genetics
Explanation
The Joke
A man shares a fun genetics fact with a couple: if you're a twin and you marry another twin, then your twin's kids are genetically as related to you as your own children would be. This is because identical twins share 100% of their DNA, so from a genetic standpoint, your twin's offspring carry the same genetic relationship to you as your own would.
The punchline comes when the man asks, "Why do you keep bringing this up?" -- revealing this is not idle trivia but a repeated suggestion. His wife, introducing herself as "Jeane" to the other woman, suggests that their siblings could "do all the work" of raising children. The other woman retorts, "Why is it always genetics with you?" -- implying this man constantly uses genetic facts to justify lazy or self-serving proposals.
The Humor
The comedy lies in the misapplication of scientific knowledge to everyday life. The genetic fact is genuinely true and interesting in an academic context, but the man is weaponizing it as a scheme to avoid the effort of parenthood by outsourcing child-rearing to his and his wife's twins. It satirizes the type of person who uses intellectual trivia to rationalize absurd or selfish behavior, and the exasperation of those around them who have clearly heard this pitch before.