Genes can be dominant or recessive. If you have two variants of a gene and one is dominant then only the dominant will be active.
Recessive genes are more commonly negative effects, as people can carry the gene without getting it’s effect it isn’t sorted out as efficiently by evolution.
If you have offspring with closely related people then the chance that your problematic recessive genes match up with an unproblematic dominant one is lower.
So you have GOOD/bad and your sibling has GOOD/bad in the same location then there is a 25% chance your kid gets the bad/bad combination. For remotely related people that is less likely because the bad genes will rarely be in the same position to meet up.
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