Inbreeding for five generations led to production of homozygous transgenic mice.However, these homozygous males or females were infertile. Which of the following approaches is most preferable and economical to obtain heterozygous transgenic animals continuously ?

Options

A. More transgenic founder (`1^(st)` animal) should be generated.
B. Crossing (breeding) of transgenic mice with wild typw mice in earlier generations should be done for continued production of transgenic heterozygous offsprings.
C. Inbreeding should be avoided after `5^(th)` generation.
D. Homozygous transgenic mice should be mated with heterozygous transgenic mice for continued production of transgenic heterozygous offsprings.

Explanation

To solve the question regarding the most preferable and economical approach to obtain heterozygous transgenic animals continuously, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Problem**:
- Inbreeding for five generations has led to the production of homozygous transgenic mice that are infertile. The goal is to find a method to produce heterozygous transgenic mice that can reproduce.

2. **Evaluate the Options**:
- **Option A**: More transgenic founders should be generated.
- This option is not viable because generating more transgenic founders will likely lead to the same issue of producing homozygous infertile mice.

- **Option B**: Crossing of transgenic mice with wild type mice in earlier generations.
- This option is promising. By crossing homozygous transgenic mice (which carry the desired transgene but are infertile) with wild type mice (which are fertile but do not carry the transgene), the offspring will be heterozygous transgenic mice. These offspring will inherit the transgene and will be fertile, allowing for continuous reproduction.

- **Option C**: Inbreeding should be avoided after the 5th generation.
- Avoiding inbreeding does not solve the issue of infertility in the homozygous mice. The problem lies in the homozygosity resulting from inbreeding, so this option does not address the core issue.

- **Option D**: Homozygous transgenic mice should be mated with heterozygous transgenic mice for continued production of transgenic heterozygous offspring.
- This option is flawed because it assumes the availability of heterozygous transgenic mice, which are not present in this scenario. Additionally, mating homozygous transgenic mice with heterozygous mice will eventually lead to infertility again.

3. **Select the Best Option**:
- After evaluating all options, **Option B** (crossing transgenic mice with wild type mice) is the most preferable and economical approach to continuously obtain heterozygous transgenic animals.

### Conclusion:
The correct answer is **B: Crossing of transgenic mice with wild type mice in earlier generations should be done for continued production of transgenic heterozygous offspring**.

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