Which among these is not a Mendelian disorder?
(I) Turner's syndrome
(II) Colour-blindness,
(III) Cystic fibrosis
(IV) Myotonic dystrophy

Options

A. (I)
B. (II) and (IV)
C. (I), (III) and (IV)
D. (IV)

Explanation

To determine which of the listed disorders is not a Mendelian disorder, we need to analyze each option based on its genetic basis.

### Step-by-Step Solution:

1. **Understanding Mendelian Disorders**:
- Mendelian disorders are those that follow the inheritance patterns described by Gregor Mendel. They are typically caused by mutations in a single gene and can be classified as autosomal dominant, autosomal recessive, or sex-linked.

2. **Analyzing Each Option**:
- **(I) Turner's Syndrome**: This is a chromosomal disorder caused by the absence of one X chromosome (45 chromosomes instead of the usual 46). It is not caused by a single gene mutation but rather by a chromosomal anomaly. Therefore, it is not a Mendelian disorder.
- **(II) Colour-blindness**: This is a sex-linked recessive disorder caused by mutations in genes located on the X chromosome. It follows Mendelian inheritance patterns.
- **(III) Cystic fibrosis**: This is an autosomal recessive disorder caused by mutations in the CFTR gene on chromosome 7. It follows Mendelian inheritance patterns.
- **(IV) Myotonic dystrophy**: This is an autosomal dominant disorder caused by mutations in the DMPK gene. It also follows Mendelian inheritance patterns.

3. **Conclusion**:
- Among the options provided, Turner's syndrome is the only one that is not a Mendelian disorder because it is a chromosomal disorder rather than a genetic disorder caused by a single gene mutation.

### Final Answer:
**(I) Turner's syndrome** is not a Mendelian disorder.

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