Chromosomes, Genes and Inheritance
How many times have you heard someone say “Oh
you look just like your father”, or “You can
see that she is your daughter”? Too many times to count
I suspect. Have you ever wondered how this actually happens?
How we inherit our characteristics or traits from our parents?
Here you will see how genetic traits are inherited
using simplified examples and trying out some GAMES. You
will see that we are the products of our parents whether
we like it or not.
You may at this stage like to remind yourself about
DNA and Genes.
In brief, DNA is the material that carries the instructions
to make a functioning human being. A walking, talking person.
This DNA is found throughout the body. In heart cells, skin
cells, brain cells and the rest. Like all good instruction
manuals it is written in a language that the user, in this
case the body, can understand. The words in this language are
the genes. It is the information in our genes that make us,
ourselves and other people themselves. It is our genes that
will determine what characteristics we will have, from hair
colour to athleticism. It is easy for us to recognise observable
traits, characteristics that we can see. However, there are
many other traits that we cannot see directly that are carried
in our genes. These may be whether we are susceptible to diseases,
such as heart disease, diabetes or certain cancers. Scientists
and doctors are studying genes to see if they can identify
what specifically in our genes may cause or contribute to this.
So, while we may not be able to identify someone who will suffer
from heart disease just by looking at them, we may be able
to identify people at risk by looking at their genes.
So, back to our parents and inheritance; after all
they gave us our genes.
Let us go right back to when we were a twinkle in our parents’ eyes,
or something along those lines. We are the product of the egg
from our mother, and the sperm from our father getting together
and fusing. They then rapidly divide to form first a fetus
and then a baby. It is at the point of fusion that we inherit
our characteristics from both our mother and father. Rather
than thinking about genes at this stage let’s think on
a bigger scale, let’s think in terms of chromosomes.
You need to remember that if you stretched out the DNA in any
one of our cells it is over 2 metres long. Clearly, given that
our cells can only be seen under magnification, using a microscope,
the cell has to somehow package this DNA. It does this by packaging
it into bundles called chromosomes. We have 46 chromosomes
arranged in 23 pairs. However, for the purposes of inheritance
in our examples we will only consider just one pair of chromosomes
in a cell. It gets a bit busy trying to keep track of 23 pairs
at one time, but the principle is the same for each pair.
Take a look at
GAME 1:
GAME
2
|
In the game you chose a chromosome
from the mother and a chromosome from the father for
the baby to inherit. This
game was just for one pair of chromosomes. In real life,
this process occurs with all the pairs of chromosomes,
so that for each pair of chromosomes one half of the
pair is from the mother, and one half of the pair is
from the
father. This pattern of inheritance obviously occurs
with each generation. Your mother received one set of
her DNA
instructions from her mother, and she from her mother,
and so on. These sets of DNA instructions in our chromosomes
have been passed on in this way since humans first existed.
This is how we can trace our ancestors. However, like
all things that are passed on down the generations, you
may
be able to think of a particular heirloom in your family,
they can acquire changes on the way. They are clearly
recognisable as the same thing that started out, but
they have altered
on the way. Often you can chart when these changes occurred.
GAME
2: Through the Generations shows the passing down of
chromosomes through generations. |
Have
a look at GAME
3
Taking a Closer Look.
|
Now let’s be more specific and
consider the inheritance of genes rather than whole chromosomes.
If we look at the chromosomes in more detail we can see
that the DNA instructions contain specific words, the
genes.
These genes are arranged in a specific
order along the chromosome and each gene has its own
map reference, showing which chromosome it is on and
whereabouts
on that chromosome. This map is the called Human Genome Map. This is much
the same as we use other maps. For example, finding a specific place on a
town map. We might first have to locate a country, then
a town and eventually a
grid
reference for a road map. At each map position, known as loci, there are
a pair of genes. One gene inherited from the mother and
one gene inherited from
the
father. There can be two different forms of a gene or many different versions
of some genes. These different versions of the same gene are called alleles.
The gene may do the same job but might have a slightly different outcome.
In some cases the change may be very minor and may result
in the change in a characteristic,
such as eye colour. In other cases the changes may have more devastating
effects and it may mean that the gene can no longer do
its job. This may result in
a genetic disorder.
So far you have looked at the simple passing down, inheritance
of a parent's
genes to a child. Like all processes it is not always quite so simple. When
the sex
cells (remember the egg in the mother and the sperm in the father) are being
formed the alleles can swap within a pair of chromosomes. You saw this taking
place in the mother’s chromosomes in GAME
3. This swapping of alleles
slightly changes the genetic composition of the chromosome that is passed
on to the child
compared to the original chromosome in the parent. Although these differences
are subtle they increase genetic variation and ensure that we are all genetically
unique. The only exceptions to this are identical twins, as they are produced
from the same egg and sperm.
You might want to go back to GAME
3: Taking a
Closer Look.
Now you have got to grips with the basics we can consider
some real examples of gene inheritance. Let us look at
a simple
genetic trait that we can
see, such as tongue rolling. A single gene causes this.
Try curling the
sides of your tongue up towards each other. Can you
roll your tongue? |
Try GAME 4
|
To Roll or Not to Roll to
get an idea of how this pattern of gene inheritance works.
Scientists think that a single gene is responsible for
the ability to roll your
tongue. We have mentioned above that genes come in pairs, one from your mother
and one from your father. There are two forms of this particular gene, one
that allows you to roll your tongue and one that makes
you unable to roll your tongue.
What happens if you inherit the tongue rolling gene from one parent and the
non-tongue rolling gene from the other? In the case
of this particular gene you would be
able to roll your tongue. This is because the tongue rolling version of the
gene is dominant. Only needing one copy of a particular
gene to display a characteristic
is called dominant inheritance. To be a non-tongue roller you need to have
two copies of the non-tongue rolling version of the
gene. This version of the gene
is recessive. Needing two copies of a particular gene to display a characteristic
is called recessive inheritance. |
Not all characteristics are caused by the inheritance of a
single gene. Some are the result of several genes, while others
are caused by genes and the environment working together. These
are more complicated and are said to be multi-factorial in
nature.
Some common characteristics that are thought to be caused
by more than one gene working together are curly hair, and
whether
we are right or left handed. An example of genes and environment
working together could be weight. Clearly, while there are
many genes that could influence weight, our “environment”,
or in other-words how much we actually eat, also plays a role. We hope
you have enjoyed this section and have found it useful. |
GAME
1
GAME 2
GAME 3
GAME 4
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