Select the incorrect statement from the following. In birds, the chromosomes composition of the sperm determines the sex of the progeny.

Options

A. In birds, the chromosomes composition of the sperm determines the sex of the progeny.
B. Cystic fibrosis is Mendelian disorder.
C. Increases in a whole set of chromosomes in an organism is known as polyploidy.
D. Down's syndrome is chromosomal disorder and is due to trisomy of chromosome `21 ^(st)`.

Answer

To solve the question of selecting the incorrect statement, we will analyze each option provided:

**Step 1: Analyze Option A**
- The statement reads: "In birds, the chromosome composition of the sperm determines the sex of the progeny."
- In birds, the sex determination system is different from that in mammals. In birds, it is the female that determines the sex of the progeny. Females have two different sex chromosomes (ZW), while males have two of the same (ZZ). Therefore, the type of egg (ovum) produced by the female (either Z or W) will determine the sex of the offspring. Thus, this statement is incorrect.

**Step 2: Analyze Option B**
- The statement reads: "Cystic fibrosis is a Mendelian disorder."
- Cystic fibrosis is indeed a Mendelian disorder, specifically an autosomal recessive disorder. It is caused by mutations in the CFTR gene. Therefore, this statement is correct.

**Step 3: Analyze Option C**
- The statement reads: "Increase in whole set of chromosomes in an organism is known as polyploidy."
- Polyploidy refers to the condition where an organism has more than two complete sets of chromosomes. This is a correct definition, making this statement true.

**Step 4: Analyze Option D**
- The statement reads: "Down syndrome is a chromosomal disorder and is due to trisomy of chromosome 21st."
- Down syndrome is indeed caused by the presence of an extra chromosome 21, which is referred to as trisomy 21. This statement is also correct.

**Conclusion:**
After analyzing all the options, the incorrect statement is:
- **A. In birds, the chromosome composition of the sperm determines the sex of the progeny.**