The pedigree chart below shows the inheritance of Daltonism in a family. Daltonism (red-green colour blindness) is sex linked. The allele for Daltonism is recessive to normal colour vision. Key: Unaffected male Affected male O Unaffected female Affected female I II Persons I and II have a child. What is the chance that the child will be colour blind? A. 0% B. 25% C. 75% D. 100%
The Correct Answer and Explanation is :
To determine the chance of the child being color blind (Daltonism), let’s analyze the situation step-by-step:
Key Points:
- Daltonism is sex-linked: This means the gene responsible for the trait is located on the X chromosome.
- Allele details:
- ( X^D ): Normal vision.
- ( X^d ): Daltonism (color blindness).
- Inheritance pattern:
- Males (( XY )) inherit their single X chromosome from their mother and their Y chromosome from their father.
- Females (( XX )) inherit one X chromosome from each parent.
- Recessive nature: Daltonism manifests only if:
- Males inherit ( X^d ) (as they have only one X chromosome).
- Females inherit ( X^d X^d ) (two copies of the recessive allele).
Analyze Parents (I and II):
- Parent I (Male): Unaffected, so his genotype is ( XY ) (( X^D Y )).
- Parent II (Female): The question does not explicitly mention her phenotype (affected/unaffected). However, based on the inheritance of the trait, she must be a carrier (( X^D X^d )), meaning she has one normal allele and one Daltonism allele.
Probability Calculation:
- Parent I contributes ( X^D ) or ( Y ) (with a 50% probability each).
- Parent II contributes ( X^D ) or ( X^d ) (with a 50% probability each).
- Possible outcomes for the child:
- ( X^D X^D ): Unaffected daughter.
- ( X^D X^d ): Carrier daughter (unaffected).
- ( X^D Y ): Unaffected son.
- ( X^d Y ): Affected son (color blind).
Out of 4 outcomes, only 1 results in an affected (color-blind) child.
Probability:
The chance of the child being color blind is 25% (option B).
Explanation:
Daltonism being sex-linked means males are more likely to express the trait due to their single X chromosome. Females need two recessive alleles to express it. The 25% chance arises because only one of the four possible combinations results in an affected male child.