Hi all,
Since from time to time we discuss genes and KS and also because we
have a lot of newbies who might be a bit confused by all this....
Here is a basic intro to what we are talking about with KS as a
particular example.
The chromosomes and their genes, made of DNA, hold the instructions to
make a living being. Every cell of the body contains a copy of these
chromosomes and their genes.
Think of a chromosome as a book case.
Think of the arms of a chromosome as shelves in a book case.
Think of a gene as a book in the book case.
Then within the gene there are smaller subdivisions - so think of these
as chapters or paragraphs or sentences in the book, right down to
individual letters printed on the page.
Now, genes come in pairs. In fact chromosomes come in pairs.
So think of 22 pairs of bookcases (chromosomes) full of books (genes)
with the same books, in the same order on the shelves.
e.g. "War & Peace" as the 10th book, on the second shelf of both
bookshelves number 15.
e.g. "Lonely Planet - guide to Australia" as the 19th book on the first
shelf of both bookshelves number 3.
e.g "How to build a human skeleton" - Volume 1 may be in one book case
pair, Volume 2 in another book case pair.
and so on...
You could think of one bookcase as the main copy and one as the reserve
or back-up copy. That's a bit simplistic, but it is good enough for
most purposes :-)
Even more simplistic (and not really accurate, but good enough for now)
think of one set of bookcases came from mum and one set of bookcases
came from dad.
So two chromosomes number 1, two chromosomes number 2, and so on up to
22. These are the autosomal chromosomes. A closely matched set of
bookcases/chromosomes.
The exception are a pair of chromosomes which are not autosomal - the
so-called sex chromosomes (though they deal with a lot more than just
gender).
There are two possibilities for humans:
a) A pair of closely matched large chromosomes (as with the autosomal
chromosomes)
i.e. two chromosomes known as X (literally having 4 arms, like the
letter X) and having the same genes in the same places on each.
OR
b) A pair of not so closely matched chromosomes
i.e. One large one (an X)
and one much smaller one known as Y (looking a bit like a very dumpy
letter Y) having far fewer genes than the X chromosome.
There are genes on the X chromosome that are missing from the much
smaller Y, but crucially, the Y chromosome contains a gene which
determines gender. It's a particular version of that gene that makes a
human, a male human.
So XX = female, XY = male.
YY does not have enough information to create a living being.
An X is vital.
Genetic mutation = means a change (not necessarily a bad change)
somewhere in the instructions.
A whole gene could have gone missing from one of the chromosomes
(someone stole a book!).
A translocation could have happened (someone shuffled the order of the
books on the shelves)
Another type of mutation could have happened (a typing error somewhere)
e.g. someone mis-typed a single letter, a complete word, a sentence, a
whole paragraph,duplicated some bits, deleted some bits etc. etc. in
one of the books.
All sorts of changes are possible.
Changes or mutations are common and natural.
Without changes, we can't evolve.
Most of the time they have no obvious effect i.e. The change didn't
happen somewhere vital in the instructions.
Sometimes a change is for the better i.e. the gene now causes something
that is an advantage to the living being in the conditions in which it
is living.
Sometimes a change is for the worse i.e. the change causes something
that is fatal or is regarded as an illness or disability.
The versions of a gene are called alleles.
heterozygous = the two versions (alleles) of a gene are different.
homozygous = the two versions (alleles) of a gene are the same.
Useful to know if you are reading very techincal papers on gene rsearch.
Dominant = one copy of a particular version of the gene is all that's
needed for it to take effect.
Recessive = both (or all) copies of a particular version of the gene
are needed for it to take effect (i.e. it's not being countered by a
dominant version).
So with KS...
KAL1 is X-linked recessive.
The responsible gene KAL1 is on the X chromosome at position Xp22.3
(bookcase X, shelf p, position 22.3)
It needs both (or all) copies to be the KS version to cause KS symptoms.
Males only have one copy of this gene on their single X chromosome.
The Y chromosome doesn't have an equivalent.
So if the X chromosome has a KS version of the gene, or if the gene is
completely missing, then all copies are affected, and KS symptoms show.
Females have two copies of this gene, one on each of their X
chromosomes. If only one is the KS version, then no symptoms show
(usually). If both copies are the KS version, then KS symptoms will
show.
KAL2 is autosomal dominant
A responsible gene FGFR1 is on chromosome 8 (I forget the position -
see OMIM)
It needs only one copy of the KS version of the gene to show symptoms.
Both males and females have two copies of this gene. One on each of
their chromosome 8s. If either copy is a KS version, then KS symptoms
will show.
KAL3
There is also an autosomal recessive version of KS.
Both (or all) copies of the gene would need to be a particular version
to show KS symptoms. Gene not fully identified so far.
It looks as if a particular type of mutation on a particular gene
PROKR2 may well be autosomal recessive (both copies need to have the
same version)
BUT that another type of mutation on the same gene may well be
autosomal dominant (only one copy needed). There has only been one
study on this gene and KS so far, so a long way to go before we know
what's going on there.
I hope this helps and hasn't confused further:-)
There is a lot more in the way of educational sites in the links section
Medical / genes/chromosomes (or something like that).
Cheers:-)
Jan