Why can't patterns of inheritance in humans be as easily as in peas or fruit flies?

Humans produce few offspring slowly.

Explanation:

One of the reasons that patterns of inheritance in humans cannot be studied as easily as in peas or fruit flies is our generation time is approximately 20 years. Is it is about three months in peas and it is about 30 days in fruit flies. All the genes in fruit flies have been mapped. Not all of the genes in humans have been mapped.