Based on prior knowledge, the Thousand seed weight (MHG) can be an important factor in determining the grain yield of soybean cultivars. Generally, a higher MHG indicates a higher weight of individual seeds, which can contribute to higher grain yield. Alternatively, a lower MHG may result in smaller seeds and potentially lower grain yield.

To create the dictionary with specific details, I will assume the following ranges for the MHG values:
- Low range: MHG values below 200g
- High range: MHG values above or equal to 200g

Here is the dictionary with typical MHG values for each target class:

```json
{
    "low": [150.2, 160.5, 170.8, 180.1, 190.3],
    "high": [210.6, 220.9, 231.2, 241.5, 251.8]
}
```

Please note that these values are only examples and should be adjusted based on specific knowledge or data available.