Disjunction is

Options

A. Chromosome aberration involving deletion
B. Modification of gene action by a non-allelic gene
C. Separation of homologous chromosomes at anaphase
D. Incompatibility of genes

Answer

### Step-by-Step Solution:

1. **Understanding Disjunction**: Disjunction refers to the separation of chromosomes during cell division. It is a crucial process that ensures that each daughter cell receives the correct number of chromosomes.

2. **Identifying the Phases**: Disjunction occurs during two specific phases of meiosis:
- **Anaphase I**: This is when homologous chromosomes (pairs of chromosomes, one from each parent) are separated and pulled towards opposite poles of the cell.
- **Anaphase II**: This is when sister chromatids (the two identical halves of a duplicated chromosome) are separated and also pulled towards opposite poles.

3. **Analyzing the Options**:
- **Option 1**: Chromosome ablation involving deletion - This is incorrect as it relates to chromosome damage rather than separation.
- **Option 2**: Modification of gene action by non-allelic gene - This is incorrect because disjunction is not about gene modification.
- **Option 3**: Separation of homologous chromosome at anaphase - This is correct as it accurately describes what happens during disjunction in anaphase I.
- **Option 4**: Incompatibility of genes - This is incorrect as disjunction does not involve gene incompatibility.

4. **Conclusion**: The correct answer is **Option 3**, which states that disjunction is the separation of homologous chromosomes during anaphase.