Okay so I got this biology assignment today. I got a pedigree and tomorrow I have to go in front of the class and analyze it. I need to know:
Which of these patterns of inheritance is shown for the disorder in the pedigree?
F. autosomal
G. sex-linked
H. codominance
J. incomplete dominance
I have to prove my answer and then I have to show why the other 3 answers couldnt be the answer. I really need help with this. Im not sure what to say and I really need this grade. Ive been trying for 3 hours now and havent gotten much done because I dont know what to do. Also, I have to have a visual aid. I have absolutely no idea what I can do for a visual aid. Thats the most important thing so please tell me some ideas of some good visual aids I can do please. Here is a picture of he exact pedigree I am analyzing. Please give me as much info on it as you can. Thanks a lot.
http://i358.photobucket.com/albums/oo21/... Let the disorder = d and normal = D.
The disorder is recessive since females can be carriers. So,
Female carrier is XDXd
Female affected is XdXd
Male affected is XdY
Male normal is XDY
Cannot be autosomal since all the female in every generation are always carriers and affected males always come from a carrier female. Assuming that the carrier females have no outward phenotype, this rules out codominance and incomplete dominance since in either of these cases the carrier female would have a different phenotype from the other unaffected females (and affected females).
This in turn rules out autosomal inheritance. In generation 2, we see that a son is affected. In order to receive an autosomal recessive trait, the son would have had to inherited a recessive allele from each parent. However, we see that the father is not a carrier nor is he affected so the son could have only received one recessive allele from his mother. Since this one allele caused the disease in the son (and not the daughter), a sex-linked condition appears to be the best explanation. The son only has one x chromosome so any recessive traits on that chromosome cannot be covered up by another dominant allele.
Hope that gets you started. One idea for one visual aid: how about drawing the pedigree and filling in genotypes as you go during the presentation? good luck Females are the circles and the males are the squares.
The colored in portion is the gene were are assessing here.
Look at Generation 1: The mom is a carrier, and her son is completely affected. So you think "hm, so the mom had it, the dad didn't and now the son is infected." You know females are XX and Males are XY, so the dad passed the Y to the son, the mom passed the X, she was a carrier, and now the son is infected. Ding! sex linked gene, on the X chromosome.
If you look, at the second generation, it has the same pattern. The left side family: The father is affected, the mother is a carrier, so they have one daughter who was affected (dad can only give an affected X, mom gives either) and one who wasn't, the son is infected because mom gave him the affected gene.
This pedigree is your visual aid, but you want want to clean it up just a little bit =) It's sex linked. Look generation 1. The female is a carrier (XX) while the male is affected (XY). That is your biggest clue.
It's not autosomal dominant because if that's the case, then all the heterozygous females would be affected as well. It's not autosomal recessive because the male in generation 1 is not a carrier.
It's not codominance because well... it's just not. Codominance inheritance would make it so that you'd see a mix of both phenotypes, so the carrier female would be kind of sick, but not as sick as a totally affected person. Both genotypes would be expressed equally because neither is dominant over the other. That doesn't make sense at all here.
Incomplete dominance in this case wouldn't make sense either because it'd also make the carrier female kind of sick, but not as sick as a totally affected person. Both genotypes would be expressed equally because the supposedly dominant allele is not "dominant enough" over the recessive. I think it's sex-linked recessive. The first generation has a mother who is a carrier and a father who is neither affected nor a carrier. They give birth to a female who is a carrier and an affected male. Going down further, you see that all of the males in the family are affected, and none of them are merely carriers. This suggests sex linkage, because in a male, if they inherit a recessive trait on the X chromosome, they will express it because they have no chance of having it "covered up" by the dominant chromosome. A female, on the other hand, has two X chromosomes, so if one has the recessive allele and the other one has the dominant allele, then the dominant allele will mask the recessive one and the female will not be affected. Thus, females can be carriers of sex-linked traits.
If it was autosomal recessive, then neither of the offspring would be affected because the father in generation 1 is not even a carrier of the trait. So, if we're calling the dominant allele 'R' and the recessive allele 'r', the mother would be Rr and the father would be RR. There is no way to get the genotype 'rr' (the 'affected' genotype) by mating these two individuals, so that rules out autosomal recessive. Meanwhile, you can't have carriers in autosomal dominant, because anyone with the dominant allele expresses the trait.
The other two inheritance patterns are more complex and are difficult to represent in a pedigree, so I don't think either of them is the correct answer. Maybe for a visual aid, you can show a different pedigree of sex-linked inheritance and contrasting it to one that shows autosomal inheritance. Or, maybe you can show a picture of a patient with a sex-linked recessive genetic disorder (e.g. Fragile X syndrome). |
