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Deleterious Genes
Introduction
• A “perfect” population would not carry any
deleterious genes—but as we’ve already
seen, natural selection does not produce a
perfect population.
• Deleterious means damaging—deleterious
genes will lead to mild to severely
disabling or even fatal conditions at some
point in life.
Intro (cont’d)
• We would expect natural selection to
remove genes with negative effects from a
population.
• Individuals who carry those genes would
not have the opportunity to reproduce as
much, so the genes should not be passed
on.
• And yet we see cases where this
expectation is not met . . .
Why might deleterious genes
exist in a population?
1. They may be maintained by
heterozygote advantage.
•
•
•
Sometimes carrying two copies of a gene
is disadvantageous and carrying only
one copy is advantageous,
In this case, natural selection will not
remove the gene from the population
i.e. the advantage conferred in its
heterozygous state keeps the gene
around.
Sickle cell anemia
• Autosomal recessive
inheritance pattern.
• The gene that causes this
condition is deleterious if you
carry two copies of it.
Normal red blood
cells (top) and
sickle cells
(bottom).
Sickle cell anemia
• If you carry only one copy of
it, and live in a place where
malaria is common, the gene
is advantageous.
• It confers resistance to
malaria.
Normal red blood
cells (top) and
sickle cells
(bottom).
• E.g. What is the probability of two
parents, heterozygous for sickle cell
anemia, having:
– a child with the sickle cell condition?
– A child who will be “immune” to malaria?
2. They may not really reduce
fitness.
• Some genetic disorders only exert their
effects late in life, after reproduction has
taken place.
• For example, the gene that causes
Huntington’s disease typically does not
exert its devastating effects until after a
person’s prime reproductive years.
• Such genes will not be strongly selected
against, because an organism’s fitness is
determined by the genes it leaves in the
next generation and not its life span.
• Often, a fetus with homozygous dominant
deleterious genes, such as Huntington’s,
will not survive.
3. They may be maintained by
mutation.
• The mutation may keep arising in the
population, even as selection weeds it out.
• For example, neurofibromatosis is a
genetic disease causing tumors of the
nervous system—it is a dominant
condition that is inherited from mutations
in gametes.
• Natural selection cannot completely
eliminate the gene that causes this
disease because new mutations arise
relatively frequently – in perhaps 1 in 4000
gametes.
• We are unsure of the cause of these
spontaneous mutations.
4. They may be maintained by gene
flow.
• The gene may be common, and not
deleterious, in a nearby habitat.
• If migration from the nearby population is
frequent, we may observe the deleterious
gene in the population of interest.
• For example, in places like the U.S.,
where malaria is not a problem, the gene
that causes sickle cell anemia is strictly
disadvantageous.
• However, in many parts of the world, the
gene that causes sickle cell anemia is
more common because a single copy of it
confers resistance to malaria.
• This becomes the advantage—thus
keeping the deleterious sickle cell gene at
a high level in the population.
Figure 2. Schematic representation of the
effect of the sickle cell hemoglobin gene on
survival in endemic malarial areas.
• People with normal
hemoglobin (left of the
diagram) are susceptible
to death from malaria.
Figure 2. Schematic representation of the
effect of the sickle cell hemoglobin gene on
survival in endemic malarial areas.
• People with sickle cell
disease (right of the
diagram) are susceptible
to death from the
complications of sickle
cell disease.
Figure 2. Schematic representation of the
effect of the sickle cell hemoglobin gene on
survival in endemic malarial areas.
• People with sickle cell
trait, who have one gene
for hemoglobin A and one
gene for hemoglobin S,
have a greater chance of
surviving malaria and do
not suffer adverse
consequences from the
hemoglobin S gene.
The sickle-cell trait has spread far and wide, outside
the Americas. Regions where the trait is more
commonly found (shown in orange stripes) overlap
areas where malaria is prevalent (shown in green).
RECESSIVE DELETERIOUS
GENE CONDITIONS
• MOST (not all) deleterious genes are
recessive traits. Heterozygous people for
the trait are carriers but are not afflicted by
the condition.
•Alkaptonuria
•Sickle cell anemia
•Cystic fibrosis
•Tay Sachs
•Duchenne’s muscular dystrophy
•Adenoleukodystrophy (ALD)
•Hemophilia
•Alper’s syndrome
•Lesch-Nyhan syndrome
•Zellweger’s syndrome
•PKU
•Aarskog syndrome
DOMINANT DELETERIOUS GENE
CONDITIONS
• Some deleterious genes are
dominant. Heterozygous people for
the trait exhibit the condition.
•Brachydactyly
•Congenital stationary night blindness
•Fascio-scapulo-humeral muscular dystrophy
•Huntington’s disease
•Pseudo-achondrophastic dwarfism
•NF-1 (Neurofibromatosis type 1)
•Marfan syndrome
•Machado-Joseph disease
Poster Project:
Precious Humans
• See handout
• Two classes
– One computer day
– One work day
• HowStuffWorks Videos "Your Body Your
Health: Sickle Cell Disease"