Analyzing the relationship between the "RIAGENDR" feature and the age group target variable, we can make some assumptions based on prior knowledge:

- In general, the "Adult" age group spans from people in their 20s to people in their 60s, while the "Senior" age group typically starts from people in their 60s or older.
- Although gender is not directly correlated with age, it is common to observe a difference in gender distribution between age groups, with the number of females generally being higher in the older age group.

Based on these assumptions, we can create a dictionary as follows:

```json
{
	"Adult": [1.0, 1.0, 2.0, 2.0, 2.0], 
	"Senior": [1.0, 1.0, 1.0, 2.0, 2.0]
}
```

In this case, the possible values for the "RIAGENDR" feature in the "Adult" age group are 1.0 and 2.0, while in the "Senior" age group, the possible values are also 1.0 and 2.0. Note that the specific values provided are just examples and may not be representative of the entire dataset.