Genetic Dominance Types Overview

Overview

This lecture covers the three types of genetic dominance—complete, incomplete, and co-dominance—explaining their definitions, how to represent them in genetic crosses, and examples for each type.

Complete Dominance

• One allele (dominant) fully masks the effect of the other (recessive) in a heterozygous organism.
• Use the same letter for both alleles: uppercase for dominant, lowercase for recessive (e.g., B = black fur, b = brown fur).
• Homozygous dominant (BB) and heterozygous (Bb) both show the dominant phenotype; only homozygous recessive (bb) shows the recessive trait.
• Mendel's Law of Dominance: Crossing two homozygous organisms with contrasting traits yields F1 offspring expressing only the dominant trait.

Incomplete Dominance

• Neither allele is dominant; heterozygous individuals show an intermediate, blended phenotype.
• Use two different capital letters representing each trait (e.g., R = red, W = white; RW = pink).
• Homozygous (RR or WW) express pure traits, while heterozygous (RW) express a mixed (intermediate) trait.
• Exam clues: if parents of different colors produce offspring of a third, new color, it indicates incomplete dominance.

Co-Dominance

• Both alleles are equally dominant and both traits are fully expressed in heterozygous individuals.
• Use a capital C with a superscript for each trait (e.g., Cᴮ = black, Cᵂ = white; CᴮCᵂ = speckled).
• Heterozygotes display both traits distinctly (e.g., black and white speckled chickens).
• No new color is made; both original phenotypes appear together.

Representation of Genetic Crosses

• Complete dominance: one capital and one lowercase letter (B and b).
• Incomplete dominance: two different capital letters (B and Y).
• Co-dominance: capital C with superscripts for each trait (Cᴮ, Cʸ).
• Always define the letter choices if not provided in the question.

Key Terms & Definitions

• Allele — Different forms of a gene.
• Complete dominance — One allele fully masks another in heterozygotes.
• Incomplete dominance — Neither allele is dominant; heterozygotes show a blend.
• Co-dominance — Both alleles in heterozygotes are fully expressed, not blended.
• Phenotype — The physical trait observed.
• Genotype — The genetic makeup (combination of alleles).
• Heterozygous — Two different alleles (e.g., Bb, RW, CᴮCᵂ).
• Homozygous — Two identical alleles (e.g., BB, bb, RR, WW).

Action Items / Next Steps

• Make flashcards of the definitions and examples.
• Practice representing genetic crosses for each dominance type.
• Watch the upcoming blood group dominance video for more advanced examples.