Types of dominance in monohybrid crosses

Complete dominance Complete dominance occurs when one allele completely dominates another allele when both are present in the genotype. A dominant allele is always expressed in the phenotype when present in the genotype. This dominant allele will always mask the presence of the recessive allele. A recessive allele will only be expressed in the phenotype when both alleles in the genotype are recessive.

Examples of complete dominance. Hitchhiker’s thumb (having hitchhiker’s thumb is dominant allele) Polydactyl hand or having extra digits on the hand (dominant allele for extra digits) Having dimpled cheeks (is dominant) Tongue rolling ability (can roll tongue is dominant) Free or attached ear lobes (having free ear lobes is dominant). Page

Incomplete dominance Incomplete dominance occurs when neither allele dominates the other. When both alleles are present in the heterozygous genotype, both contribute to produce a phenotype that is a blend of the other two phenotypes. Therefore, three different phenotypes can occur (as opposed to only two with complete dominance). Upper case and lower case letters are still used for incomplete dominance, even though neither allele is dominant.

Examples of incomplete dominance. Snapdragon flowers can be red, white and pink (a blend of red and white). Andalusian chickens can be black, white or grey.

Co-dominance Co-dominance occurs when both alleles are equally dominant. When both alleles are present in the heterozygous genotype, they are both expressed in the phenotype. Three different phenotypes can occur. Some animal coat-colour patterns and plant flower-colour patterns are the results of alleles showing co-dominance. With co-dominant alleles, some cells of the organisms show one phenotype and some cells show the other phenotype.

Examples of co-dominance. Shorthorn cattle and horses can be white, red and roan (red roan is a mixture of white and red hair) or black, white and roan (blue roan is a mixture of black and white hairs to give an overall blotchy bluish-greyish coat). In human blood groups I A and I B are regarded as co-dominant because they are both expressed phenotypically when they are together genotypically as I A I B (note blood groups is an example of multiple alleles). We will cover multiple alleles next.

Co-dominant alleles could produce unusual petal colour pattern – e.g. some could be red and some could be white. Alleles that are co-dominant are both shown using the same capital letter but with a different superscript: e.g. P R for petal colour red and P W for petal colour white.

Example co-dominance The seed-coat pattern of lentils is an example of co-dominance inheritance. It is controlled by the alleles C S, which produces spotted seeds, and C D, which produces dotted seeds. A pure bred spotted seed lentil plant is crossed with a pure bred dotted seed lentil plant. Draw a punnet square to show the phenotypes and genotypes of the F 1 generation.

Two spotted dotted seeded plants from the F 1 generation were crossed. Draw a punnet square to show the genotypes and phenotypes of the F 2 generation.