Albinism is caused by a recessive gen "c". A normal man marries an albino woman. The first son happened to be albino. What are the possible phenotypes of the parents? What is the chance that their other kids will be albino?

Alright, since "c" is recessive and the woman is albino, then the woman must be "c c" right? And since the son is albino, he must be "c c" too. Hence the father must have at least one "c". But since he is normal, then the other allele must NOT be "c" too (otherwise he would be albino).

My question is, when writing the Mendel table, how do I express that allele of the father (the one that is not "c")?

$$\begin{bmatrix} & c & c\\ ? & ?c & ?c\\ c & cc & cc\end{bmatrix}$$

I marked it with $?$ because I'm not sure what to put there. My first thought was "well, it could be a capital C I guess..." - but that doesn't sound right to me. A capital C would mean there is a dominant albinism allele (but I am told that albinism is recessive only...)

I can see clearly that there is a 50-50 chance of having albino kids. I just don't know how to draw that inheritance table.


1 Answer 1


Super oober short answer: The father is Cc, the mother is cc, there is a 50% chance the children will be albino (as you predicted).

An explination on Mendelian genetics:

First let's look at (what a lot of people consider) the normal Mendel table from here:

enter image description here

Since the dad is normal, that means he has at least one Normal Gene. Which makes him C ? . We also see that mom is normal, so that means she also carries the gene C ? . In this case, they wind up with one child that is affected which means the child has the trait cc. That means both parents had to give the child one little c so that makes both of the parents Cc.

Your answer:

Now, in your problem the mother is albino, which you correctly put as cc.

The dad is normal, which means his C ?. He could eaither be CC (homozygous dominant - an uneffected non-carrier of the affected gene) or he could be Cc (heterozygous - an uneffected carrier of the affected gene). So at this point all we know about the father is that he is C ? . Which means you were on the right path.

Now we look at their first son. We are told he is albino. That makes him cc. That means that the father HAS to have at least one affected gene making him Cc.

So you were correct in thinking that it was Cc. You tripped yourself up by saying that it would make albanism the dominant. As you were told Albanism is a recessive gene. The big C is the normal gene which would hide the albanism trait.

To draw your table, just change all of your ? to big C.


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