Sex-Linked Inheritance—Predicting Color Blindness in Offspring?

I need some help with my Biology lab, here's some background info and stuff:

Recall that females have two X chromosomes: one from the mother and one from the father. Males have one X chromosome and one Y chromosome; the X chromosome comes from the mother, and the Y chromosome comes from the father. Traits associated with genes located on the sex chromosomes are called sex-linked traits.

The dominant allele for normal color vision is N. Individuals with the sex-linked condition called red-green color blindness do not perceive the colors red and green. Red-green color blindness is caused by the recessive allele n and is carried on the X chromosome. When an X chromosome contains the dominant allele, the allele is written as XN. When an X chromosome contains the recessive allele, the allele is written as Xn.

1.What is the genotype of a male with red-green color blindness?

2.What is the genotype of a female who is not color-blind but is a carrier of red-green color blindness?

3.Even though red-green color blindness is a recessive trait, can a female have red-green color blindness? Explain.

4.A man who is color-blind marries a woman who is not color-blind and is not a carrier of the allele for color blindness. Create a Punnett square to predict the possible genotypes of their children.

5.Will all of the female children be carriers of the color-blind allele, or will none be carriers? Explain.

6.Will all of the male children be color-blind, or will none be color-blind? Explain.

Thanks. :)


1. Xn Y

2. XN Xn

3. Yes, the color blind woman would have to have a father with color blindness (Xn Y) and a mother who is either a carrier (XN Xn) or also color blind (Xn Xn). She would have to receive a recessive color blind trait from both the mother and father (Xn Xn)

4. Xn Y



5. All the girls would be carriers because the father only has the one color blind allele to give them

6. You would have no color blind boys because the father gives them the Y chromosome which doesn't contribute to color blindness and the mother has all normal alleles to pass on


Ans1. Genotype of father is- (Xn Y)

Ans2. Genotype of carrier female is- (XN Xn)

Ans3. Even though this disease is recessive, females can have this disease if they have recessive alleles or the alleles for disease, on both the X-chromosmes and this is possible only when the father of female is colour blind (Xn Y) and the mother is atleast the carrier of the disease (XN Xn)

Ans4. Man's genotype- (Xn Y)

and Woman's genotype- (XN XN)

Punnet Square-


--XN---XN Xn-----XN Y-------

--XN---XN Xn-----XN Y-------

Offsprings are- 2 normal males and 2 Carrier females

Ans5. Yes, in the above punnet square you can see that all the female children are carriers of the color blind allele.

Ans6. You can see in Ans 4 that none of the male children is color blind.


1. XnY.

Since the copy of X chromosome can only be inherited from the mother, the fact that he has red-green color blindness means the allele passed down from his mother is Xn, thereby making him an XnY. A father can only pass a Y chromosome to his son.

2. XNXn.

Carriers are normally heterozygous for a condition and do not exhibit recessive phenotypes despite having a recessive allele in their genotype, owing to the dominant allele masking the effects of the recessive allele.

3. A female can only have red-green colorblindness if she has inherited two copies of the recessive alleles, Xn, from each parent.

E.g. If the mother is XNXn and the father is XnY (color blind), the daughter can have any of the following genotypes:

-XNXn (no color blindness, heterozygous, carrier for color blindness)

-XnXn (color blindness, homozygous recessive)





5. All daughters will have the genotype XNXn. None of the daughters will be color blind, but they will be carriers for the recessive allele. The mother is not a carrier of the allele for color blindness, hence her genotype must be XNXN and she can only pass on an XN allele to her daughters. The father, however, is color blind. Since he only has a copy of the X chromosome which happens to be recessive for color blindness to pass on to his daughters, they will thus be XNXn.

6. None of the sons will be color blind as they will have the genotype XNY. Male offsprings inherit a copy of the X chromosome only from their mother while their father passes on the Y chromosome to them. So since the mother is XNXN, her sons will all have color vision.