So, a color-blind man mates with a heterozygous woman. Using a Punnett square, determine the percentage of sons and daughters that will be color blind.
The Correct Answer and Explanation is:1
Let’s break this down using a Punnett square.
Color blindness is an X-linked recessive trait, which means the gene responsible for color blindness is located on the X chromosome. Men have one X and one Y chromosome (XY), while women have two X chromosomes (XX). The color blindness allele is represented by “X^c” (for color blind) and the normal vision allele is “X^C” (for normal vision).
Parents’ Genotypes:
- Man: Since he is color blind, his genotype must be X^cY. He has only one X chromosome, which carries the color-blind allele.
- Woman: The woman is heterozygous, meaning she carries one normal X and one color-blind X. Her genotype is X^CX^c.
Punnett Square:
We will use the genotypes of both parents to set up the Punnett square.
| X^C (Mom) | X^c (Mom) | |
|---|---|---|
| X^c (Dad) | X^cX^C (Daughter, Normal Vision) | X^cX^c (Daughter, Color Blind) |
| Y (Dad) | X^CY (Son, Normal Vision) | X^cY (Son, Color Blind) |
Analysis:
- Daughters: From the Punnett square, we can see that daughters will inherit one X chromosome from each parent. If they inherit the X^C from their mother and the X^c from their father, they will have normal vision (X^CX^c). If they inherit the X^c from their mother and the X^c from their father, they will be color blind (X^cX^c). Thus, 50% of daughters will be color blind, and 50% will have normal vision.
- Sons: Sons will inherit the Y chromosome from their father and one X chromosome from their mother. If they inherit the X^C, they will have normal vision (X^CY). If they inherit the X^c, they will be color blind (X^cY). Thus, 50% of sons will be color blind, and 50% will have normal vision.
Conclusion:
- 50% of daughters will be color blind.
- 50% of sons will be color blind.
This is a simple example of how X-linked recessive traits are inherited, with a male always passing his Y chromosome to his sons and one of his X chromosomes to his daughters.
