Which one is de novo mutation?

Options

A. TT X TT
B. tt X tt
C. Tt X tt
D. Tt X Tt - TT,Tt,tt

Answer

### Step-by-Step Solution:

1. **Understanding De Novo Mutation**:
- A de novo mutation refers to a genetic alteration that is present for the first time in a family member. This mutation occurs in a germ cell (egg or sperm) and is not inherited from either parent.

2. **Identifying the Source of Mutation**:
- The mutation can arise in one of the parent's germ cells (either the egg or the sperm). This means that the genetic change is not found in the parent's somatic cells but occurs in the cells that contribute to the offspring.

3. **Mutation During Early Development**:
- The genetic alteration can manifest in the fertilized egg during early embryogenesis. This means that the mutation can affect the offspring's genotype right from the beginning of its development.

4. **Example of Genotype Change**:
- In the context of the question, if we consider two parents that are both homozygous recessive (small t, small t), they can only produce offspring that are also homozygous recessive. However, if a de novo mutation occurs, it could lead to the production of a heterozygous offspring (capital T, small t).

5. **Conclusion**:
- Therefore, the correct example of a de novo mutation in this scenario is the production of a heterozygous genotype (capital T, small t) from two homozygous recessive parents (small t, small t). This indicates a genetic alteration that is present for the first time in the offspring.

### Final Answer:
The correct answer for the de novo mutation is the production of the genotype **capital T, small t** from the parents that are both **small t, small t**.

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