In a monohybrid cross, 120 plants are obtained . The ratio of homozygous and heterozygous will be

Options

A. `40 : 80`
B. `60:60`
C. `20:100`
D. `10:110`

Answer

To solve the problem of finding the ratio of homozygous to heterozygous plants in a monohybrid cross where 120 plants are obtained, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Monohybrid Cross**:
A monohybrid cross involves a single trait that is controlled by two alleles. In this case, we can consider the tall plant (T) as dominant and the dwarf plant (t) as recessive.

2. **Gametes Formation**:
From the parents, we can derive the gametes. For example, if we have one parent as homozygous tall (TT) and the other as homozygous dwarf (tt), the gametes produced will be T and t.

3. **Punnett Square**:
We can set up a Punnett square to visualize the offspring from a monohybrid cross. If we self-cross the heterozygous offspring (Tt), the Punnett square will look like this:

```
T t
---------
T | TT | Tt |
---------
t | Tt | tt |
```

4. **Offspring Genotypes**:
From the Punnett square, we can see the genotypes of the offspring:
- 1 TT (homozygous tall)
- 2 Tt (heterozygous tall)
- 1 tt (homozygous dwarf)

5. **Counting the Ratios**:
The ratio of the genotypes is:
- Homozygous (TT + tt) = 1 + 1 = 2
- Heterozygous (Tt) = 2

Therefore, the ratio of homozygous to heterozygous is 2:2 or simplified to 1:1.

6. **Calculating the Number of Plants**:
Given that there are 120 plants, we can divide them based on the ratio:
- Total parts = 2 (homozygous) + 2 (heterozygous) = 4 parts
- Each part = 120 plants / 4 parts = 30 plants

Therefore, we have:
- Homozygous plants = 30 (TT) + 30 (tt) = 60
- Heterozygous plants = 60 (Tt)

7. **Final Ratio**:
Thus, the final ratio of homozygous to heterozygous plants is 60:60, which simplifies to 1:1.

### Conclusion:
The ratio of homozygous to heterozygous plants in this monohybrid cross is **1:1**.

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