The babies of close (say, brother and sister, or cousin and niece) family members have a higher chance of being born with an abnormality than babies from non-family related men and women. Why?

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    $\begingroup$ Who says this is a homework question? $\endgroup$ Jan 21 '20 at 15:52
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    $\begingroup$ Please do not edit in content to questions that is not part of the question. The homework policy here doesn't only cover assignments specifically given as homework, it covers other types of school work as well as questions that need further research effort. "This isn't homework" does not make a question subject to different rules. $\endgroup$
    – Bryan Krause
    Sep 11 '20 at 18:35

Many disease/abnormality causing alleles of a genes are maintained in populations through their recessive nature. That is to say, in order for the disease to manifest itself in offspring they must in have TWO copies of the disease causing allele - they must be homozygous for the disease causing allele. Inbreeding leads to a far greater chance of being homozygous at a disease locus if a recent common ancestor of the parents was affected or a carrier. The likelihood of this occurring is measured by the Inbreeding Coefficient.


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