Dominant and Recessive Phenotypes

How do you define dominance? What are the 3 main types of dominance?

Dominance: The relationship between the alleles of a gene and the observable phenotype

• The dominance of a trait determines the phenotype

3 main types of dominance:

• Complete dominance

• Codominance

• Incomplete dominance

What is complete dominance? What phenotype does this produce in which individuals?

Complete dominance: A pattern of dominance in which the recessive trait is completely masked by the dominant trait in heterozygotes

 

• Describes the phenotype that always appears when there is at least one allele for the trait

• Dominant phenotypes: phenotype seem in homozygous dominant and heterozygous individuals

What is the pattern of dominance where there is an intermediate phenotype produced?

Incomplete dominance: A pattern of dominance in which a heterozygous individual displays an intermediate phenotype that is combination of the two alleles

• The heterozygote presents an intermediate because both alleles are dominant and therefore neither is completely dominant

EG Red and White are equally dominant and produce pink flowers 

What is the allelic notation in incomplete dominance?

• A capital letter is used for the gene as neither gene is completely dominant

EG C for Colour of flowers

• The alleles are written as capital superscripts

How do you define codominance? What is the phenotype of individuals with codominant traits?

Codominance: A pattern of inheritance in which two traits are equally dominant and the heterozygote displays both traits

• The heterozygote displays both phenotypes simultaneously as both alleles are fully expressed

• The conventions for writing genotypes are the same as for incomplete dominance 

What are the key differences between incomplete and codominance?

Feature	Incomplete Dominance	Codominance
Heterozygous phenotype	Blend/intermediate phenotype	Both traits shown together
Allele expression	Neither allele completely dominant	Both alleles fully expressed
Appearance	Traits mix	Traits remain separate

• Humans have 4 blood group phenotypes: A,B,AB or O

• Blood type is determined by three possible alleles:

•  and  are codominant

• Both  and  are completely dominant over

• Therefore, the ABO system demonstrates both codominance and complete dominance

What are the types of inheritance in the ABO blood grouping system?

Codominance

•   Iᴬ and Iᴮ are both fully expressed together

• IᴬIᴮ = Blood type AB

Complete dominance

•  i is recessive and is masked when paired with Iᴬ or Iᴮ

•   Iᴬi→ blood type A  and Iᴮi→ blood type B

What are the key concepts of the ABO blood grouping system?

• Humans have 4 blood group phenotypes: A,B,AB or O

• Blood type is determined by three possible alleles:

•  and Iᴬ, Iᴮ and i

•  Iᴬ and Iᴮ are codominant

• Both  Iᴬ and Iᴮ and are completely dominant over i

• Therefore, the ABO system demonstrates both codominance and complete dominance