SUPPLEMENTARY GENES

Supplementary Genes

Supplementary genes are genes that both contribute to a single characteristic, where one gene can mask the effect of the other. You may also think of supplementary genes in terms of one gene producing a characteristic and the second as only being able to ‘supplement’ this characteristic.

The dominant allele for gene A produces an enzyme responsible for the production of a pigment melanin (chemical compound 2). Thus as long the individual has one dominant allele A, they will be brown. However, the dominant allele for gene B produces an enzyme which increases the level of Melanin expression. Thus if an individual has at least one dominant allele for both gene A and gene B, they will be black. In a heterozygous cross (AaBb x AaBb) the following offspring are produced:

Note that although three of the albino individuals have the dominant allele B (which increases the expression of melanin), no melanin is produced to begin with and thus no colour can be produced. Therefore the final phenotypic ratio for Supplementary genes is always:

9 : 3 : 4