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It is folk knowledge that in-breeding causes birth defects due to similarity of genetics between parents. Consequently, is there a similar correlation between genetic distance of parents and probability of birth defects?

The best reference I can find of this is this handout by the Australian Centre for Genetics Education which says:

If parents are unrelated, their chance of having a child with a birth defect or disability is between 2% and 3%.

If parents are first cousins, the chance is a little higher at 5% to 6%. This is due to the increased chance that they will both carry the same autosomal recessive mutation, passed down through the family.

However this doesn't talk about genetic distance in particular.

This question is really just a more specific version focused on a single metric (genetic distance) and outcome (birth defects) of two previously answered questions:

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  • $\begingroup$ Are you looking for any evidence of the effect of inbreeding on fitness? Are you specifically interested in humans? What is your goal? Are you trying to estimate something specific such as the number of lethal equivalents of inbreeding for example, or are you just looking for any reference showing evidence of inbreeding depression to cite? $\endgroup$
    – Remi.b
    Commented Oct 30, 2017 at 21:00
  • $\begingroup$ @Remi.b yes, I am specifically interested in humans, as per the tag. I'm looking for any evidence that can show a correlation between genetic distance and "inbreeding depression". Although, I must admit I had not heard of the term "inbreeding depression" until now and am embarrassed by this. $\endgroup$
    – Seanny123
    Commented Oct 30, 2017 at 21:56
  • $\begingroup$ It depends how much the source populations share the same defects. If chinese and euro villages parents both had high prevalence of clubfoot, they would have worse outcome than close source parents. ... besides, an Italian village of 400 people genetics was studied because they have the lowest heart problem percentage in europe. They found that nearly all the village was inbred from 2 ancestors. they were all similar and healthy. Secondly, another study found that cousins breeding is about as troubled as late motherhood at the age of 38 onwards, i.e. having one after 38 is as good as a cuzin. $\endgroup$
    – bandybabboon
    Commented Nov 2, 2017 at 17:38

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For humans the closest argument may come from observational studies (since experiments aren't possible due to ethical reasons).

One example for the correlation of genetic distance and "defects" comes from a recent study of Chagnon et al. PNAS, 2017 http://www.pnas.org/content/114/13/E2590.full . Rather than asking for birth defects directly they however survey the number of children, and survival of children to age 15 (which would be one example of the term "inbreeding depression" that is more general than "birth defects")

Note that their study also sheds some light on the underlying big question, why certain types of kin relationships can be common despite their effect on the creation of healthy offspring - and why certain types of cousin relations are different from others.

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  • $\begingroup$ I was hoping to extend the level of observation to an even larger dataset/population. Is there reasons why this dataset wouldn't exist? $\endgroup$
    – Seanny123
    Commented Nov 2, 2017 at 2:29
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    $\begingroup$ Suggestions, if want to look for large dataset: check Danish statistics, as Denmark has good records for several generations and is thus a model system for population-wide studies of health (note: kin relations will be unusual - nowadays). Since question relates to a phenomenon that isn't that exciting to contemporary research, but would seem to fit the 20ies-60ies, and large scale studies tend to get many citations: Look for old reference in the paper mentioned above. Search for reference in google scholar, select citing paper with most citations -> look at oldest reference --> iterate $\endgroup$
    – tsttst
    Commented Nov 2, 2017 at 2:47
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    $\begingroup$ Legit. Currently don't have the motivation to make the time to do that, but that's my problem. $\endgroup$
    – Seanny123
    Commented Nov 2, 2017 at 2:50
  • $\begingroup$ ad larger dataset/population: Being able to observe something within a small population makes an even stronger argument than something that is only seen in large populations (since it effect size must be relatively large) $\endgroup$
    – tsttst
    Commented Nov 2, 2017 at 14:03

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