I want to know is marriage with half-cousin's girl more risky than marriage with a non-family girl?

Also I like to know if her parent are cousins(but from mother side), does it add more risk to this marriage or not?

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    $\begingroup$ The answers are yes and yes. However, I will have to dig up some sources before I can answer. $\endgroup$
    – Eff
    Aug 2, 2018 at 7:26
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    $\begingroup$ The marriage is obviously not an issue. The reproduction can be an issue. $\endgroup$
    – Remi.b
    Aug 2, 2018 at 15:12
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    $\begingroup$ Why is a girl connected with a girl? And she in turn marries a guy? $\endgroup$ Aug 2, 2018 at 17:15
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    $\begingroup$ @TheLastWord it's an awkward diagram, but if you read the text of the post (her parents are cousins), you'll see that the lines connecting the family on the right side of the figure and the family on the left side of the figure are parent-child relationships, like all the black lines connecting boxes in this figure. The girl at the base of the figure is the child of the boy she is connected to with the black line and the girl she is connected to with the black line. $\endgroup$
    – De Novo
    Aug 2, 2018 at 20:20
  • $\begingroup$ @TheLastWord: The (right side)girl is her mother and she is cousin with her father. $\endgroup$ Aug 6, 2018 at 15:25

1 Answer 1


The medical term here is consanguinity, or a consanguineous union. The risks of consanguinity are well studied, and based on the principle that children born to parents with shared genes are more likely to be homozygous for recessive alleles. Population level measurements of specific risks for, e.g., fetal loss, major birth defects, or other problems vary depending on the population studied. Many of these studies look at first cousins, where typical numbers are between a 1-5% greater risk for major birth defects and 4-5% greater risk for late miscarriage or death of a child before age 10. The data on earlier fetal or embryonic loss isn't conclusive. You can read more about this in the review I linked to at the end of this answer. The specific risk for a specific individual isn't on topic here, since it is a personal medical question. If you are looking for an answer to that question, I'd suggest scheduling a visit with a medical geneticist.

Your diagram depicts marriage between half first cousins once removed, with an expected shared percentage of genes of 3.125%, equivalent to second cousins, which fits the definition of consanguinity generally used in the medical literature and allows application of studies that use that definition. The fact that one of the potential parents in this marriage was also the product of a consanguineous union may increase the risk.

Consanguinity is an important risk factor for many birth defects and other diseases (see Cecil Medicine Chapter 41 and this review)

  • $\begingroup$ Thank you De Novo, but I can not understand why relationship between the girl's father and mother affects on her marriage, when that relation doesn't any issue with the boy who likes to marry with. I mean, why relationship of the right side family, can affect on this marriage when the boy has no relationship with the right side family? $\endgroup$ Aug 8, 2018 at 7:52
  • $\begingroup$ @Hasani it suggests both potential parents are part of a subpopulation with a higher rate of consanguinity and less genetic variability. This means there is a slightly higher risk that the potential parents are both carriers of the same recessive allele for some disease. Empirically, when there is more than one consanguineous union in the family tree, we see a higher rate of birth defects and other diseases than you would expect form the expected shared percentage of genes alone. $\endgroup$
    – De Novo
    Aug 8, 2018 at 16:18
  • $\begingroup$ Dear De Novo, May you explain how percent the risk of this marriage is more than a non-family marriage? $\endgroup$ Nov 8, 2018 at 16:41
  • $\begingroup$ @Hasani the specific question about the risk of defects in your child should be asked in a visit with a doctor, ideally a medical geneticist. I've edited my answer to include more about what studies have reported in general. $\endgroup$
    – De Novo
    Nov 8, 2018 at 17:19

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