Codominance
By Jeanne Smith, Contributor

MENDEL in his experiments examined several characteristics that showed simple dominance, that is, one allele dominant and the other recessive. However not all genes exhibit dominance. In a cross of red flowered zinnia plants with white flowered ones, only pink flowers were produced in the F1 generation. A cross of the F1 progeny produced a ratio of 1 red : 2 pink: 1 white in the F2 generation. These results mean that neither allele is dominant over the other, they both show equally in the phenotype. This is called codominance (see figure 1).

BLOOD GROUPS

The inheritance of A B O blood groups in humans is also an example of codominance. A and B are equally dominant and O is recessive to both. The possible genotypes and phenotypes are shown below.

TESTCROSS

A testcross is used to reveal unknown genotypes. In pea plants, purple colour is dominant to white. Purple flowers could therefore be represented by the following combination of alleles:

PP - homozygous dominant
Pp - heterozygous

When a part of the genotype is known it is represented by a hyphen (-), eg P-. The known part of the genotype may be determined by a testcross or back cross. The unknown genotype may be PP or Pp. Assuming the plant is the genotype PP, then the possible gametes from such a plant will bear only P; if bred with a white plant, pp, all the offspring will be Pp.

If the unknown genotype is Pp then half the gametes will bear the P allele and the other half the p allele. If this plant is crossed with a white plant then half the offspring will be purple and the other half white.

Crossing the unknown with the homozygous recessive therefore reveals its genotype. Possible matings with a pair of alleles

The table shows that the testcross cannot be carried out using an
organism which shows the
dominant phenotype.

*Jeanne Smith teaches Biology at the Queen's school. Send your questions and comments to the CXC Study Guide, the Gleaner Company Ltd., 7 North Street, Kingston; or email us at jcampbell@gleanerjm.com