overall frequency of the developmental defects.
People have interpreted these in complex and
emotive ways in the past. The Romans regarded them
as warnings from the Gods. They had effectively,
I made this name up I admit, a bureau of two-headed
calves. If the number of animals born with
abnormalities increased, they interpreted
that as a sign of the wrath of the Gods and
could actually take political action as a
consequence of what they feared was going on.
At the same time, continuing too much
later, there was the idea that a developmental
abnormality was a consequence of some wrongdoing
on the part of the parents, of the mother,
or indeed, the father that it was a consequence
of sin or immorality. Then more recently still,
many people may still believe that there are,
what I’ve called, proximate emotional causes.
So the idea that if you are frightened by
a spider, that you might have a spider-shaped
birthmark, or in the classic story of the
elephant man, it was suggested that his mother
had been frightened by an elephant during the
course of pregnancy. And somehow, by sympathetic
magic that would lead to a defect. So, eating
strawberries might give that kind of skin
mark called a strawberry nevus. All of these,
of course, are completely untrue.
In fact, I find animal analogies quite unhelpful in
talking about developmental events. So there’s
still a tendency to draw parallels between
human developmental defects and other forms
of animals, and this is irrelevant. There’s
no connection between them, whatsoever.
So in looking at the causes, we know very
little, but what do we know? About 10% of
an identifiable major chromosomal defect and
about 8% have a single gene cause that we’ve
already identified. Environmental factors,
which often attract the most attention in
people’s minds, we only know of about 7%
of cases directly caused by an environmental
factor. About 25%, we know to be an interaction.
So, it’s between the genes and something
in the environment. And the whole 50%, we’re
still largely guessing at what’s going on.
One thing we do know is that there seems to
be an association with poverty, that societies,
where people are living in social deprivation,
have higher numbers of developmental defects
and as a result, we can conclude that there
may be in part a political solution to some
of these kinds of issues. Very powerful techniques
have a reason to identify genes which might
be involved, and as our understanding of the
genetic control of development increases,
so in turn, we begin to understand better what
some of the factors influencing developmental
abnormalities might be as well. So, human
genome project was a huge step forward to
identifying all of the genes that are involved
in possible developmental abnormalities.