To properly understand what the genetic vulnerability factors are,
we need to start at the beginning and make sure that we are clear that
we know the answers to questions like: What is DNA?
What is a chromosome? What is a gene?
DNA stands for
deoxyribonucleic acid. This complicated name just describes the chemical
structure of the molecule. DNA is
made up 4 types of smaller unit called nucleotides. These 4 types of
DNA unit (the nucleotides) each have
a letter, they are A C G and T. The DNA in
each cell of our bodies contains 6 billion (6,000,000,000) of these
units, A, C, G and T!
To get an idea of how much that is, imagine that you are typing
out the letters. You can probably get about 1500 letters on a page,
so if you were to make a book every time you had 400 of these pages,
you would need 5,000 books for all the letters in one cell!
DNA is found in
almost every cell in our bodies. Inside each of our cells, we have 6
If you were to lay out all the DNA in
your body, from every cell, from end to end, it would stretch 9 million
kilometres! That is enough to reach to the moon and back 13 times!
is a chromosome?
This much DNA needs
to be carefully and tightly packaged so that it can fit into the cell.
So, inside our cells, DNA is
tightly packaged up as chromosomes. Most types of cell in the body have
23 pairs of chromosomes 46 in all.
The only type of cells that dont have 46 chromosomes
are the egg and the sperm cells, these cells each have just one copy
of 23 chromosomes. This is so that when the egg and the sperm cell meet
to make a new embryo, the new embryo now has 23 pairs of chromosomes
46 in all.
So, we each get one copy of each chromosome from our mothers
and one copy of each chromosome from our fathers.
The first 22 pairs of chromosomes have numbers (1-22) and are
called the autosomes. The last pair of chromosomes are
called the sex chromosomes, males have an X chromosome and a Y chromosome,
while females have no Y and instead have 2 copies of the X chromosome.
The different chromosomes can be distinguished from each other
by their size, and by the particular pattern of light and dark bands
that can be seen when they are treated with a special stain.
is a gene?
A gene is a special section of DNA
that makes a protein. Proteins are the building blocks that make up
every cell of our bodies. By making proteins, genes tell our bodies
how to grow and develop. Every chromosome has lots of these special
sections of DNA, or genes.
The genes are the instructions that tell the body whether to
make proteins for black or blonde hair, and proteins for brown or blue
eyes and whether to make proteins for A, B, or O type blood.
The genes are also the instructions that tell our bodies how
to make proteins which are different components of the brain, like the
chemicals that transmit signals from one brain cell to another (neurotransmitters),
and the special receptors for these signals that are found on the surface
of brain cells.
Within a gene, each set of three DNA subunits
(e.g. ACC or GTA), corresponds to a different protein subunit (or amino
acid). So, when the sequence of letters in the gene is read, the body
knows what amino acids to put together to make the proper protein.
It is estimated that humans have about 25,000 genes.
Each set of 23 chromosomes has a single copy of each of the
25,000 genes. So, we have two copies of each of these 25,000 genes,
as we have 2 copies of every chromosome.
So, because we inherit 23 chromosomes from our mothers and
23 chromosomes from our fathers, we also inherit genes (the instructions
telling out bodies how to grow and develop) from both our mothers and
our fathers. This explains how we have characteristics of both of our
Summary so far
DNA is packaged
inside the cell as 46 chromosomes. Genes are the special sections of
that DNA that contain the instructions
to the body about how to make a protein
do genes cause diseases?
Genes can have mistakes in them. This is quite common and everyone
will have at least some genes with mistakes in them. However, in some
people, these mistakes can sometimes cause health problems. We call
these genetic mistakes mutations.
Mutations can cause health problems because they can change
the instructions for making a protein contained within the gene. A mutation
may make the genes instructions incorrect, or it might make the instructions
make no sense. This is shown below.
you remember from above, genes are read in groups of 3 letters (like
ACG, or TGA), and each group of 3 letters corresponds to an amino acid,
which the body then assembles to make a protein. So, imagine this is
theoldmansat the old man sat
Reading it in groups of 3 letters it makes sense, and so if
this were a gene, the protein would be made correctly. However, if we
make a difference in the gene (and there are a number of ways in which
this can happen, shown below) reading in groups of 3 letters no longer
makes sense, and then the protein would not be made correctly.
A letter in the gene can be changed:
old man sat
a letter in the gene can be lost:
the old ans at
a letter in the gene can be gained:
tgh eol dma nsa t
Mutations in genes can cause proteins to be made incorrectly
(this is shown in the picture below) or sometimes, the protein will
not be made at all. Mutations can happen in genes that make proteins
which are important for sending messages between brain cells. These
proteins include neurotransmitters as well as transporters and receptors
for these neurotransmitters. Researchers believe that mutations in these
genes might be an important contributing factor to a person developing
a psychosis. Mutations in these genes are the genetic vulnerability
A DNA mutation can cause a protein
to be made incorrectly
a) Correct protein
b) Incorrect Protein
You will hear psychiatrists and other health
professionals telling you that psychosis is caused by chemical imbalance
in the brain. The chemical imbalance in the brain can be caused by mutations
in genes that make these chemicals.
ASIDE you may have heard on the news
recently that if an Individual has a father who is older (over the age
of 50 at the time of conception) this can lead to an increased risk
of schizophrenia in the child. Researchers think that this is because
of the increased chance for mutations (just like those shown above)
to happen in the fathers sperm as the father gets older. The increased
chance of schizophrenia in a child who has an older father is very small.
Researchers think that usually, these genetic vulnerability
factors are not enough on their own for a person to develop a psychosis.
It is thought that several different mutations in different genes AND
some environmental vulnerability factors are needed for a person to
develop the illness. To put this another way,
it seems that usually having a single mutation is not enough to make
a person develop a psychosis.
Because having a mutation in one gene is not enough for a person
to develop a psychosis, the genes that are involved in psychosis DO
NOT CAUSE psychosis, they only confer susceptibility or increase
vulnerability to psychosis.
Genes that can increase vulnerability to psychosis
In the last couple of years, research to find mutations
in genes that might increase vulnerability to psychosis has come a long
way. Researchers around the world have been trying to find the genes
that might increase a persons vulnerability to psychosis.
Genes that make proteins that are involved with sending messages
in the brain have been found on many different chromosomes. Recently,
mutations in some of these genes (on chromosomes 1, 6, 8, 12, 13, and
22) have been found to increase the chance of a person developing certain
types of psychosis like schizophrenia or bipolar disorder. However,
each of these genes is thought to increase a persons vulnerability
to psychosis by a small proportion.
Although we can be quite confident that some of the genes that
have been found are involved with increasing susceptibility to psychosis,
we dont yet have a clear understanding of how these genes increase
susceptibility. This is because we dont yet fully understand how