Should states repeal their laws banning first cousin marriage?
-- Effects of first cousin marriage on genetic disease and mortality of offspring
Dr. Ingrid Waldron, Department of Biology, University of Pennsylvania, 20111
You may want to use the following figure to help students understand that the increased risk for
offspring of first cousin marriages is that both parents will have inherited the same harmful
recessive allele from their shared grandparents resulting in increased risk that their children will be
homozygous for a harmful recessive allele. The increase in risk does not affect conditions due to
dominant alleles or X-linked recessive alleles for male offspring since in both those cases only one
copy of the allele is required to produce the condition.
(Figure from Klug and Cummings, Concepts of Genetics; note that filled in squares and circles indicate
shared ancestors of the first cousin parents who may be heterozygous carriers, as well as the potentially
affected child of the first cousin marriage.)
A modified version of this figure can also help students understand why the increase in risk of birth
defects and genetic diseases for offspring of first cousin marriages is higher in populations where
first cousin marriage have been common for many generations. In that case, the great grandparents
may also be cousins who are both heterozygous for the same harmful allele, thus substantially
increasing the probability of homozygous recessive conditions in the offspring of the first cousin
marriage.
(Continued on next page)
1
These teacher notes, the related student handout, and other activities for teaching biology are available at
http://serendip.brynmawr.edu/exchange/bioactivities.
Current evidence indicates the following levels of risk:
% for Difference Relative risk
offspring of (% for offspring of (ratio: offspring of first cousin
non-first cousin first cousin - non-first marriages / non-first cousin
marriages cousin marriages) marriages)
significant birth
defect ~1-2% ~2-2.5% ~2-3
mortality by age
10
- developed ~1-4% ~3-4% ~1.5-2.5
countries
- poorer
countries ~20-30% ~3-4% ~1.1-1.2
- high risk
populations ~10-30% ~10-25% ~1.5-3
Notice that, for birth defects and child mortality in developed countries, the level of risk may seem
larger when considering relative risk (a doubling of risk) than when considering the difference (an
increase of 2-4%). For poorer countries, the mortality for non-first cousin marriages is higher, so a
similar increase in mortality for offspring of first cousin marriages results in a lower relative risk.
In the discussion of students’ policy opinions, you may want to include the information that women
in their early 40s have an equivalent risk of birth defects as first cousin couples, but there are no
legal restrictions on older women reproducing. Even more striking, a person with Huntington's
disease (due to a dominant allele) has a 50% risk of passing this on to each child, but again there are
no restrictions on their reproduction.
The Commission on Uniform State laws has recommended repeal of bans on first cousin marriages.
Genetic counselors recommend that first cousin couples and offspring receive the usual genetic
screening for genetic problems common in their ethnic population and family and also neonatal
screening for metabolic errors and hearing loss, since early intervention is helpful for these
conditions.
In some poor countries there appear to be significant social advantages for first cousin marriages,
including reduced dowry, domestic violence and divorce rates. However, if first cousin marriages
have been common in previous generations, there may be substantially increased biological risk for
the offspring of first cousin marriages.
Teaching Points:
factors that affect level of risk of first cousin marriage in different populations
relative risk versus absolute increase in risk can result in different impressions of whether
risk is acceptable
evaluating evidence pro and con in making decisions about social policy
Bibliography
-- Kershaw, "Shaking off the Shame", New York Times (section D), November 26, 2009
-- Bittles and Black, "Consanguinity, Human Evolution, and Complex Diseases", PNAS Early
Edition, www.pnas.org.cgi.doi/10.1073/pnas.0906079106