Co-dominance

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

Homozygous (2^nd 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

C^R = Red pigment

C^W= White pigment

e.g. C^RC^R  + C^W C^W

	CR	CW
CR	CRCR	CRCW
CW	CRCW	CWCW

Phenotypic Ratio: 1
Pink

   e.g. C^RC^W + C^RC^W

	CR	CR
CW	CRCW	CRCW
CW	CRCW	CRCW

Phenotypic ratio: 2:1:1

 Pink: Red: white