Pedigree analysis problem with solution (Example 14)

Pedigree analysis problem with solution (Example 14)

Q) Joe is color-blind. His mother and father both have normal vision, but his mother’s father (Joe’s maternal grandfather) is color-blind. All Joe’s other grandparents have normal color vision. Joe has three sisters—Patty, Betsy, and Lora—all with normal color vision. Joe’s oldest sister, Patty, is married to a man with normal color vision; they have two children, a 9-year old color-blind boy and a 4-year-old girl with normal color vision.

a) Using correct symbols and labels draw a pedigree of Joe’s family.

b) What is the most likely mode of inheritance for color-blindness in Joe’s family?

Answer: X-linked recessive inheritance.

Reason:

  • Only males have the trait.
  • Male inherit the trait from their mothers, who are carriers.
  • The trait is never passed from father to son.

(c) If Joe marries a woman who has no family history of color-blindness, what is the probability that their first child will be a color-blind boy?

Answer:  Zero (If there is no mutation or non disjunction of chromosome)

  • Joe cannot pass his color-blind X chromosome to his son. The trait never passed from father to son.

(d) If Joe marries a woman who is a carrier of the color-blind allele, what is the probability that their first child will be a color-blind boy?

Answer: The probability is ¼.

Probability of having a boy child = ½

Probability that the first child will inherit the color-blind X chromosome from the carrier mother = 1/2

Probability of having a color blind boy child = ½ X ½ = ¼.

(e) If Patty and her husband have another child, what is the probability that it will be a color-blind boy?

Answer: The probability is ¼.

Patty is a carrier because she had a color-blind son.

Probability of having a boy child = ½

Probability of having color blind child = ½

Therefore probability of having color blind boy child = ¼ (1/2 X 1/2)

 

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