Download Sickle Cell Anaemia

Codominance and
Multiple Alleles
Codominant alleles
Alleles are codominant if they are both expressed in the
phenotype of a heterozygote. They can be represented by two
capital letters superscript to the letter representing the gene.
For example, flower colour in
snapdragons Antirrhinum majus.
CR = red flowers CW = white flowers
Genotype
Phenotype
CRCR
homozygous
red flowers
CRCW
heterozygous
pink flowers
CWCW
homozygous
white flowers
Now try the exam questions.
2 of 45
© Boardworks Ltd 2009
ABO blood group
Some genes have multiple alleles (i.e. more than two), but
only two can be present in an individual. For example, the
ABO blood group gene (immunoglobulin) in humans.



IA produces antigen A on the
surface of red blood cells
Genotype
Phenotype
IAIA and IAIO
blood group A
IB produces antigen B on the
surface of red blood cells
IBIB and IBIO
blood group B
I OI O
blood group O
IO produces no antigen.
IAIB
blood group AB
A and B are codominant and O is recessive to both.
3 of 45
© Boardworks Ltd 2009
Use this information to explain why it is so
important that blood type is tested before
transfusions take place.
4 of 45
© Boardworks Ltd 2009
When a person receives a blood transfusion, it is important that the
blood received does not contain antibodies which attach to the blood
cells and cause them to clump together (agglutination).
For example, if a person with type A blood was given a transfusion of
type B red blood cells, anti-B antibodies in their blood plasma would
stick to the B antigen on the surface of the red blood cell and cause
agglutination.
Complete the table to show which blood types can donate to and
receive blood from the other blood groups (N.B. when blood is
donated, only red blood cells are used, not the plasma that
potentially contains antibodies).
Blood
Group
Can receive
from:
Can donate
to:
A
B
O
When a person receives a blood transfusion, it is important that the
blood received does not contain antibodies which attach to the blood
cells and cause them to clump together (agglutination).
For example, if a person with type A blood was given a transfusion of
type B red blood cells, anti-B antibodies in their blood plasma would
stick to the B antigen on the surface of the red blood cell and cause
agglutination.
Complete the table to show which blood types can donate to and
receive blood from the other blood groups (N.B. when blood is
donated, only red blood cells are used, not the plasma that
potentially contains antibodies).
Blood
Group
Can receive
from:
Can donate
to:
A
B
O
A, O
B, O
O
A, AB
B, AB
AB
A, B, AB,
O
A, B, AB,
AB
O
If 2 homozygous dominant individuals for different alleles (IAIA and
IBIB) produced offspring, then all would be heterozygotes with AB
blood group.
If this F1 generation interbreeds:
Parental genotypes:
X
F2 genotypes:
F2 phenotypes:
Ratio:
This ratio is typical for examples of co-dominance.
Now try the questions on horses on page 112.
8 of 45
© Boardworks Ltd 2009
Sickle Cell Anaemia


This is an inherited
disease.
It affects the red
blood cells that
carry oxygen to our
cells.


Red blood cells are
normally biconcave
discs.
In sickle cell
anaemia, the
haemoglobin
molecules are
distorted, causing
them to stick to each
other and form long
fibres inside the red
blood cells.




This pulls the red blood
cells out of shape into a
“sickle”.
Inefficient at carrying
oxygen.
Block small capillaries.
Prevents healthy red
blood cells passing and
results in severe anaemia
(lack of oxygen supply to
body cells) or death.
Questions to consider whilst you
watch the video…
1.
2.
3.
What is unusual about the incidence of sickle
cell anaemia?
What is the benefit of being a carrier for the
sickle cell gene?
What type of inheritance do you believe is
responsible for sickle cell anaemia?
Video
S – “Normal” gene (dominant)
s – Sickle-cell gene (recessive)
Parents (both carriers)
Ss


Ss
Possible genes in the babies:
SS
Ss
sS
ss
Clear
Carrier
Carrier
Sickle
Cell
Therefore a 1 in 4 (25%) chance
of a sickle-cell sufferer.
2 in 4 (50%) chance of a carrier.

Sickle cell disease is
passed on by a
recessive allele.
A child therefore
needs to inherit the
faulty allele from
both parents.
Both parents must be
carriers.

Because the gene
needs to be
inherited from both
parents, there is a 1
in 4 (25%) chance of
a child of two
carriers having the
disease.



Carriers of sickle cell
disease are particularly
common in Africa.
The sickle cell gene that
they carry gives some
protection against malaria,
which is common in Africa.
This is because their
“sickled” red blood cells
carry fewer of the
Plasmodium organisms
that cause malaria.
a) Despite the fact that few patients live beyond
40, the sickle cell allele is still present in the
population. Explain why.
b) Answer the questions on pages 126 and 127 of
the textbook.