Refer the given statements
(i) Incomplete or mosaic inheritance is an example of pre-Mendelian concept of blending inheritance
(ii) Test cross is a special type of back cross
(iii) Chromosomal aberrations are commonly observed in cancer cells
(iv) Thalassaemia is a Mendelian disorder
Which of the above statements are correct ?

Options

A. (i) and (ii) only
B. (ii),(iii) and (iv)
C. (ii) and (iv) only
D. (i) and (iv) only

Explanation

To determine which of the given statements are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i)**: "Incomplete or mosaic inheritance is an example of pre-Mendelian concept of blending inheritance."
- **Analysis**: Incomplete inheritance refers to a situation where the phenotype of the heterozygote is intermediate between the phenotypes of the two homozygotes. This concept is indeed different from Mendelian inheritance. However, it does not strictly fall under the pre-Mendelian concept of blending inheritance, as it adheres to Mendelian principles. Therefore, this statement is **incorrect**.

2. **Statement (ii)**: "Test cross is a special type of back cross."
- **Analysis**: A test cross involves crossing an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual to determine the genotype of the former. This is indeed a specific type of back cross, where the offspring are analyzed to determine the genetic makeup of the parent. Thus, this statement is **correct**.

3. **Statement (iii)**: "Chromosomal aberrations are commonly observed in cancer cells."
- **Analysis**: Chromosomal aberrations, which include changes in chromosome number or structure, are frequently observed in cancer cells due to errors during DNA replication and cell division. This statement is **correct**.

4. **Statement (iv)**: "Thalassaemia is a Mendelian disorder."
- **Analysis**: Thalassemia is indeed a genetic disorder that follows Mendelian inheritance patterns, specifically autosomal recessive inheritance. Therefore, this statement is **correct**.

### Conclusion:
The correct statements are:
- Statement (ii): Test cross is a special type of back cross.
- Statement (iii): Chromosomal aberrations are commonly observed in cancer cells.
- Statement (iv): Thalassaemia is a Mendelian disorder.

Thus, the final answer is that statements (ii), (iii), and (iv) are correct.