Download 4.1. chromosomes, genes and alleles

Genetics
Biology 5
4.1 Chromosomes, genes, alleles and mutation
Variation
Genetics is concerned with explaining how some characteristics are passed from generation to generation, i.e.
heredity, or inheritance. However, it is important to firstly deal with some background information about the
characteristics themselves. This may then be tied in with evolution and the development of different species of
living organisms. .
Like most living organisms, humans show variation. If you consider almost any characteristic, you will find
differences between various people (or other animals or plants) in a population. There are two forms of variation:
continuous and discontinuous variation.
Characteristics showing continuous variation vary in a general way, with a broad range, and many intermediate
values between the extremes. If you consider a large enough sample from a population, perhaps plotting frequency
as a histogram, you will find that most of the values are close to the average (mean), and extreme values are actually
rather rare.
Height is an example of a continuously variable characteristic, as long as you consider a consistent sample, for
example a large number of people of a particular age and sex.
It is usually difficult to give a straightforward explanation of the genetic basis for these continuously variable
characteristics because they result from a combination of genetic factors as well as environmental influences.
Characteristics showing discontinuous variation fall into a few very distinct classes. The ability to roll the tongue,
and blood groups, are examples of discontinuous variation. These characteristics can be explained much more easily
by simple rules of genetics and are less likely to be affected by other factors.
Causes of variation
Some of the characteristics possessed by an individual in a population can be said to be inherited - i.e. derived from
the previous generation. These characteristics are passed on, in a fairly predictable way, as a result of sexual
reproduction. Sexual reproduction also introduces an element of randomness, so that variation is brought about in a
population. These two almost contradictory factors - dependable inheritance of characteristics from parents, and
variation within the population - are essential to an understanding of the process of evolution.
It is certainly too simplistic to imply that characteristics like facial features of children can merely be attributed to
parents by looking at them, without knowing the background of previous generations.
The examples often chosen give the impression that inheritance covers only trivial features, such as the shape of the
human chin, or ear-lobes. However, an extremely wide range of characteristics are known to be passed on in this
way. In fact, practically every aspect of normal human body functioning is under hereditary (genetic) control,
because there are many examples of fairly rare "conditions" (diseases which cannot be transmitted from one person
to the next, but which are caused by defective functioning of certain cells) which can be inherited in exactly the same
way as hair or eye colour. These inherited forms of disease" may also be called inborn errors of metabolism.
Other characteristics are said to be acquired during life (non-inherited). These may also be said to be caused by
environmental effects.
It must also be said that some characteristics probably have both an inherited and an environmental basis, such as
I.Q. - intelligence quotient. The balance between them is the answer to the "nature versus nurture" question.
Similar considerations apply in all living organisms; for example different plants grown in different conditions of light
or temperature may show differences in growth rate and vigour, and understanding the causes of this variation is
quite fundamental in controlling or increasing agricultural and horticultural productivity.

