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 153-154

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 IA and IB are regarded as co-dominant because they are both expressed phenotypically when they are together genotypically as IA IB (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. PR for petal colour red and PW 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 CS, which produces spotted seeds, and CD, 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 F1 generation.

• Two spotted dotted seeded plants from the F1 generation were crossed.• Draw a punnet square to show

the genotypes and phenotypes of the F2 generation.