So that is that family. So he is colorblind. His brother has normal vision. His sister has normal vision. Okay. His father has normal vision and his mother has normal vision, but he has a colorblind allele right there. Remember that males only get their color blind alleles from their mothers, so she must have at least one color blind allele. But because she has normal vision, she also has a dominant normal vision allele. So she is a heterozygote. These women are commonly called carriers because they carry the gene, but they don't have the phenotype. So she is a carrier. She has a colorblind son, a normal son. What about the daughter? Her vision is normal. Now, we're told that means that she had to get this normal allele from her dad, and we don't know if she got the normal allele from her mom, or the color blind allele from her mom. Either way, she would have normal vision. So we're gonna put a question mark here. And because she doesn't have any children, we can't verify for sure which allele she has. Okay? So there's a question mark there because we don't know what her other allele is. So we finished with that family. Now, let's move down the tree. Okay. So let's look at another family. We know that this mother here is color blind. Right? And we know that her husband is normal. So his genotype has to be this with the normal allele. Okay. So now we're also told that this daughter is normal, which means she has to have at least one dominant allele. And she got that one from her father. Because if you notice, the mother, right here, can only donate the recessive color blind allele, so the daughter must have a recessive allele, so that normal daughter must be heterozygous. Okay? Again, this whoops. Let me get my highlight. This normal daughter here, she has normal vision, so she got her dominant normal allele from her father. And because her mother can only give her the recessive alleles, she has one recessive allele, so she is a carrier. What about their son? What about this guy right here? Well, remember that males only get their x chromosome from their mothers. Alleles to give, so her son is colorblind. So we're gonna write cb right there. Okay? Oh, I also forgot up here, I'm sorry, I forgot this. Remember that this woman, because she's heterozygous, has to be normal. Let me write I'm just gonna write norm. She has to be normal because she's heterozygous. Okay? Okay. So we've finished almost all of the families. We have one family left. So let's get rid of these highlights. Okay. So now we don't know anything about these 2 parents here. But the thing that we do know is we know that their daughter is color blind, which means she only has one genotype, and that means she is homozygous recessive. So that means that the father gave his only X chromosome to her, which means his X chromosome must be recessive, which means he must be color blind. Okay? Alright. Now, the other thing we know is right here is this son is normal. What is his genotype? It has to be x his x allele is dominant. He has the normal allele, and he has a y. So what does this tell us about this mother? Well, that tells us because her son has a normal genotype, that she gave him that normal genotype. But because she has a daughter who is color blind, she had to give a recessive allele to her daughter. So she has normal vision, but she is is a heterozygous carrier. Okay, everyone. We have filled out all of the genotypes and all of the phenotypes for this family. And this family does, in fact, carry color blindness. And like I said, these sex linked ones are a little bit more complicated. Practice these, and I know you will be very good at them. Okay, everyone. Let's move on to our next video.

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