Genetic Dominance Types Overview

Jun 14, 2025

Overview

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

Types of Dominance in Genetics

  • There are three kinds of dominance: complete, incomplete, and co-dominance.
  • Dominance describes how alleles (gene versions) interact to determine an organism's phenotype (physical trait).

Complete Dominance

  • Complete dominance: one allele is dominant and completely masks the other (recessive) allele in heterozygous individuals.
  • Use the same letter for both alleles, uppercase for dominant (e.g., B = black fur, b = brown fur).
  • Homozygous dominant (BB) and heterozygous (Bb) individuals show the dominant phenotype; homozygous recessive (bb) shows the recessive phenotype.
  • Governed by Mendel's law of dominance: crossing two homozygous parents with contrasting traits yields offspring showing only the dominant trait.

Incomplete Dominance

  • Incomplete dominance: neither allele is dominant; heterozygous individuals have an intermediate phenotype.
  • Use different capital letters to represent each allele (e.g., R = red, W = white in snapdragon flowers).
  • Homozygous red (RR) and white (WW) flowers produce pink (RW) flowers.
  • Intermediate phenotypes indicate an incomplete dominance cross.

Co-dominance

  • Co-dominance: both alleles are equally dominant and both are fully expressed in the heterozygous phenotype.
  • Use a letter (e.g., C) with superscripts for each trait (CB = black, CW = white).
  • Heterozygous chickens (CBCW) are black and white speckled, not a new blended color.
  • Co-dominance is seen in other animals like cows with spotted fur patterns.

Blood Group Dominance (Preview)

  • Blood types show both complete dominance and co-dominance: alleles A and B are co-dominant, O is recessive.
  • Further details will be covered in a separate lesson.

Lettering Conventions in Crosses

  • Complete dominance: capital letter for dominant, lowercase for recessive (B/b).
  • Co-dominance: capital C with superscripts (CB, CW).
  • Incomplete dominance: different capital letters for each trait (B/Y).
  • Always provide a key for chosen allele symbols.

Key Terms & Definitions

  • Allele — Different versions of a gene.
  • Dominant — Allele that masks the effect of a recessive allele.
  • Recessive — Allele masked by a dominant allele.
  • Heterozygous — Having two different alleles for a gene.
  • Homozygous — Having two identical alleles for a gene.
  • Phenotype — Observable physical trait.
  • Genotype — The allele combination (genetic makeup) for a trait.
  • Law of Dominance — Mendel's principle that dominant alleles mask recessive ones in heterozygotes.
  • Incomplete Dominance — Both alleles blend to create an intermediate phenotype.
  • Co-dominance — Both alleles are fully expressed in the phenotype.

Action Items / Next Steps

  • Review blood group dominance and co-dominance examples in the upcoming video.
  • Create flashcards using the key terms for exam preparation.
  • Practice genetic crosses using proper allele notation and Punnett squares.