The zygosity of an organism for a particular phenotype can be identified by crossing it with

Options

A. AaBb
B. aabb
C. aaBb
D. Aabb

Answer

To determine the zygosity of an organism for a particular phenotype, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Zygosity**: Zygosity refers to the genetic constitution of an organism regarding a specific trait. It can be either homozygous (having two identical alleles for a trait) or heterozygous (having two different alleles for a trait).

2. **Identify the Phenotype**: The phenotype is the observable characteristic or trait of the organism. For example, if the organism is tall, we need to determine if it is homozygous tall (TT) or heterozygous tall (Tt).

3. **Perform a Test Cross**: The most effective way to determine the zygosity is to perform a test cross. This involves crossing the organism in question with a homozygous recessive individual (which expresses the recessive phenotype).

4. **Analyze the Results**:
- If the organism is homozygous (e.g., TT), all offspring will exhibit the dominant phenotype (e.g., tall).
- If the organism is heterozygous (e.g., Tt), the offspring will show a 1:1 ratio of dominant to recessive phenotypes (e.g., tall and dwarf).

5. **Select the Correct Option**: From the provided options, we need to identify which one includes a homozygous recessive individual that can be used for the test cross.

- **Option A**: A, a, B, b - No recessive individual.
- **Option B**: a, a, b, b - This is a homozygous recessive condition (aa bb).
- **Option C**: a, a, B, b - This is not a recessive condition.
- **Option D**: A, a, B, b - This includes a dominant allele.

The only option that provides a homozygous recessive individual is **Option B** (a, a, b, b).

### Conclusion:
Thus, the correct answer is **Option B**: small a, small a, small b, small b.