How do X-linked recessive alleles work?

Question

There is one kind of muscular dystrophy, caused by an X linked recessive allele, that causes loss of muscle function and ultimately death by the age of about twenty. The disorder usually occurs in males. Can you explain why?

Answer 1

Females are XX

Males are XY



it's recessive so for a female to have muscular dystrophy, it must be on both of her X chromosomes.. That's why for X-linked recessive disorders, females tend to be carriers more often then affected by the actualy disease.

For males it's impossible for them to be carriers so if they recieve an affected X chromosome, they're screwed. They have it no matter what.

Answer 2

Sorry about this, this is gonna be a long answer, and I'm not sure if I can explain it easily without drawing a punnets square diagram. Right, If we first of all name the different alleles so you know what I'm talking about: XC = X chromosome, normal wings Xc = X chromosome, cut wings Y = Y chromosome (Because males have one X and one Y) F = Not Fuzzy f = Fuzzy Ok, first of all: Genotype of the male = Xc,Y,F,F it has cut wings, and a normal body, and is male. Genotype of the female = XC,XC,f,f it has normal wings and a fuzzy body Now, with these genotypes, the male can produce Xc,F and Y,F gametes in equal proportions, and the female produces all XC,f gametes. When you put these into a 4x4 punnets square you get an F1 generation all with the phenotype of normal wings and a normal body (the wild type), however they will have within their genotype the recessive genes that their parents carried, they will have the genotypes Xc,XC,F,f if they're female, and the genotype XC,Y,F,f if it's male, with equal proportions of male and female. When you cross the F1, your gonna have to cross male and female (obviously) so there will be male gametes XC,F XC,f Y,F and Y,f, and female gametes Xc,F Xc,f XC,F and XC,f. These can then also be put into a 4x4 punnets square and give offspring with the genotypes Xc XC F F, Xc XC F f, XC XC F F, Xc XC f f, XC XC F f, XC XC f f, Xc Y F F, Xc Y F f, XC Y F f, XC Y f f, Xc Y f f and XC Y F F in the ratios 1:3:1:1:1:1:1:2:2:1:1:1, giving the phenotypes of Normal, Cut wings-normal body, Normal wings-Fuzzy body and Cut wings-Fuzzy Body in the ratios 9:3:2:2. Hope that explains how to get to the answer well enough, This really isn't the easiest way to explain genetic crosses like that. I apologise now that there may be some error it my calculations, it is quite early here, and I was using a different notation when working it out, so many not have copied it onto here correctly. Edit: Looked at this when more awake, same as below, my working is right, but I agree with him, 9:3:3:1, but please, look through this for yourself, as he and I have both said, and done, this stuff is easy to trip up on

Answer 3

Men are XY, and women are XX. X-linked recessive alleles are usually more prominent in men because they only need to get the recessive allele one time on their only x chromosome. For women to have a disease that is x-linked recessive, they must have the recessive allele on both of their X chromosomes.

Answer 4

because males have x and y and all he needs is the x to have the recessive trait...however, in females we have x and x sex chromosomes so we need it on both x's. using a punnett square this means that your mom must be a carrier and your dad must have the disease. i hope this helped