Consider the following human disorders :
i. Haemophilia
ii. Down's Syndrome
iii. Cystic fibrosis
iv. Colour bilindness
v. Night blindness
Which of these disorders exihibit 'Mendelian' pattern of inheritance .

Options

A. ii, iii, and iv
B. i, ii, and iii
C. i, iii, and v
D. i, iii, and iv

Answer

To determine which of the listed human disorders exhibit a Mendelian pattern of inheritance, we need to analyze each disorder based on its genetic basis. Here’s the step-by-step solution:

### Step 1: Understand Mendelian Inheritance
Mendelian inheritance refers to the inheritance patterns that are based on the laws formulated by Gregor Mendel. These patterns typically involve traits that are controlled by a single gene, which can be dominant or recessive. Disorders that follow this pattern are often classified as Mendelian disorders.

**Hint:** Review the basic principles of Mendelian inheritance, including dominant and recessive traits.

### Step 2: Analyze Each Disorder
1. **Haemophilia**: This is a sex-linked disorder caused by a mutation in a gene on the X chromosome. It is inherited in a recessive manner. Therefore, it follows a Mendelian pattern of inheritance.

**Hint:** Remember that sex-linked disorders are still considered Mendelian if they follow the dominant/recessive inheritance pattern.

2. **Down's Syndrome**: This is a chromosomal disorder caused by an extra copy of chromosome 21 (trisomy 21). It does not follow Mendelian inheritance because it involves chromosomal abnormalities rather than single gene mutations.

**Hint:** Distinguish between chromosomal disorders and Mendelian disorders.

3. **Cystic Fibrosis**: This is an autosomal recessive disorder caused by mutations in the CFTR gene. Since it is controlled by a single gene and follows the recessive inheritance pattern, it is considered a Mendelian disorder.

**Hint:** Identify whether the disorder is autosomal or sex-linked, and check if it is recessive or dominant.

4. **Colour Blindness**: This is also a sex-linked recessive disorder caused by mutations in genes on the X chromosome. Like haemophilia, it follows a Mendelian pattern of inheritance.

**Hint:** Consider whether the gene responsible for the disorder is located on a sex chromosome.

5. **Night Blindness**: This condition can be caused by various factors, including vitamin A deficiency. However, certain types of night blindness can be inherited in a Mendelian manner, often as an autosomal recessive trait. Thus, it can be considered a Mendelian disorder.

**Hint:** Investigate the genetic basis of night blindness to determine if it follows Mendelian inheritance.

### Step 3: Conclusion
Based on the analysis:
- **Mendelian Disorders**: Haemophilia, Cystic Fibrosis, Colour Blindness, and possibly Night Blindness (depending on the specific type).
- **Non-Mendelian Disorder**: Down's Syndrome.

Thus, the disorders that exhibit Mendelian patterns of inheritance from the given list are:
1. Haemophilia
2. Cystic Fibrosis
3. Colour Blindness
4. Night Blindness (conditional)

**Final Answer**: Haemophilia, Cystic Fibrosis, Colour Blindness, and potentially Night Blindness exhibit Mendelian patterns of inheritance.