How does inbreeding in humans cause birth defects?
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Its tied to the concept of Dominant and Recessive gene variants. Each of your genes is a combination of half from your Mom and half from your Dad. For many genes, there can be two variants, or mutations of that half. If you get a Dominant type from EITHER your Mom or your Dad, then its characteristic dominates. Sometimes, undesirable characteristics are carried by the Recessive variant. You only see that characteristic if you get the Recessive variant from BOTH your Mom and Dad.
So, (and this is a made up example), lets say that a certain type of deafness is caused by a particular (recessive) variant. Lets also say that a sister and her brother both carry this recessive variant, but they aren’t deaf, because they also carry the non-deaf Dominant variant. If the sister and brother were to mate, there would be a higher chance of their offspring getting both halves of this gene in the “deaf” (recessive) variant, resulting in a deaf child.
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