Identical twins are born when:

Options

A. One spermatozoan fertilizes two ova
B. One ovum is fertilized by two spermatozoa
C. Two eggs are fertilized
D. One fertilized egg divides into two blastomeres and they become separate

Answer

### Step-by-Step Solution:

1. **Understanding Twin Types**:
- There are two main types of twins: identical (monozygotic) and fraternal (dizygotic). Identical twins come from a single zygote, while fraternal twins come from two separate zygotes.

2. **Formation of Identical Twins**:
- Identical twins are formed when one sperm fertilizes one egg (ovum), resulting in the formation of a single zygote.

3. **Cleavage and Separation**:
- After the zygote is formed, it undergoes cleavage (cell division). In the case of identical twins, during the early stages of development, the zygote divides into two separate blastomeres. These blastomeres then develop into two separate embryos.

4. **Characteristics of Identical Twins**:
- Since they originate from the same zygote, identical twins share the same genetic material and are therefore genetically identical. They will either be both males or both females.

5. **Elimination of Other Options**:
- The other options provided in the question can be eliminated:
- One sperm fertilizing two ova is not possible.
- One ova being fertilized by two spermatozoa (polyspermy) is also not allowed.
- Two eggs being fertilized would result in fraternal (non-identical) twins.

6. **Conclusion**:
- The correct answer to the question is: Identical twins are born when one egg is fertilized by one sperm and then divides into two embryos.