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Powerpoint Presentation: Codominance

 

Genetics Index

Chromosomes
Meiosis
Meiosis: Crossing over
Mitosis and Meiosis
Introduction to Mendelian Genetics
Test Cross
Multiple Alleles
Pedigree Charts
Twin Studies
The dihybrid cross
Dihybrid Cross : Test Cross
Autosomal Linkage
The Genetic Diagram for Linked Genes
Calculating the cross over value using a test cross
Sex determination and sex linkage
Sex linkage
Genetic diagram for sex linked genes
Blood Clotting and Haemophilia
The Retina and Daltonism
Genetic Modification
Cloning Animals
Cloning Plants

Topic Chapters Index

 

GENETICS

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Co-dominance

Not all genes have dominant and recessive alleles. Some have alleles that are both expressed together in the heterozygote individuals.

Co-dominant alleles have three phenotypes, one for each genotype.

In humans there are a number of conditions that are co-dominant.

 

Example: Sickle Cell Anaemia

Sickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells. Haemoglobin is normally a ball-shaped molecule but the sickle cell allele makes it form long strands. The red blood cell carrying these molecules distorts into characteristic long shapes.

The shape of the haemoglobin molecule is controlled by two alleles:

  • Normal Haemoglobin allele

  • Sickle Cell Haemoglobin allele

There are three phenotypes

  • Normal : Normal individuals have two normal haemoglobin alleles

  • Sickle cell anaemia : A severe form where all the red blood cells are affected. Sickle cell anaemia patients have two sickle cell alleles in their genotype

  • Sickle cell trait : A mild condition where 50% of the red blood cells are affected. Sickle cell trait individuals are heterozygotes, having one of each allele.

 

Symbols for codominant alleles

Because both alleles are expressed in the heterozygote they are considered codominant and both take a capital case letter. An index letter identifies the allele.

Therefore:

Normal haemoglobin allele is HbN

Sickle cell allele is HbS

sickle cell alleles

 

Unusual proportions

Because the heterozygotes have their own phenotype this gives rise to different proportions amongst their offspring compared with crosses between heterozygotes for dominant and recessive alleles.

sickle cell alleles

 

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