Create a Punnett square to determine the possibility of a couple having a color-blind child if the mother is color blind but the father has normal vision. HINT: Use Xb to indicate an X with the color-blindness trait.
The Correct Answer and Explanation is:
Punnett Square for Color Blindness
Let’s define the alleles:
- X = normal vision X chromosome
- Xb = X chromosome with color blindness
- Y = Y chromosome (does not carry the gene for color vision)
Parents’ Genotypes
- Mother is color blind: XbXb
- Father has normal vision: XY
Punnett Square
| X (Father) | Y (Father) | |
|---|---|---|
| Xb (Mother) | XbX | XbY |
| Xb (Mother) | XbX | XbY |
Results
- XbX = Female carrier (normal vision but carries the gene)
- XbY = Male child with color blindness
Offspring Possibilities
- 50% chance of XbX = daughters with normal vision but carriers
- 50% chance of XbY = sons who are color blind
Explanation
Color blindness is a sex-linked recessive trait, meaning the gene responsible for it is found on the X chromosome. Males only have one X chromosome (XY), so if they inherit the Xb chromosome, they will express the trait because there is no second X chromosome to mask it. Females have two X chromosomes (XX), so they need two copies of the Xb allele to be color blind. If a female has one normal X and one Xb, she is a carrier but not color blind herself.
In this case, the mother is color blind, which means her genotype must be XbXb. This is because females need two Xb chromosomes to express color blindness. The father has normal vision, so his genotype must be XY with a normal X chromosome.
To determine the possible combinations of their children’s genotypes, we use a Punnett square. When we cross XbXb with XY, each child will inherit one Xb chromosome from the mother. The father can contribute either an X or a Y chromosome. If the child inherits his X, the child will be a carrier daughter (XbX). If the child inherits his Y, the result is a color-blind son (XbY).
Therefore, there is a 50% chance of having a color-blind son, and a 50% chance of having a daughter who is a carrier. All daughters will be carriers, and no daughters will be color blind, but all sons will be affected if they receive the Xb chromosome.
