This pedigree chart represents the inheritance of colour blindness in humans Knowing that colour blindness is a sex-linked recessive disorder; if individuals Il-1 and Il-2 have another boy, what is the probability he will be colour blind? Express your answer as percent_
The Correct Answer and Explanation is:
Correct Answer: 25%
Explanation:
Key information:
- Colour blindness is a sex-linked recessive trait, located on the X chromosome.
- Males (XY) express the trait if they inherit one affected X (they don’t have a second X to compensate).
- Females (XX) must inherit two affected X chromosomes to express the trait but can be carriers if they have one affected X.
Step-by-step analysis:
- Individual II-1 is a male (square) and not shaded → not colour blind, so his genotype is XY (normal X).
- Individual II-2 is a female (circle), not shaded, but she has a colour blind son (III-2), meaning she must be a carrier (XᴺXᶜ).
- Why? Because her son III-2 is colour blind (XᶜY), and he must have inherited the Xᶜ from his mother, as males inherit their Y from their father.
- Genotypes:
- Father (II-1): XᴺY (normal male)
- Mother (II-2): XᴺXᶜ (carrier female)
Punnett square for a son:
| Xᴺ (Father) | Y (Father) | |
|---|---|---|
| Xᴺ (Mother) | XᴺXᴺ | XᴺY (normal son) |
| Xᶜ (Mother) | XᴺXᶜ (carrier daughter) | XᶜY (colour blind son) |
So, for a son:
- 50% chance of inheriting Xᴺ → XᴺY → normal
- 50% chance of inheriting Xᶜ → XᶜY → colour blind
But the question asks if they have another boy, so we only consider male offspring:
- Among boys: 1 out of 2 will be colour blind = 50%
However, the probability of the baby being a boy is 50%. So:
Final probability = 50% (chance of being male) × 50% (chance of being colour blind if male)
= 25%