Stephen Matheson

[[![ResearchBlogging.org](http://www.researchblogging.org/public/citation_icons/rb2_large_gray.png){style="border: 0;"}](http://www.researchblogging.org/)]{style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"}[Genetic
hitchhiking](http://sfmatheson.blogspot.com/2011/09/genetic-hitchhiking-in-english.html)
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](http://hapmap.ncbi.nlm.nih.gov/thehapmap.html.en), 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](http://www.1000genomes.org/about), 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](http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1000293).  
  
[Continue
reading\...](http://sfmatheson.blogspot.com/2011/09/harmful-genes-and-sneaky-too-genetic.html#more)


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.

Harmful genes, and sneaky, too: Genetic hitchhiking in the human genome

The next post will discuss [recent evidence for genetic hitchhiking in
humans](http://www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1002240).
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.  
[Continue
reading\...](http://sfmatheson.blogspot.com/2011/09/genetic-hitchhiking-in-english.html#more)


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.

Genetic hitchhiking in English

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.  
>   
>
> ::: {style="text-align: center;"}
> -- *Hamlet*, Act I, Scene IV, [The Oxford
> Shakespeare](http://www.bartleby.com/70/4214.html)
> :::

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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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Reflecting on the reality of inherited dysfunction, we might be tempted
to assume that a \"vicious mole of nature\" is something seen only \"in
particular men,\" and that those who are not so characterized (let\'s
call them \"normal people\") have been dealt a genetic hand that lacks
such devilish cards. Normal people don\'t have bad genes.

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Okay, so in the real world I suspect that most people are not so naïve;
if you\'re reading this blog, then you probably know that bad genes can
be carried by normal, healthy people. Nevertheless, when we think about
bad genes -- or more technically, deleterious mutations -- we are likely
to think that they are not very common.

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[Continue
reading\...](http://sfmatheson.blogspot.com/2011/09/stamp-of-one-defect-endless-series-on.html#more)


Not surprisingly, Hamlet weighed in on the nature vs. nurture question, at least once. 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.

Rogue Scholar Beiträge

Harmful genes, and sneaky, too: Genetic hitchhiking in the human genome

Genetic hitchhiking in English

"The stamp of one defect": an endless series on harmful mutations