Mendelian disorders are mainly determined by

Options

A. Alteration or mutation in single gene
B. Absence of one chromosome
C. Excess of one or more chromosome
D. All the above

Answer

**Step-by-Step Text Solution:**

1. **Understanding Mendelian Disorders**: Mendelian disorders are genetic conditions that are primarily caused by alterations or mutations in a single gene. These disorders follow the principles of inheritance first described by Gregor Mendel.

2. **Analyzing the Options**:
- **Option 1: Alternation and Mutation in Single Gene**: This option suggests that Mendelian disorders arise from changes in a single gene. This is indeed the primary cause of these disorders, making this option a strong candidate.
- **Option 2: Absence of 1 Chromosome**: This refers to conditions like monosomy, where one chromosome is missing. While this can lead to genetic disorders, it is not the primary cause of Mendelian disorders.
- **Option 3: Access of 1 or more Chromosome**: This refers to conditions like trisomy or polyploidy, where there is an extra chromosome or set of chromosomes. Similar to option 2, this is not the main cause of Mendelian disorders.
- **Option 4: All of the above**: This option suggests that all the previous options contribute to Mendelian disorders, which is incorrect as only option 1 directly relates to them.

3. **Conclusion**: After analyzing the options, the correct answer is **Option 1: Alternation and Mutation in Single Gene**. This is because Mendelian disorders are primarily caused by mutations or alterations in a single gene.