Sex-linked traits/ color-blindness?

A sex-linked gene 'c' produces red-green color blindness in humans. A normal woman whose father is color-blind marries a color-blind man.


a)What genotype(s) are/is possible for the mother of the color-blind man the normal woman has married?


b) What are the chances that the first child from this marriage will be a color-blind boy?


c) Of the daughters of these parents, what fraction are expected to be color-blind?

Sex-linked traits/ color-blindness?
Well, that first answer was helpful, wasn't it? And there are a few problems with the above answers:


Mindclover did not account for the fact that we know the genotype of the wife.


Dr.Dave’s calculations don’t account for the fact that the husband cannot be C and C, as he only has ONE X chromosome to carry a C/c, not two.








Sometimes X-linked traits can confuse newbies to genetics, it's okay.





Let's call the colourblindness X(c) - we're looking at the X chromosome here, and it has the recessive c allele of the colour-vision gene on it. X(c) is recessive to X. Without another X present, a man with X (c) is colourblind - turn this around and the man in this Q is X(c)Y.





Then you have to figure out the genotype of the wife. She has two X chromosomes, one from her mom and one from her dad. Her dad only has one type to give as he was a colourblind male X(c)Y : that X chromosome is X(c). This means she is X X(c).





NOW you can do the punnet square.





.........X(c)....Y (colourblind man)


X(c)


X





Remember that each box on the square is of equal probability, and you can answer the questions. One quarter of the offspring will be X(c)X(c) (colourblind female), one quarter X(c)Y (colourblind male), one quarter normal vision male and one quarter n ormal vision female.





Make SURE you go through this step by step and understand how to get the answer, okay? I can guarantee you this will turn up on your exam.





(and remember that this isn't an out-there-improbable situation! I know I'm X(c)X because my dad is red/green colourblind! My son's dad has normal vision. My son has a fifty percent chance of having red/green colourblindness)
Reply:it looks like this.





XCXC or XCXc = normal female


XcXc = colorblind female


XCY = normal male


XcY = colorblind male





b. The boy has a 50/50 chance of being colorblind


c. The girl has a 75% chance of being colorblind
Reply:a) the mother of the color blind man is definitely XCXc because he has to have inherited color blindness gene on the X chromosome (Xc) to be colorblind. Remember, boys inherit their X chromosome from the mother and their Y chromosome from dad. Since colorblindness gene is an X-linked gene, for a boy to be color blind, his mother has to be carrier.





b) Since the father of the woman is colorblind, then she must be carrier: XCXc


XCXc x XcY


1/4 XCXc + 1/4 XCY +1/4 XcXc + 1/4 XcY


1/4 carrier girl + 1/4 healthy boy + 1/4 sick girl + 1/4 sick boy





Therefore there is a 50% chance that their first child is either a colorblind girl or a color blind boy.





c) 25%. See the answer to b.
Reply:it is hard to do a punnett square here as this box will not support graphics...


but lets say that the woman is C (colour blind) and c not colour blind) {she carries the trait only} marries a man that is C and C


do a punnett square and it will show that is will be 50% of CC and 50% will be Cc. {CC, CC, Cc, Cc}. (2 withdisease and two with trait)


Men are usually colour blind because the trait is sex linked so (one chromosome is X and the other Y) with the one X linked, they will have it


the woman is X and X so in order for her to be colour blind, the mother and father both have to be colour blind, or she will just have the trait (and maybe just tint blind)


1st child be a colour blind boy, about 48%


daughters colour blind, not, unless both parents were...


now if the woman was colour blind CC and so was the father, then all 4 kids would be CC