>>74662251NTA but it's all about risk at first. You risk some congenital defects because they are mostly coded by recessive genes, some of which you might share with your blood relatives. Recessive genes come into play only when your child has a copy of that gene from you and your partner.
High school-grade genetics problem: you have two plants with genotype Aa ("A" means dominant gene of, let's say, green leaves, "a" means recessive gene of black leaves). After breeding them, you get offspring with one of three possible permutations of genes: AA, green leaves with 25% probability; Aa, green leaves but carrying the black leaf gene with 50% probability; aa, black leaves with 25% probability. It's massively oversimplifying but the same kind of dice roll is going on with every gene in your body.
Habsburg-tier deformities begin over many generations of accumulating failed dice rolls, ignoring all the children who died in the womb and shortly after birth.