From an experimental point of view, how may the effects of these environmental factors be shown, as
distinct from effects due to different genetic make-up of the plant in question?
Chromosomes, Genes and Alleles
Genetics is the study of heredity, that is,
the transfer of characteristics or traits
from one generation to another.
Inside the nucleus of a eukaryotic cell
there are many long pieces of DNA known
as chromosomes each composed of DNA
and proteins. The proteins are concerned
with the packing of the DNA. Packing is
necessary because the chromosomes
contain huge amounts of DNA. The
largest chromosome in man contains
200,000,000 base pairs and the DNA is
7cm long. All this DNA must be capable of
condensing to 10um during cell division, a
packing ratio of 10,000:1.
It is the DNA which carries information
about the design of thousands of
different proteins.
A gene is the name we give to a small
length, or section, of chromosome that
carries information about one particular
polypeptide. If a protein contains more
than one polypeptide then more than one
gene will be involved in its production.
Between genes, and making up most of
the chromosome’s length, there is a lot of
junk DNA which appears to have no function.
Gene – a heritable factor that controls a specific
characteristic.
Allele: A particular form of a gene. Alleles usually
occur in pairs, one on each homologous
chromosome in a diploid nucleus. Alleles differ by
one or a few base and occupy the same gene locus.
Genome: the complete complement of genetic
material in a cell
Gene mutation: a change in the genetic material of
an organism. Mutations that cause an alteration to
a gene result in a new allele. Mutations can also
affect the number and size of chromosomes.
Mutations
Alterations in the number or structure of chromosomes are called chromosome mutations. Gene mutations are
changes in the nucleotide base sequence of a gene. A change of a single nucleotide base pair is called a point
mutation. There are a number of different point mutations, including:
Substitution: replacement of one nucleotide with another containing a different base.
Deletion: the loss of a nucleotide
Insertion: addition of an extra nucleotide
Sickle-cell anaemia is an example of an inherited condition that results from a single base substitution. Single
mutations can have a serious effect on the polypeptide produced, just as it has a serious effect on the sentence
below.
Intended Message:
SUSAN IS NOW ARRIVING BY AIR FORM NEW YORK
Actual Message:
SUSAN IS NOT ARRIVING BY AIR FROM NEW YORK
Two genes are responsible for determining the type of haemoglobin (quarternary
protein) we have: one for the pair of alpha chains and the other for the pair of beta
chains. Research shows that sickle-cell anaemia results from a mutation in just one
nucleotide base, causing a difference in one amino acid between the abnormal beta
chain and the normal alpha chain.
Sickle cell anemia is a serious condition in which the red blood cells can
become sickle-shaped (that is, shaped like a “C”). Normal red blood cells
are smooth and round like a doughnut without a hole. They move easily
through blood vessels to carry oxygen to all parts of
the body. Sickle-shaped cells don’t move easily
through blood. They’re stiff and sticky and tend to
form clumps and get stuck in blood vessels.
The clumps of sickle cells block blood flow in the
blood vessels that lead to the limbs and organs.
Blocked blood vessels can cause pain, serious
infections, and organ damage.
Figure A shows normal red blood cells flowing freely in a blood vessel.
The inset image shows a cross-section of a normal red blood cell with
normal hemoglobin. Figure B shows abnormal, sickled red blood cells
clumping and blocking the blood flow in a blood vessel. The inset image
shows a cross-section of a sickled red blood cell with abnormal strands
of hemoglobin.
Sickle cell anemia is an inherited, lifelong condition. People who have sickle cell anemia are born with it. They inherit
two copies of the sickle cell gene, one from each parent. People who inherit a sickle cell gene from one parent and a
normal gene from the other parent have a condition called sickle cell trait.
Sickle cell trait is different from sickle cell anemia. People with sickle cell trait don’t have the condition, but they
have one of the genes that cause the condition. Like people with sickle cell anemia, people with sickle cell trait can
pass the gene on when they have children.
Anemia is a condition in which a person’s blood has a lower than normal number of red blood cells, or the red blood
cells don’t have enough hemoglobin . Hemoglobin is an iron-rich protein that gives blood its red colour and carries
oxygen from the lungs to the rest of the body.
In sickle cell anemia, a lower-than-normal number of red blood cells occurs because sickle cells don’t last very long.
Sickle cells die faster than normal red blood cells, usually after only about 10 to 20 days. The bone marrow can’t
make new red blood cells fast enough to replace the dying ones. The result is anemia.
Sickle cell anemia affects millions of people worldwide. There are excellent treatments for the symptoms and
complications of the condition, but there’s no cure.
Genetic basis for sickle cell
The gene responsible for sickle-cell anaemia has two possible forms or alleles. The sickle cell allele is given the
symbol Hbs, the normal allele is HbA. As a person usually has two alleles for a particular gene three combinations are
possible: HbsHbs, HbAHbA or HbAHbs.
HbsHbs: sickle cell anaemia (abnormal red blood cells causing sickle cell anaemia)
HbsHbA: sickle cell trait (may have problems when oxygen demand is high)
HbAHbA: normal blood
As the Hbs allele is harmful it would be expected that it would be removed from a population due to natural
selection. However, in some parts of the world this allele is common and many babies are born with sickle-cell
anaemia.

Make brief notes on the link between sickle cell anaemia and Malaria. Emphasise how the sickle cell allele
has been able to survive

The pictures below show sickle cell haemoglobin in high oxygen concentration and low oxygen
concentration. What conditions would cause low oxygen concentrations in the blood? What happens to the
pH of the blood under these conditions? How does the change in blood composition and the change in one
amino acid lead to the change in shape of the blood cell?