Question

Last one, I swear.

Color blindness is a sex-linked recessive trait. A mother with normal color vision and a color blind father have a color blind daughter. Which statement is true about the offspring?
A)
All of their daughters must be color blind.
B)
Some of their sons can have normal color vision.
C)
Their daughters might carry a gene for color blindness.
D)
All of their sons will carry a gene for color blindness.

Answer 1

Cool. So the way to start a genetics problem is to write down the genotypes for each parent. It has given you information about the phenotypes, or appearances. And it has told you that the gene for color blindness is carried on the sex chromosomes. Would you like to take a stab at interpreting those phenotypes as genotypes?

Let me know if that was moving too fast, and I'll slow down.

Answer 2

I don't really understand sex-linked traits at all, so slow down, please.

Answer 3

OK. So forgetting the problem and just looking at the basic science, humans carry genes on chromosomes.

Most chromosomes are the same in men and women, but one pair of chromosomes, called 'sex chromosomes' are not. Women have two X chromosomes. Men have one X chromosome and one Y chromosome. Don't worry about the nitty gritty of whether having a Y imparts maleness, or whether maleness is caused by the absence of a second X.

Just remember:

Women are XX
Men are XY

So far so good?

Answer 4

Yes, so sex-linked traits are carried on the X chromosome.

Answer 5

From what I barely know, I believe it is D since males don't have another X to cancel out the the X with the color blindness trait. Even if that is correct, please continue typing; I want to hear all of it.

Answer 6

Indeed they are.

The X chromosomes are big and appear like normal chromosomes - they have many genes on them. The Y chromosomes in men are small, shrivelled up little things with a very few genes (some important for sex determination, none important in this context).

So the idea is, if a woman has two X chromosomes and they have different alleles for something like color blindness (one dominant, one recessive), she will not be color blind because the X chromosome with the dominant (not color blind) allele will mask the effect of having the other X chromosome with a recessive allele.

Whereas a man has only one X chromosome. If that X chromosome has a dominant, normal allele - he will not be color blind. But if it has a recessive allele, then he will be color blind.

Basically women have two XX, so they can be heterozygotes. But it is impossible for men to be heterozygous for the X chromosome (because their other sex chromosome is a Y).

Answer 7

Yep, I understand. :)

Answer 8

The next step is to interpret the problem. It has given you the parents appearances and what you want to infer from that are the parents genotypes.

Answer 9

The father is the easy one. It tells you that he is color blind. So you know that he has only one X chromosome, and it has a recessive color blind allele on it. His other sex chromosome, because he is a man, is a Y.

The woman is not so clear...

Answer 10

@blues does the x male chromosome carry any allele for color blindness

Answer 11

@telijahmed, yes it does.

Answer 12

sorry i meant the Y

Answer 13

But if the woman does not have color blindness, so wouldn't she just be a normal XX?