Incomplete Dominance and Codominance

Exceptions to Mendel’s principles

• Some alleles are neither dominant nor recessive

  • Instead show “blending,” or differential expression in different parts of the organism.

• Polygenic traits

Incomplete dominance

• No allele is dominant over the other

• Both alleles are expressed in each and every cell that carries them

• When both alleles are present a “new” phenotype appears that is a blend of each allele.

• Alleles will be represented by capital letters with superscripts.

According to Mendel, What happens when a red flower is crossed with a white flower?

• Some white and some red or 

• All offspring red (if red is dominant) or

• All offspring white (if white is dominant)

Sickle Cell Anemia

• An inherited blood disorder 

• Oxygen circulation is diminished due to defective haemoglobin (Hb)

• Haemoglobin is coded for by 2 different alleles

Codominance

• ONE allele is expressed in each and every cell that carries the two alleles but which allele is expressed alternates between cells

• Produces patches, stripes, variegation in colour

• Alleles will be represented by capital letters with superscripts.

Multiple Allele Inheritance

• One gene for the characteristic BUT two or more alleles exist in the population. 

• The alleles continue to be related:

  • dominant, 

  • recessive, 

  • incompletely dominant, or

  • codominant

The human blood types (phenotypes)

• A

• B

• AB

• O

A and B are codominant to each other

Both A and B are dominant over O

How are human blood types classified?

• Classified based on the specific agglutinogens on the surface of red blood cells (erythrocytes)

• We designate agglutinogens as A or B

Agglutinogens

Antigens found on the surfaces of red blood cells

Antigens

Structures on cells surfaces that can cause a immune response in the body

Antibody

A large, Y-shaped protein made by plasma cells that is used by the immune system to neutralize foreign pathogens like bacteria or viruses

Human blood types- how do humans develop antibodies against antigens not on their own red blood cell?

someone with:

• blood type A will develop anti-B antibodies

• blood type B will develop anti-A antibodies

• Blood type AB will develop neither antibody

• type O will develop both anti-A and anti-B antibodies

What happens if incompatible blood is given in a transfusion?

• Donor cells are treated as foreign invaders (since plasma antibodies attach to matching surface proteins) and the patient's immune system attacks them accordingly. 

• Blood transfusion is useless and a potentially massive activation of the immune system and clotting system (agglutination) can cause shock, kidney failure, circulatory collapse, and death. 

Type A Blood

• Allele = I^A

• Blood cells have small antigens on the surface.

Type B Blood

• Allele = I^B

• Cells coated with type B antigens

Type AB Blood

• genotype = I^AI^B

• Blood cells contain both types of antigens

• Known as universal recipient

Type O Blood

• Allele = i 

• No antigens on the surface of the blood cells

• Known as universal donor

Polygenic Traits

• Traits controlled by two or more genes.

• (I.e. Human height, eye and skin colour)