Red-green color blindness, in humans, is a sex-linked trait controlled by alleles on the X chromosome. Normal color vision (X+) is dominant to colorblindness (Xc). (NOTE: You could write this as XC for the normal allele and Xc for the recessive colorblind allele. Unfortunately, capital C and lowercase c are hard to tell apart as superscripts, especially in type. I will use C — “X-plus” — for the normal allele.) If a colorblind man marries a woman with normal vision and they have a colorblind son, what are the genotypes of the father and the mother?
The correct answer and explanation is :
In this case of red-green color blindness being a sex-linked recessive trait, the key is to understand the genetic inheritance and how the alleles on the X chromosome determine the outcome.
Given information:
- Colorblindness (Xc) is recessive and located on the X chromosome.
- Normal color vision (X+) is dominant to colorblindness (Xc), and also located on the X chromosome.
- The man is colorblind, which means his genotype must be XcY (since males have one X and one Y chromosome).
- The woman has normal color vision, but she must be a carrier for colorblindness (X+Xc), as she has a colorblind son.
Genotypes:
- Father’s Genotype: XcY
- Since the father is colorblind, his X chromosome must carry the colorblind allele, and he also has a Y chromosome, which does not carry an allele for color vision.
- Mother’s Genotype: X+Xc
- The mother has normal color vision but is a carrier of the colorblind allele. Therefore, one of her X chromosomes carries the normal color vision allele (X+), and the other carries the colorblind allele (Xc).
Inheritance of the child:
The son must inherit his Y chromosome from his father (since only males inherit the Y chromosome from their father), and he must inherit one of the X chromosomes from his mother. Since the mother is a carrier (X+Xc), the possible X chromosome that the son could inherit from her is Xc (which is the colorblind allele).
Thus, the son’s genotype is XcY, making him colorblind.
Summary of the family’s genotypes:
- Father: XcY (colorblind)
- Mother: X+Xc (normal vision but carrier)
- Son: XcY (colorblind)
Explanation:
In sex-linked inheritance, the X chromosome is crucial because females have two X chromosomes while males have one X and one Y chromosome. The father can only pass on his Y chromosome to his son, while the mother passes one of her X chromosomes. Since the mother is a carrier, she has a 50% chance of passing on the colorblind allele to each child. This combination results in the son being colorblind, inheriting the Xc from his mother and the Y chromosome from his father.
Here is the diagram illustrating the genetic inheritance of colorblindness in this family. It shows the genotypes of the father (XcY), the mother (X+Xc), and the son (XcY). The arrows indicate how the alleles are passed on from the parents to the child.