Intersex

Hermaphroditus,
wall painting from Herculaneum

In the previous post I mentioned that in biological chimeras (individuals formed by the fusion of two independent fertilized eggs) it may happen that most of the body belongs to one sex, but the genital organs belong to the other. This phenomenon is called pseudohermaphroditism. It can also be the case (although it is rarer) that the same individual has the two genital organs, complete or incomplete. This phenomenon was formerly called hermaphroditism, name of a son of Hermes and Aphrodite in Greek mythology, but these cases have recently been included in the concept of intersex, a more general term that covers all cases that do not fit the usual definition of male or female bodies, reserving the word hermaphroditism for animals or plants where that condition is normal.

To clarify things, normal members of the human species have 46 chromosomes (23 pairs), with pair 23 formed by the two sex chromosomes, which are responsible for the differences between the sexes. They can be either two X chromosomes (one inherited from the mother, the other from the father); this genetic endowment is called XX and the individual is female. Or they can be an X chromosome (inherited from the mother) and a Y chromosome (inherited from the father); this genetic endowment is called XY and the individual is male. But in addition to these two cases there are others, much less frequent. Let us quote a few (frequency figures are taken from Wikipedia):

The slaughter of the innocents

This image shows a possible result of the tests
described in the post. Black figures represent
pregnant women who tested negative.
Red figures are false positives.
Only the white figures are true positives.
Above-left are the results
for the Down syndrome

A study performed a few years ago [1] describes a delicate situation regarding certain widely used medical tests. The triple test has been performed on many pregnant women in order to detect whether the fetus will have the Down syndrome and other deficiencies. When the test is positive, gynecologists often recommend an amniocentesis or a chorionic villus sampling (biopsy), but as these tests involve some risk, many pregnant women do not want to do it, and some may decide to have an abortion based only on the results of the triple test.

The Down syndrome affects approximately 0.13% of fetuses. The problem is that the triple test used to detect it has a 70% sensitivity (or what is the same, the probability of a false negative is 30%) and a 91% specificity, which means that the probability of a false positive is 9%.

How should we interpret these numbers?

Suppose the triple test is performed on 1000 pregnant women. By applying the above data we conclude that:

•         The likely number of fetuses affected by Down syndrome will be 1 or 2.

•         The probability of detecting them with the test is 70%.

•         90 women (9% in 1000) will test positive, even if their child won’t be affected by the Down syndrome.