Question: Red-Green color blindness is inherited as an X-linked recessive trait in humans. A woman, whose mother had red-green color blindness, marries a colorblind man. What are the chances that their child will be a colorblind female? Answer Here: colorblind female? Answer Here: normal male? Answer Here: Explain why males cannot “carry” the colorblindness recessive allele.
The Correct Answer and Explanation is:
Answer:
- Chance of colorblind female: 50%
- Chance of normal male: 50%
Explanation:
Red-green color blindness is caused by an X-linked recessive allele, meaning that the gene responsible for this condition is located on the X chromosome. To understand the inheritance pattern in this case, let’s break down the parental genetic makeup:
- Mother’s Genotype: The woman’s mother has red-green color blindness, which means the mother carries the X-linked recessive allele for color blindness on both of her X chromosomes (genotype: XcXc, where “Xc” represents the X chromosome carrying the colorblind allele). Since the woman inherits one X chromosome from her mother, her genotype must be XcX (heterozygous, where one X carries the colorblind allele and the other is normal).
- Father’s Genotype: The man is colorblind, meaning he has the X-linked colorblind allele on his single X chromosome (genotype: XcY, where “Xc” is the colorblind allele and “Y” represents the Y chromosome, which does not carry the colorblindness gene).
Inheritance to the Children:
- For a female child: A female child inherits one X chromosome from each parent. Since the father can only pass down his X chromosome (Xc), and the mother has the potential to pass either an Xc or an X, the child can inherit:
- One Xc from the father (colorblind allele)
- One X from the mother (either normal or colorblind depending on the specific allele she passes)
Since the mother is heterozygous (XcX), she has a 50% chance of passing the Xc chromosome. Therefore, there’s a 50% chance that the daughter will inherit two Xc chromosomes (XcXc), making her colorblind.
- For a male child: A male child inherits his Y chromosome from his father (since males are XY), and one of the X chromosomes from his mother. Since the mother is heterozygous (XcX), there is a 50% chance that she will pass down the Xc chromosome (colorblind allele) to the son, and a 50% chance she will pass the normal X chromosome. Therefore, there is a 50% chance that the son will inherit the Xc chromosome and be colorblind, and a 50% chance that he will inherit the normal X chromosome and be unaffected.
Why Males Can’t “Carry” the Colorblindness Recessive Allele:
- Males have only one X chromosome (XY). If that X chromosome carries the colorblind allele, they will express the condition because they do not have a second X chromosome to “mask” the effect. In contrast, females have two X chromosomes (XX). If one X chromosome carries the colorblind allele, the other X chromosome may carry a normal allele, which could prevent the expression of color blindness. This is why females can “carry” the colorblind allele without being affected, but males cannot—they either have the colorblind allele or they do not.
