Gr 11 Modes of Inheritance

Dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects

The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype

• The genotype of a particular gene will typically be either homozygous or heterozygous

The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype

• The phenotype is determined by both the genotype and environmental influences

Complete Dominance

Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other

• The dominant allele will mask the recessive allele when in a heterozygous state
• Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
• The recessive allele will only be expressed in the phenotype when in a homozygous state

When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele

• An example of this mode of inheritance is mouse coat colour – black coats (BB or Bb) are dominant to brown coats (bb)

Co-dominance

Multiple Alleles and Co-Dominance - inheritance of Blood Groups

Challenge Question:

Is it possible for a couple to have four children, each child showing a different blood type?

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ABO Blood Group Inheritance shows Co-Dominance AND Multiple Alleles

Non-Mendellian Ratios