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Step-by-Step Explanation
Step 1: Understanding the Family Pedigree
• The father (male) is diseased.
• The mother (female) is normal.
• They have three daughters, and all of them are diseased.
• They have five sons, and all of them are normal.
Step 2: Recognizing the Inheritance Pattern
• Since the father is the only parent affected and all his daughters (but none of his sons) show the disease, it suggests a link to the X-chromosome.
• If a trait is on the X-chromosome and is dominant, any daughter receiving the father’s affected X-chromosome will inherit the disorder because only one copy of the mutated allele is sufficient to express the disease.
• The sons, on the other hand, receive the Y-chromosome from the father and thus remain unaffected.
Step 3: Concluding the Mode of Inheritance
• Because the father passes the trait to all daughters but to no sons, this pattern is indicative of Sex-Linked Dominant inheritance.
• If it were X-linked recessive, the daughters would not necessarily show the disease (they could be carriers), and some sons could be affected if they inherited the mutant X from the mother.
• Autosomal inheritance (dominant or recessive) would not produce this distinct father-to-daughter but not father-to-son pattern.
Final Conclusion
The gene for this disease is Sex-Linked Dominant.
