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:
In this case, we have a woman whose mother had red-green color blindness, and she marries a colorblind man. Red-green color blindness is inherited as an X-linked recessive trait, meaning that the gene responsible for the condition is located on the X chromosome.
1. Chances of a colorblind female:
- The woman, since her mother was colorblind, must have inherited the colorblind allele from her mother. Therefore, her genotype is X^cX (where X^c represents the X chromosome carrying the colorblind allele).
- The man is colorblind, so his genotype is X^cY.
- For a child to be a colorblind female, she must inherit a colorblind allele from both parents (i.e., X^cX^c). The woman can pass on either an X^c or a normal X chromosome, and the man can only pass on the X^c chromosome. Thus, there is a 50% chance the daughter will inherit the X^c chromosome from the mother and be colorblind.
2. Chances of a normal male:
- A normal male requires the inheritance of a normal X chromosome from the mother and a Y chromosome from the father. The woman can pass on either a normal X or an X^c chromosome, and the man can only pass on the Y chromosome.
- The chances of having a normal male are 50% since the child can inherit a normal X from the mother and the Y from the father.
3. Why males cannot carry the colorblindness recessive allele:
- Males have only one X chromosome (XY). Since the colorblindness allele is recessive, if a male inherits the X chromosome with the colorblind allele (X^c), he will express the trait because there is no second X chromosome to potentially carry a normal allele to mask the effect. Therefore, males cannot be carriers of X-linked recessive traits. They either have the condition or they don’t, depending on which X chromosome they inherit.
To summarize:
- The chances of a colorblind female are 50%.
- The chances of a normal male are also 50%.
- Males cannot “carry” the colorblindness allele because they only have one X chromosome and will express the trait if they inherit the X^c chromosome.
