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Should first cousins be allowed to have children?
Introduction
2009 sees a significant double anniversary in science: Charles Darwin was born 200 years ago
and his groundbreaking theory of evolution by natural selection was first published 150 years
ago. Understandably therefore, the media spotlight has been pointing in his direction rather a
lot recently. However, the focus of this attention has been largely on his work rather than his
personal life. So it may come as a surprise to some to learn that he was married to his first
cousin, Emma Wedgwood, with whom he fathered 10 children. Albert Einstein was also married
to his first cousin.
Today, the practice of marrying one’s first cousin is illegal in several countries (and many states
in the US) and whilst it is currently legal in the UK, there is a social stigma attached to it. The
reasoning behind these feelings is supported by scientific evidence and explanation, which we
will look at in the course of this article. However, two scientists from the US and New Zealand
have called for the taboo to be lifted. This is following research showing that the risk of having a
baby with genetic defects is no smaller in women who become pregnant over the age of 40 than
it is in first cousin families. Their argument is that few would question the right of a 40 year old
woman to have a baby, so why should anyone question first cousin families?
The science
Incest is very rare in wild mammals. Humans also instinctively avoid such relationships. I’m
sure that most of us would agree that the idea of copulating with a member of our close family is
repulsive! What is the genetic basis for this?
We all inherit 22 pairs of homologous chromosomes. We inherit one homologue of each pair
from our mother and the other from our father. The same genes are present at the same
positions (loci) on each homologue, although the form of the genes (alleles) differs.
The process of copying DNA is complicated and therefore often leads to mistakes in replication
called mutations. These mutations are often harmful (deleterious). However, the negative effects
of the mutated alleles are usually masked by a normal allele, present on the other chromosome
of the homologous pair. Having two different copies of an allele makes the individual
heterozygous. Deleterious alleles are usually recessive, which means that the harmful effects will
only be seen if two copies are inherited, one on each chromosome. If this happens, the
individual is said to be homozygous recessive for that allele.
If our parents come from different families the chance of such homozygosity is very low.
However, first cousins, by definition, share one set of grandparents. Therefore, the chance that
they both receive the same recessive allele from their grandparents is much higher. Neither
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cousin exhibits the negative effects of the recessive allele because they are likely to be
heterozygous. However, the problem arises if they decide to have children, because each cousin
has a 50% chance of passing on the recessive allele to their offspring. Therefore, the chance of
the baby inheriting the recessive allele from both parents is 25%. If this happens, the baby will
be homozygous recessive for that allele and will exhibit the effects of the genetic defect.
In Bradford, around 55% of people of Pakistani descent marry their first cousins. Peter Corry, a
paediatrician at St Luke’s Hospital in Bradford, estimates that children born in these first cousin
families have a 10 to 15 times greater risk of genetic disorders than the general population.
Another study reported a lower, but still significantly increased risk, of a little less than 3%,
whereas it has also been shown that infant mortality is increased by 1.2% among children of
first cousins.
Discussion
It would be interesting to see how much greater the risk of having children with genetic defects
would be if the mother was over 40 and the father was her first cousin. It may well be true that a
first cousin couple in their early twenties have a similar risk of conceiving a child with a
congenital abnormality as a woman in her forties, but that is still significantly higher than a nonfirst cousin couple of their own age.
A different argument against banning first cousin families is that genetic screening technology is
developing very quickly and it will soon be relatively easy to detect any genetic abnormalities in
a developing foetus. However, the religious beliefs of the very societies that tend to encourage
first cousin families may prohibit such screening and even if they didn’t, the question of whether
to terminate pregnancies might not be open to discussion. Furthermore, people’s ability to
interpret the results of such screening may be limited so education would to be required. Yet,
this raises ethical issues: do we have the right to ‘educate’ people in a way that might question
their beliefs? First cousin marriages aren’t always a result of cultural practices. For example,
some rural communities are very small and first cousin partnerships are almost a necessity in
order to maintain the population.
Genetic screening also brings up moral dilemmas. For example, the results of the screening
procedure would also reveal the gender of the foetus. In some societies, a baby of one gender is
more highly valued than that of the other, so parents may feel under pressure to terminate a
pregnancy regardless of whether the foetus has congenital defects. Also, where is the line
drawn? One genetic abnormality may be seriously detrimental or even fatal, whereas another
may have relatively innocuous effects on the child. Who decides what is viable and what isn’t?
This isn’t such a big step away from the ethical minefield of ‘designer babies’.
In conclusion, first cousin couples have a significantly increased risk of having a baby with
genetic defects. However, some societies rely on first cousin marriages, whether for cultural
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reasons or because of practicality. Furthermore, scientists have shown that the risk may not be
as high as once thought. Indeed, it may be no higher than in a woman of 40 in a non-first cousin
relationship. Do we really have the right to ban the most fundamental animal instinct,
procreation? Medicine has now developed ways to reduce the risk of giving birth to babies with
genetic abnormalities, but the procedures involved raise certain ethical questions. Is the longheld stigma associated with first cousin marriages out of date, or should we protect children
from the increased risk of genetic abnormality?
Excercises
Two first cousins decide to have a child together. Unknown to them, they are both carriers of the
same three deleterious recessive alleles, which are present at different loci on different
chromosomes. Work out what the chance (probability) is of the baby being born:
1. Homozygous recessive for just one of the alleles
2. Homozygous recessive for two of the alleles
3. Homozygous recessive for all three of the alleles
4. Without showing any genetic defects
5. Exhibiting at least one genetic defect
Sexual reproduction is very effective at promoting intraspecific variation by shuffling genes.
6. What does intraspecific mean?
7. Name three features of sexual reproduction that generate genetic variation.
8. What is the evolutionary advantage of this?
9. Endangered species may suffer from inbreeding depression. What does this mean?
10. If you were a doctor, working for the NHS, having to treat a baby with the symptoms of a
genetic disorder, caused because the parents were first cousins, how would you feel
about it? This theme would also suit a classroom discussion.
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