Hi there!
Thanks for your question. The term allele is from the world of
genetics. As you will be aware, our genes are inherited from our
parents and determine all of our individual characteristics. Read this
extract from the BBC's Gene Stories site:
"The gene for a particular feature may come in several alternative
versions called alleles. Although they will both govern the same
feature - eye colour or hair colour, for example - different alleles
will have slightly different DNA sequences, so that they code for
slightly different versions of a protein.
Some are dominant (or over-riding) and others are recessive (or beaten
by more dominant genes). If an individual has two alleles that are the
same they are said to be homozygous. Those with different alleles for
the same gene are said to be heterozygous. The existence of different
alleles for a gene is known as genetic variation. Genetic variation is
responsible for much of the physical variety we see between all living
things, including ourselves. The total variety of all the alleles of
all the genes within a population of individuals is known as the 'gene
pool'.
For example if you inherit an allele for black hair from each of your
parents then your hair colour will be dark because the hair colour
alleles your parents passed on to you were homozygous, or the same
(i.e. both dark).
But you could have two parents with black hair yet have red hair
yourself. This would be because, despite each carrying one of the
black hair alleles and having dark hair themselves, your parents both
also carried the allele for red hair (inherited from their parents)
and passed it on to you. The allele for black hair is dominant while
red is recessive, so red didn't come out in your parents. But the red
allele came out in you because your hair colour alleles were both for
red (homozygous) and there was no black to dominate.
It may seem strange that genes should be so varied, but in fact it's
vital to the way the natural world functions. Genetic variation is the
fuel of evolution: without it and the driving force of natural
selection ('survival of the fittest') we wouldn't see the massive
diversity of life on earth today. In any situation, the most fitting
genetic variation has an advantage - and so may survive where others
do not."
- http://www.bbc.co.uk/health/genes/basics/inheritance_2.shtml
Read the BBC's full pages on genes at
http://www.bbc.co.uk/health/genes/
This explains how traits can skip generations. If I was an albino
(which is a recessive characteristic), then my two alleles would be
'aa'. I need to have them both to have the condition. Let's say I
marry a non-albino woman. She is not albino, but she still might
'carry' the condition. If so, her alleles would be 'Aa'. We would each
pass on a random one of our two alleles to our child. There would
therefore be a 50% chance of our son being albino, since if my wife
passes on her 'A' then he will not be albino, and if she passes on her
'a' then he will be. If an albino man has a child with an albino
woman, there is a 100% chance of their son being an albino.
This is why it's generally a bad idea to have children with people
you're already related to - recessive alleles are more likely to bump
into each other.
Here is a good page that uses diagrams to explain how this operates:
Mount Sinai Hospital
- http://www.mtsinai.on.ca/pdmg/Genetics/inheritance.htm
So what's the relevance of all of this to the apoE4 allele? Well, this
is discussed here:
"To understand some of the subtleties one needs to consider just what
we know and don't yet know about the Apolipoprotein E gene. We know
that this gene exists in three major isoforms or common subtypes which
have been creatively named apoE2, apoE3 and apoE4 (I don't know what
happened to apoE1). Since we have two alleles or copies of each of our
autosomal genes there are six different combinations of common alleles
found in humans. ApoE4 has been identified to be a risk factor for
both late onset Alzheimer's disease and coronary artery disease."
- http://www.umdnj.edu/genesatwork/topics/adult_medicine/01_adult.htm
So to put that in simpler English, as well has having alleles that
give us black, brown, fair or ginger hair, we have them for the
fundamentals such as protein manufacture. Alleles in a certain
combination will mean proteins are created in a different way. There
are three possible combinations for alleles to exist in the
Apoliprotein E gene. The third of these, confusingly called apoE4, is
more common in people who develop Alzheimer's disease and coronary
artery disease. Here are two other articles about this.
APOE4 allele in Alzheimer's Disease
- http://www2.umist.ac.uk/optometry/dept/molecular.htm
Is the apoE4 allele an independent predictor of coronary events?
- http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=0011152862&dopt=Abstract
I hope the information I've posted here is what you were after. All
Google Answers researchers pride themselves on the quality of their
answers. If you believe there is something that I've missed, or you
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for your question.
Yours
jumpingjoe-ga
Google Answers Researcher |
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