Dominance: Incomplete Dominance, Codominance and Overdominance

What is dominance?

Interaction between genes at the same locus

• 2 versions of the same gene

What are the 3 subgroups of dominance?

• Incomplete dominace

• Codominace

• Overdominace

How does this relate to the Mendel theory?

It acknowledges dominace, but not the different types of dominance

How are alleles in the different types of dominance, different from each other?

Due to slight differences in DNA sequence at same loci

What is the phenotype of heterozygotes in incomplete dominance?

It falls within a range (intermediate) between the phenotypes of the 2 homozygotes

Therefore, which theory does incomplete dominance represent?

Blending theory

What is the phenotypic ratio of incomplete dominance?

1:2:1

2 diagrams showing incomplete dominance

What is codominance and what is observed in heterozygote?

Neither allele masks the other

In heteozygote: Effects of both alleles are observed without blending

What is a real life example of codominace and how does it present it?

Example: Blood type

Represent by:

• IA and IB = codominant

• BUT IA and IB are completely dominant over i

How are different blood types determined?

By the antigen type

• Antigen A (controlled by allele IA)

• Antigen B (controlled by allele IB)

• Antigen O (controlled by allele i)

Therefore, type A and type B phenotype can be resulted from which genotypes?

• Homozygous dominant

• Heterozygous with i

  • IA i

Table that represents codominance

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What is overdominance?

In which a heterozygotes is more vigorous than both types of homozygotes

What is overdominance also called?

Heterozygous advantage

What is an example of overdominance?

Sickle cell anemia

• Autosomal recessive disorder

• Affected individuals produce abnormal forms of hemoglobin

Sickle cell anemia

• HbA = Normal hemoglobin, hemoglobin A

• HbS = Abnormal hemoglobin, hemoglobin S

Q: How does it represent overdominance

Because

• HbA HbS individuals are “better” than

  • HbS HbS because they do not suffer from sickle cell anemia

  • HbA HbA, because they are more resistant to malaria

What is heterosis (hybrid vigor)

When offspring is

• Phenotypically stronger

• Larger

• More vigorous