I need thelp please.?

Question

F. SEX LINKAGE
10. In humans, dark hair (D) is dominant over blondness (d), and color blindness (c) is a sex-linked recessive trait. A woman has a blond brother, a blond mother, and a dark-haired father. Her brother and her parents have normal vision. She bears the following three children by her blond, normal-vision husband:
a dark-haired son with normal vision,
a dark-haired daughter with normal vision, and
a dark-haired color blind son.
What is the probability that her next (fourth) child will be a colorblind boy?

Answer 1

By looking at the various offspring, you should be able to determine the genotype of the female. You know that her husband is homozygous recessive for blondness, and has a normal color vision X chromosome. Since she had one kid with colorblindness, she must be heterozygous for the colorblindness gene.

Once you get her genotype, you can answer the problem.

Answer 2

Let's go through it step by step:

1. Nothing is asked about the hair color of the son. So for this question we can forget about it, because it doesn't relate to color blindness.

2. It doesn't matter what the other children are, because with every new child the chromosomes are freshly mixed up.

3. Because sons get the Y sex chromosomes from their fathers and color blindness is encoded on the X sex chromosome, sons inherit their color blindness always from their mothers.

4. She already has a colorblind son and a son with normal vision. This tells us, that she must have a colorblind X chromosome and a normal one.

5. Now the calculation, looked by the chromosomes possible combinations:
5.1. It's always 50% to get a boy or a girl.
5.2. It's again 50% to inherit the corrupted X or the normal X.

This all together makes a 25% possibility, that her next son will be colorblind.

Answer 3

Womans genotype XBcXb ( dark hair heterozygous with colourblind sex related gene)
Cross with husband XbYb
Do a cross
Xb Yb
XBc XBcXb XBcYb
Xb XbXb XbYb
The probability of having a colourblind boy (XBcYb) is 25%