• Where both alleles are equally dominant
  • Therefore, both alleles are expressed in the phenotype

e.g. Snapdragon plant has one allele codes for enzyme which catalyses formation of red pigment and other allele codes for enzyme that lacks this catalytic activity

If alleles showed the usual pattern of 1 dominant and 1 recessive the flower would be either red and white

But as they are co-dominant three colours of flower are found:

Homozygous (1st allele): Both alleles code forthe enzyme and so plant is
red

Homozygous (2nd allele): No enzyme is produced and so plant is white

Heterozygous: Their single allele for the functional enzyme produces some red and so plant is pink

CR = Red pigment

CW= White pigment


e.g. CRC + CW CW


  CR CW
CR CRCR CRCW
CW CRCW CWCW

Phenotypic Ratio: 1
Pink

   e.g. CRCW + CRCW

  CR CR
CW CRCW CRCW
CW CRCW CRCW

Phenotypic ratio: 2:1:1


 Pink: Red: white