Types of dominance in monohybrid crosses

1. Types of dominance in monohybrid crosses

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

3. 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

4. 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 phenotypethat 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 alleleis dominant.

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

6. Co-dominance • Co-dominance occurs when both alleles are equally dominant. • When both alleles are present in the heterozygousgenotype, they are both expressed in the phenotype. • Three different phenotypescan 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.

7. Examples of co-dominance. • Shorthorn cattle and horses can be white, red and roan (red roan is a mixture of white and redhair) or black, white and roan (blue roan is a mixture of black and whitehairs 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 phenotypicallywhen they are together genotypically as IA IB (note blood groups is an example of multiple alleles). We will cover multiple alleles next.

8. 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.

9. 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.

10. 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.