Carl Zimmer has produced a remarkably rewarding book on
the subject of heredity: She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity. It is a brick of a book stretching to almost
600 pages, unusual for a volume aimed at a general audience, but there is not a
dull stretch in it. The length was
necessary to try to do justice to all the topics that arise. Here we will consider Zimmer’s discussion of
human height and its heritability.
Height, along with intelligence, and longevity, are desirable
characteristics that appear to have some degree of heritability. This has raised the issue that we might one
day have the technology to perform genetic engineering that could control positive
and negative characteristics in our offspring.
Zimmer’s investigations provide some insight into how likely that is to
be possible.
Part of Zimmer’s findings were covered in Average Height as an Indicator of National Social Success where the dependence on health and nourishment was examined
in its historical context. There we
learned that we in the United States were once the tallest and healthiest people
on earth. However, over the past
century, Europeans with their “misguided” notions of welfare and social
assistance, have left us in the dust as we have descended, height-wise, to
mediocrity among the wealthy nations of the world.
“How tall children grow depends
intimately on their health and diet. A child’s
growing body demands fuel both to stay alive and to build new tissues. A healthy diet—especially one rich in
protein—can meet both demands. If the
diet falls short, the body sacrifices growth for survival. Diseases can also stunt a child’s growth,
because the immune system needs extra resources to fight off infections. Diarrheal diseases are especially brutal
because they also rob children of the nutrients in their food. This fate can get locked in tightly in
infancy. As a result, the height of
children at age three correlates well with their height in adulthood.”
Our personal observations lead us to believe that tall
people tend to have tall children, but descriptions like “tend to” do not constitute
good science. And the effect of
nurture—nourishment and health—complicates the determination of the degree to
which height is heritable. The advent of
genetic mapping has provided researchers with a powerful tool to assist in
their investigations.
“Peter Visscher and his
colleagues found that pairs of siblings can vary tremendously in their genetic
similarity, sharing as little as 30 percent of their genetic variants in common
to as much as 64 percent. If a trait is
highly heritable, Visscher reasoned, then it should be more similar in siblings
who have more DNA in common.”
“In 2007, Visscher and his
colleagues examined the height of 11,214 pairs of regular [non-twin]
siblings. They found that…those who
shared more than half of their DNA—tended to grow to more similar heights. Siblings with less genetic similarity were
not so similar. The scientists used
these correlations to calculate the heritability of height. They ended up with an estimate of 86 percent.”
“That’s an exceptionally high
figure. Nicotine dependence has a
heritability of 60 percent. The age at
which women go into menopause is 47 percent.
Left-handedness is at a mere 26 percent.
In the world of heritability, height stands tall.”
Given that our heights are determined to a considerable
factor by our genetic inheritance, does that suggest that it is a quantity that
could be subject to genetic manipulation?
Could parents one day pay to provide their children with the advantage
of added height?
Genetic scanning has had some success at pinpointing causes
for a few diseases that can be associated with defects in a single gene or in
just a few genes. However, for most
diseases any correlations are too small to be easily isolated and
identified. So far that has been the
case for the attribute of height as well.
The approach is to compare genetic data at various locations
in the genome for as many people as possible in order to search for
correlations that can be linked to the height of the individuals. Only a couple of genes have been easily identified
that appear to have an effect on height, but these only represent a small
fraction of the dispersion in heights.
Very detailed studies indicate that height is correlated with numerous
locations and these are distributed throughout the genome. These studies have required examining
millions of locations in the genome of hundreds of thousands of individuals in
order to identify statistically significant correlations. Consider one ambitious study.
“…Hirschhorn’s team scanned 2.4
million genetic markers in a quarter of a million people. They looked for variants at each of the
markers with a very strong link to height—so strong that they could confidently
reject the possibility that the links were just coincidences.”
“That study gave Hirschhorn and
his colleagues a list of about seven hundred strongly supported genes. But they also found many other ambiguous
variants that didn’t quite meet their strict standards.”
Subsequently, a researcher named Pritchard went back and
applied a more rigorous statistical approach to evaluating these ambiguous
cases.
“What made this study startling
was just how many of these variants Pritchard and his colleagues found. At 77 percent of the markers they
studied—almost two million spots in people’s DNA, in other words—they could
detect an influence on height. The
markers were not clumped around a few genes on one chromosome. They were instead spread out across all the
chromosomes, encompassing the entire human genome.”
“Traditionally, geneticists have
called height polygenic—meaning ‘many genes.’
Pritchard thinks a new name is called for: omnigenic.”
It would appear that if one wished to create taller people
one must revert to the traditional breeding methods long used on plants and
animals. That is probably a good thing.
The interested reader might find the following articles
informative:
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