Consanguinity Fact Sheet -- Debunking Common Myths

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An article by Robin Bennett, MS, CGC debunking common myths of consanguinity.

Consanguinity -- Debunking Common Myths

Robin Bennett, MS, CGC

Background

Consanguinity is a term used to describe the relationship between couples who share at least one common ancestor. The term incest is defined differently in biological and legal settings. Incest usually refers to relationships between first-degree relatives, for example parent-child or sibling-sibling, but may include relationships between grandparent-grandchild or uncle-niece and aunt-nephew relationships. Legal definitions of incest may include unions between nonbiological relatives, such as stepfather and stepdaughter or between stepsiblings.

Table: Example -- Consanguineous Relationships and the Proportion of Shared Genes

Table: Example -- Consanguineous Relationships and the Proportion of Shared Genes

Relationship Example Relationships Shared Genes Pedigree
First degree Parent-child

Siblings
1/2
Second degree Half siblings

Uncle-niece

Aunt-nephew
1/4
Third degree First cousins

Half uncle-niece

Half aunt-nephew
1/8
Fourth degree First cousins once removed

Half first cousins
1/16
Fifth degree Second cousins 1/32

(Adapted from Bennett et al., 2002, Figure 1)

Although marriages between close relatives are discouraged (or even illegal) in North America, in many cultures (particularly in the Middle East, Asia, and Africa) preferred marriages are between first cousins or, less commonly, between an uncle and niece or double first cousins. In fact, in some parts of the world, 20% to 60% of all marriages are between close biological relatives (Bennett et al., 2002).

In the United States, there is significant stigma associated with consanguineous marriage. Consanguineous couples may keep their relationship hidden because of fears of stigma, discrimination, ostracism, and even legal prosecution. These beliefs are often promulgated in the medical community, where the roots of misconceptions about the consequences of consanguineous unions are deeply ingrained. A New England physician in 1859 noted:

"Perhaps no opinion upon subjects of a medical character is more widely diffused among the public, or more tenaciously held, than that the results of the marriage of blood relations are almost uniformly unfortunate. This opinion has been so long held and so often reiterated by sheer force of these circumstances alone it has come to be regarded as an unquestioned and unquestionable fact" (Bell, 1859).

First Cousins

(Third-Degree Relatives)

Risk above general population:

  • 1.7% to 2.8% for significant congenital defects
  • 4.4% for pre-reproductive mortality

Consanguineous Relationship Risk Data

In fact, couples who are first cousins are estimated to have about a 1.7% to 2.8% increased risk for significant congenital defects above the population background risk. There is approximately a 4.4% increased risk for pre-reproductive mortality above the background risk, some of which includes major congenital defects (Bennett et al., 2002). The general population risk for significant birth defects varies among populations because of their varied sociodemographic characteristics and how various studies define adverse health outcomes in the first years of life.

The offspring of consanguineous unions may be at increased risk for genetic disorders because of the expression of autosomal recessive gene mutations inherited from a common ancestor. The closer the biological relationship between parents, the greater is the probability that their offspring will inherit identical copies of one or more detrimental recessive genes (see table above). Offspring of consanguineous unions may also be at increased risk for disorders of multifactorial or complex inheritance; however, well-controlled studies evaluating the effect of consanguinity on multifactorial diseases of childhood and adulthood have not been conducted (Bennett et al., 2002). Chromosome anomalies do not occur more commonly in the offspring of consanguineous unions.

Incestuous Unions

(Usually First-Degree Relatives)

Risk above general population:

  • 31.4% for death and severe defect (4 data sets)
  • 6.8% to 11.2% for significant birth defects (extrapolated from first-cousin data)

Incestuous Relationship Risk Data

The published data on the risks to the offspring of incestuous unions are fraught with major ascertainment biases (such as lack of paternity documentation, young maternal age, possible parental diseases and/or intellectual impairment, parental socioeconomic status or lack of reporting this variable, and complications of unsuccessful attempted pregnancy termination) (Bittles, in press). The 4 most comprehensive studies published on the consequences to the offspring of incestuous relationships include only 213 children! These studies are limited in the number of years that the incestuous progeny are followed. Combining the 4 sets of data, the excess level of death and severe defect in the offspring of incestuous unions (a proportion of which may have been nongenetic in origin) was 31.4%. Another method of predicting this risk is to extrapolate from the data on the predicted risk to the offspring of first-cousin unions and amplify the risk based on the closer degree of relationship, thus estimating a 6.8% to 11.2% risk above the population background for significant birth defects (Bennett et al., 2002).

Genetic Counseling

The simplest and most comprehensive tool for providing genetic screening to consanguineous couples and their offspring is to obtain a medical family history covering 3 to 4 generations from the couple (Bennett et al., 2002). A referral to a genetic specialist (genetic counselor or medical geneticist) is important to help identify appropriate testing based on the family history and the ethnic background. A listing of over 1,000 references to inherited disorders that have been described in specific population groups is available at Consang.net . When a known or suspected genetic condition is identified in a fetus or newborn of a consanguineous union, the genetic evaluation can proceed as it would for a nonconsanguineous couple.

In addition to taking a family history and offering appropriate carrier screening for autosomal recessive disorders (based on family history and ethnic background), the screening plan for the offspring of parents related as second cousins or closer includes the following:

  • Pregnancy
    • high-resolution ultrasound at 20 to 22 weeks
    • maternal-serum marker screening at 15 to 18 weeks
    • consideration of first trimester screening as the sensitivity and specificity of this technology improves
  • Newborns
    • in addition to the standard neonatal screening tests, supplemental neonatal screening by tandem mass spectrometry should be offered by age 1 week, with the goal of identifying potentially treatable inborn errors of metabolism
    • hearing screening should be offered by age 3 months to identify hearing loss and to implement subsequent language intervention
    • assure standard pediatric follow-up care as outlined for all children by the American Academy of Pediatrics

A summary of these recommendations can be found in the consensus document of the National Society of Genetic Counselors, which is summarized on the National Guidelines Clearinghouse website (Bennett et al., 2002).

Given the stigma associated with cousin unions in the United States and Canada that has little biological basis, health providers should provide supportive counseling to these families and respect cultural belief systems. It is legal to marry a cousin in about half of the states.

See CousinCouples.com for information on states that allow cousin marriage.

The laws are quite varied as to what relationships are allowed. Each state's genetics coordinator can provide information about the laws by individual states (see the Coalition of State Genetics Coordinators ). Couples often find contact with the support organizations for cousin couples reassuring (see CousinCouples.com).



References

Bell J. The effects of the consanguinity of parents upon the medical constitution of the offspring. Boston Med Surg J. 1859;60:473-484.

Bennett RL, Motulsky AG, Bittles A, et al. Genetic counseling and screening of consanguineous couples and their offspring: recommendations of the National Society of Genetic Counselors. J Genet Couns. 2002;11:97-120.

Bittles AH. Genetic aspects of inbreeding and incest. In: Durham W, Wolf A, eds. Incest, Inbreeding, and the Knowledge at the Turn of the Century. Stanford, Calif: Stanford University Press; in press.

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