The following pedigree diagram shows the inheritance of a rare genetic disorder (filled shapes depict affected individuals).
Which of the following is the most likely pattern of inheritance of the disorder?
Show Hint
In pedigree analysis, look first at the offspring of affected males.
If all daughters of an affected male are affected and all his sons are unaffected, the trait is almost certainly X-linked dominant.
This simple rule helps save valuable time during competitive examinations.
Step 1: Understanding the Question:
The question presents a pedigree diagram representing the transmission of a rare genetic disorder across three generations.
We need to determine the mode of inheritance by analyzing the transmission of the trait between parents and offspring. Step 2: Key Formula or Approach:
To identify the mode of inheritance, we test the given choices against key genetic rules:
1. X-linked dominant: Affected fathers must pass the trait to all of their daughters (since they receive his only X chromosome), but none of their sons (since they receive his Y chromosome). Affected mothers can pass the trait to both sons and daughters with a 50% probability.
2. X-linked recessive: Affected mothers must pass the trait to all of their sons. Unaffected parents cannot have affected daughters unless the father is affected.
3. Autosomal dominant: The trait typically does not skip generations, and affected parents can have unaffected offspring.
4. Autosomal recessive: The trait can skip generations, and unaffected parents can have affected offspring. Step 3: Detailed Explanation:
Let us analyze the pedigree diagram step-by-step:
- In Generation II, we observe two affected males (black squares).
- Let's look at the rightmost affected male in Generation II. He is married to an unaffected female (white circle).
- Their offspring in Generation III consist of:
- One affected daughter (black circle).
- Two unaffected sons (white squares).
- Let's also look at the leftmost affected male in Generation II. He is married to an unaffected female (white circle).
- Their offspring in Generation III consist of:
- Two affected daughters (black circles).
- Two unaffected sons (white squares).
- In both cases, the affected fathers transmit the disease to 100% of their daughters and 0% of their sons.
- This is the signature pattern of an X-linked dominant inheritance, because a father passes his X chromosome to all his daughters and his Y chromosome to all his sons.
- Additionally, the affected female in Generation I (black circle) married to an unaffected male (white square) passes the trait to approximately 50% of her children:
- She has one affected daughter, one unaffected daughter, one affected son, and two unaffected sons.
- This 50% transmission rate is expected for a heterozygous mother carrying a dominant allele ($X^D X^d$). Step 4: Final Answer:
Based on the perfect correlation of the pedigree with the rules of X-linked dominant inheritance, the correct option is (A).
