Codominance and Codominance and Multiple AllelesMultiple Alleles

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Genotype Phenotype

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

CRCR homozygous red flowers

CRCW heterozygous pink flowers

CWCW homozygous white flowers

Now try the exam questions.

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Genotype Phenotype

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

IB produces antigen B on the surface of red blood cells

IO produces no antigen.

A and B are codominant and O is recessive to both.

IAIA and IAIO blood group A

IBIB and IBIO blood group B

IOIO blood group O

IAIB blood group AB

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Use this information to explain why it is so important that blood type is tested before transfusions take place.

Blood Group

A B O

Can receive from:

     

Can donate to:

     

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

A B O AB

Can receive from:

 A, O B, O   O A, B, AB, O

Can donate to:

 A, AB B, AB  A, B, AB, O 

AB

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).

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.

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Now try the questions on horses on page 112.

Sickle Cell AnaemiaSickle Cell Anaemia

This is an inherited This is an inherited disease.disease.

It affects the red It affects the red blood cells that blood cells that carry oxygen to our carry oxygen to our cells.cells.

Red blood cells are Red blood cells are normally biconcave normally biconcave discs. discs.

In sickle cell In sickle cell anaemia, the anaemia, the haemoglobin haemoglobin molecules are molecules are distorted, causing distorted, causing them to stick to them to stick to each other and each other and form long fibres form long fibres inside the red blood inside the red blood cells.cells.

This pulls the red blood This pulls the red blood cells out of shape into cells out of shape into a “sickle”.a “sickle”.

Inefficient at carrying Inefficient at carrying oxygen.oxygen.

Block small capillaries. Block small capillaries. Prevents healthy red Prevents healthy red

blood cells passing and blood cells passing and results in severe results in severe anaemia (lack of anaemia (lack of oxygen supply to body oxygen supply to body cells) or death. cells) or death.

Questions to consider whilst you Questions to consider whilst you watch the video…watch the video…

1.1. What is unusual about the incidence of sickle What is unusual about the incidence of sickle cell anaemia?cell anaemia?

2.2. What is the benefit of being a carrier for the What is the benefit of being a carrier for the sickle cell gene?sickle cell gene?

3.3. What type of inheritance do you believe is What type of inheritance do you believe is responsible for sickle cell anaemia? responsible for sickle cell anaemia?

Video

Sickle cell disease is Sickle cell disease is passed on by a passed on by a recessive allele.recessive allele.

A child therefore A child therefore needs to inherit the needs to inherit the faulty allele from faulty allele from both parents.both parents.

Both parents must Both parents must be carriersbe carriers..

Ss Ss

S – “Normal” gene (dominant)s – Sickle-cell gene (recessive)

Parents (both carriers)

sssSSsSS

Possible genes in the babies:

Clear Carrier Carrier SickleCell

Therefore a 1 in 4 (25%) chance of a sickle-cell sufferer.2 in 4 (50%) chance of a carrier.

Because the gene Because the gene needs to be needs to be inherited from both inherited from both parents, there is a parents, there is a 1 in 4 (25%) 1 in 4 (25%) chance of a child of chance of a child of two carriers having two carriers having the disease.the disease.

Carriers of sickle cell Carriers of sickle cell disease are particularly disease are particularly common in Africa.common in Africa.

The sickle cell gene that The sickle cell gene that they carry gives some they carry gives some protection against protection against malaria, which is malaria, which is common in Africa.common in Africa.

This is because their This is because their “sickled” red blood cells “sickled” red blood cells carry fewer of the carry fewer of the PlasmodiumPlasmodium organisms organisms that cause malaria.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.