GENETIC MUTATION
Learning Outcomes:
After completing this lesson, you will be able to:
 Identify which substances may cause genetic mutations
 Identify which substances may cause changes in development
 Identify the environmental factors and individual choices over which you have
some control
 Describe preventative measures you may take to reduce the risk of genetic
mutations or changes in development
Human Reproduction
You have learned in the section on reproduction that humans reproduce sexually with
both parents contributing a haploid set of 23 chromosomes through meiosis. The
offspring has 23 pairs of chromosomes from both parents.
On each chromosome are many genes. Each gene is responsible for one trait in the
offspring. Recall that the 23 pairs of chromosomes are homologous. Two chromosomes
produce similar characteristics. This means, in turn, that two genes are responsible for
each characteristic in the offspring.
Genes are either dominant or recessive. Using an
example, a curly hair gene (C) is dominant and straight
hair gene (c) is recessive. An offspring receiving a
dominant gene for curly hair from one parent and a
recessive gene for straight hair from another parent will
exhibit the curly hair trait. The dominant gene trait will
mask the recessive gene trait.
When everything is working normally, one generation
follows the next with inherited traits manifesting
themselves in the phenotype.
Sometimes something goes wrong and the offspring show unexpected traits. Where do
these variations come from?
First, recall that we have two kinds of reproduction taking place. Sexual reproduction is
responsible for creating the next generation. Meiosis reduces the number of
chromosomes from each parent from 46 chromosomes (or 23 pairs) to 23 individual
chromosomes. In this way, the offspring will also have 23 pairs – 23 chromosomes from
each parent. Our bodies also use asexual reproduction to produce new cells as soon as
the sperm and egg unite. We also use mitosis to produce new cells for repair and
replacement in our bodies.
Whenever an offspring has an unexpected trait caused by an abnormality in meiosis or
mitosis, we can say there has been a mutation in the gene responsible for the trait in the
individual or the function of the cell. Mutations to somatic cells are not inherited by
offspring.
The reproductive process, either meiosis or mitosis, is not flawless. There are many
opportunities for errors to occur in both processes. A list of genetic mutations is shown
below.
Chromosome Abnormalities
Chromosome abnormalities are the most common
form of genetic mutation. During meiosis,
chromosomes or parts of chromosomes can be
lost, changed, or mixed up. Down syndrome and
Turner syndrome are both examples of the effects
of chromosome dysfunction.
Single Gene Disorders
Single gene disorders are caused by a single gene losing or
altering part of its structure by mutation. An example of a
single gene disorder is sickle-cell anemia. A mutation causes
blood cells to look like a sickle rather than the normal
doughnut-without-a-hole shape of a blood cell.
Cystic fibrosis and Huntington’s disease are also single gene
disorders.
Multifactorial Disorders
Multifactorial disorders result from mutations in multiple genes, often combined with
environmental causes. The complicated bases of these diseases make them difficult to
study and to treat. Heart disorder, diabetes, and cancer are examples of this type of
disorder.
Cancer is a disease that arises from a combination of factors. One cause of cancer is
an inherited gene mutation. Another cause of cancer occurs after birth from a
combination of factors, such as overexposure to ultraviolet light, certain chemicals, or
viruses. Spontaneous genetic mutation or the aging process can also lead to cancer.
Any cancer involves a mutation that interferes with a cell’s division regulator. A normal
cell has detectors that indicate when a cell should stop replicating, when it is in contact
with other cells and should stop, or when it is occupying space that doesn’t belong to it.
When these signals fail to work properly, the cell is in danger of reproducing itself
spontaneously without stopping.
Other diseases called diabetes, Tourette
syndrome, and lupus are a result of more than
one factor giving rise to an illness. All of these
diseases involve a genetic mutation.
It may seem that we are surrounded by genetic
mutations. Genetic mutations are common, but
serious mutations are often expelled by the body
and never show up. Sexual reproduction also
has the ability to deal with mutations as they are
usually recessive and do not show up in the
offspring.
The medical and scientific community is researching ways of dealing with genetic
diseases when they do manifest themselves. There have been advances in recent
years in many diseases using treatments that offer relief.
Environmental Factors
Genetic mutations can be the result of an environmental impact. There are different
types of mutagens which are capable of altering the genetic code. A list of some
mutagens is shown below.
 Ultraviolet/radiation overexposure
 Toxins
 Carcinogens
 Food additives
 Hormone mimics
 Pollution
 Pesticides
Lifestyle
There are some lifestyle decisions that can have an impact on genetic mutation. Some
of the factors that have been listed as environmental can also be listed as lifestyle.
One lifestyle choice that can have an extreme impact on the development of a child is a
mother’s use of alcohol during her pregnancy. Fetal Alcohol Syndrome is a disease that
affects children for their whole life. The effects of FAS are predictable and devastating
for the affected children, and those who must care for them.
Definition of Mutations
A chromosomal mutation is a major chance occurring in one or more of the
chromosomes, or in the number of chromosomes.
A gene mutation is a chemical change occurring in an individual gene (an individual
length of DNA).
Various factors cause mutations:
 Radiation – direct damage to DNA
 Gamma radiation – why atom bombs are so dangerous
 X-rays – why radiographers wear lead aprons
 Ultraviolet rays – why skin tans, and people wear sun
blockers
 Certain chemicals – called mutagens
 Copying errors – mistakes during DNA replication
Genes controlling rates of cell division are particularly susceptible to mutations. If
these genes mutate, cells may begin to divide too quickly, causing a tumour; if it
then spreads through the body it is a cancer. Factors causing these mutations are
said to be carcinogenic.
Significance of Mutations
 They may not affect phenotype (subtle point), but they may do so, sometimes
catastrophically.
 They are the fundamental source of all genetic variation.
 Meiosis and fertilization merely shuffle up genes.
 They are random. Although they may be caused by an environmental effect,
they are not a specific response to a change in the environment, and they
are, therefore, mostly harmful.
 Any mutation of a body cell can have serious consequences for the individual,
but a mutation in a sex cell could have consequences for future generations
since the mutation can be passed on to the next generation.