Thank you for the opportunity to help you with your question!
Hi, SAO5423! Thanks for letting me answer your question.
1. There would be less genetic variation among gametes. Less genetic variation = less diversity (in an extreme sense, it could lead to "clones"). Keep in mind though that people wouldn't be 100% the same because random mutations can still take place.
2. Higher risk of extinction due to being too genetically similar. Think about it this way, if there's no "survival of the fittest" because everyone is the same, then something drastic such as enviornmental change could wipe out an entire population. This is known as the "bottleneck effect"
Please let me know if you need any clarification. I'm always happy to answer your questions.