AMOEBA SISTERS: VIDEO RECAP SEX-LINKED TRAITS Sex-Linked Punnett Square Practice Bob is color blind, but he knows that neither of his parents were color blind. He is wondering if he received the gene for color blindness from his mother, his father, or both of his parents. What would you tell Bob? Show Punnett square to prove your answer. Bob is color blind, but he knows that neither of his parents were color blind. He is wondering if he received the gene for color blindness from his mother, his father, or both of his parents. What would you tell Bob? Show Punnett square to prove your answer.
The Correct Answer and Explanation is:
Based on the principles of sex-linked inheritance, here is the correct answer and explanation for the question.
Answer:
Bob received the gene for color blindness from his mother.
Explanation:
Color blindness is an X-linked recessive trait, which means the gene causing it is located on the X chromosome. Since males have one X chromosome and one Y chromosome (XY), they inherit their Y chromosome from their father and their X chromosome from their mother.
Here’s a breakdown of the genetics:
- Bob’s Genotype: Bob is a color blind male, so his genotype must be X^c Y. (We will use ‘c’ to represent the recessive colorblind allele and ‘C’ for the dominant normal vision allele). He received the Y chromosome from his father and the X^c chromosome from his mother.
- Father’s Genotype: His father is not color blind. Since he is male, his genotype is X^C Y. He passed his Y chromosome to Bob, not his X chromosome. Therefore, the father could not have given Bob the gene for color blindness.
- Mother’s Genotype: His mother is not color blind, but she must carry the recessive allele to have a color blind son. Her genotype is X^C X^c. She is a “carrier” because she has the allele but does not show the trait herself, as her normal vision allele (X^C) masks the colorblind allele (X^c).
Punnett Square Proof:
The Punnett square below shows the possible offspring from a cross between Bob’s carrier mother (X^C X^c) and his non-color blind father (X^C Y).
| X^C (from Mother) | X^c (from Mother) | |
| X^C (from Father) | X^C X^C (Daughter, normal vision) | X^C X^c (Daughter, normal vision carrier) |
| Y (from Father) | X^C Y (Son, normal vision) | X^c Y (Son, color blind) |
This Punnett square clearly demonstrates that there is a 25% chance for these parents to have a color blind son. It proves that the son must inherit the Y chromosome from his father and the X chromosome from his mother. For the son to be color blind (X^c Y), he must have received the X chromosome carrying the recessive allele (X^c) from his mother.
