Genetic hitchhiking is thought to be an inevitable result of strong
positive selection in a population. The basic idea is that if a particular gene is strongly selected for (as opposed to selected against), then the chunk of the genome that carries that gene will become very common in the population. The result is a local loss of genetic diversity: all (or nearly all) of the individuals in the population will have that same chunk of genetic information, whereas before the selection process acted, there might have been a lot of
variation in that chunk throughout the population. And this means that areas of the human genome that are less variable between people are suspected sites of recent positive selection. Within that chunk, there are potentially many genes and genetic elements that became more common in the population by virtue of their placement near the gene that was actually selected for. Those other genes are the hitchhikers. And it's likely that some hitchhikers are bad news – they're harmful mutations that would normally become rare or extinct in the population, but instead have become common by hitchhiking.
In the last few years, large amounts of genetic information have become available that have enabled biologists to look for evidence of such phenomena in the human genome. Specifically, two major projects have collected genetic data for the purpose of analyzing genetic variation among humans. One project, the International HapMap Project, mapped and quantified sites in the human genome that are known to vary among humans by a single genetic letter. These sites are called single nucleotide polymorphisms, or SNPs (pronounced "snips"). The project has mapped millions of these sites in a group of 270 humans representing various lineages. Another project that has made the news recently is the 1000 Genomes Project, which also seeks to provide a picture of human genetic variation using more people (more than 1000 at present) and slightly different technology. Efforts like these have taken analysis of the human genome to a new level. No longer do we merely wonder what "the" human genome is like – we can begin to learn about how genetic differences give rise to biological differences such as susceptibility to particular diseases.
23 September 2011
Harmful genes, and sneaky, too: Genetic hitchhiking in the human genome
19 September 2011
Genetic hitchhiking in English
The next post will discuss recent evidence for genetic hitchhiking in humans. So, what do we mean when we say that genes can hitchhike? To make sense of this phenomenon, we
first need to review chromosomes and sexual reproduction.
Most people know that sexual
reproduction creates offspring that are genetically distinct from
both of the their parents. That's true, but the genetic scrambling
that occurs is more significant than is sometimes reported. Let's
start by looking at chromosomes.
Like every other animal (or plant or pretty much any other organism),
your genetic endowment is carried in chunks of DNA called
chromosomes. You have 23 of these chunks, which are rather like
volumes in a set of encyclopedias. More completely, you have 23 pairs
of these volumes; one set was contributed by your mother and the
other by your father. Each of your parents had a complete set, also
consisting of a set from Mom and a set from Dad. When your mother
made the egg that became the zygote that became you, she provided you
with one copy of each volume in the set, and she chose those copies
randomly. For example, she may have chosen her dad's copy of
chromosome 1, but her mom's copy of chromosome 2. Just by virtue of
this random picking process, she made an egg with a shuffled version
of her own genetic cards. Dad did the same when he made his sperm,
and so your genetic complement is an amalgamation of your parents'
genomes which were amalgamations of your grandparents' genomes, and
so on.
13 September 2011
"The stamp of one defect": an endless series on harmful mutations
Not surprisingly, Hamlet weighed in on the nature vs. nurture question, at least once.
So, oft it chances in particular men,
That for some vicious mole of nature in them,
As, in their birth,―wherein they are not guilty,
Since nature cannot choose his origin,―
By the o’ergrowth of some complexion,
Oft breaking down the pales and forts of reason,
Or by some habit that too much o’er-leavens
The form of plausive manners; that these men,
Carrying, I say, the stamp of one defect,
Being nature’s livery, or fortune’s star,
Their virtues else, be they as pure as grace,
As infinite as man may undergo,
Shall in the general censure take corruption
From that particular fault: the dram of eale
Doth all the noble substance of a doubt,
To his own scandal.
– Hamlet, Act I, Scene IV, The Oxford Shakespeare
It is certainly true that "the stamp of one defect" can wreak havoc on the scale that Hamlet describes, and whether the result is a debilitating physical limitation or damage to "the pales and forts of reason," the outcome is tragic by any measure.
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