Codominance And Incomplete Dominance Lesson

Lesson Overview

• Genes, Alleles, and Patterns of Inheritance
• Incomplete Dominance: When Traits Blend
• Codominance: When Both Traits Show Up Together
• Comparing Incomplete Dominance and Codominance
• Applying the Concepts with Punnett Squares
• Identifying Patterns Based on Descriptions

Why do some offspring show traits that are a blend of both parents, while others show both traits side by side? These outcomes are explained by two non-Mendelian inheritance patterns-incomplete dominance and codominance. 

This lesson explores these concepts in depth, helping to understand how traits are passed down in ways that differ from simple dominant and recessive rules.

Genes, Alleles, and Patterns of Inheritance

Understanding how traits are inherited begins with the concepts of genes and alleles. These determine how characteristics are passed from parents to offspring.

• Genes are segments of DNA that carry instructions for specific traits.
  
• Alleles are different forms of a gene found at the same location on chromosomes.
  
• In classic Mendelian inheritance:
  
  • One allele may be dominant (expressed) over a recessive one (hidden).
    
  • Example: In pea plants, yellow seeds dominate green seeds.

However, not all traits follow this dominant-recessive model. Some traits show more complex patterns, like incomplete dominance and codominance.

Incomplete Dominance: When Traits Blend

Incomplete dominance occurs when neither allele completely dominates the other, so the traits blend together in the heterozygote. Think of it as a "compromise" between the two alleles.

Instead of one trait winning out, the offspring shows an intermediate phenotype - a mix or blend of the parents' traits.

Characteristics

• Offspring show a third phenotype that is a blend of the parent traits.
  
• Neither allele is completely dominant or recessive.
  
• The heterozygous genotype results in an intermediate phenotype.

Genotype and Phenotype Relationship

• Symbols like R and W are used to show two equally dominant alleles.
  
• The heterozygous genotype RW expresses a mixed trait (pink).

Key Examples

• Flowers (snapdragons):
  
  • RR = red
    
  • WW = white
    
  • RW = pink
    
• Hair Texture in Humans:
  
  • CC = curly
    
  • SS = straight
    
  • CS = wavy
    
• Tail Length in Animals:
  
  • SS = short
    
  • LL = long
    
  • SL = medium

Typical Phenotypic Ratio in Offspring

Codominance: When Both Traits Show Up Together

Codominance is a different twist: both alleles are expressed fully and simultaneously in the heterozygote. Instead of blending, the offspring shows both traits side by side. In other words, the two alleles act as co-leaders – co-captains – neither one masks the other, and neither one blends with the other. They coexist and you can observe each one's effect clearly.

Characteristics

• Offspring do not blend traits but show both traits distinctly.
  
• Each allele contributes to the phenotype equally.
  
• Traits appear as patches, stripes, or dual expression.

Genotype and Phenotype Relationship

• Superscripts may be used (e.g., C^B for black, C^W for white).
  
• Heterozygous genotype (BW or C^B C^W) shows both traits.

Key Examples

• Cattle Coat Color (Roan):
  
  • RR = red
    
  • WW = white
    
  • RW = roan (red and white hairs intermixed)
    
• Feather Color in Chickens:
  
  • BB = black
    
  • WW = white
    
  • BW = checkered or spotted black and white
    
• Blood Type in Humans:
  
  • IA = A antigen
    
  • IB = B antigen
    
  • IAIB = AB blood type (both A and B antigens expressed)

Typical Phenotypic Ratio in Offspring

Comparing Incomplete Dominance and Codominance

It's easy to mix up incomplete dominance and codominance at first, because in both cases there's no clear dominant allele overshadowing the other. The key difference lies in the appearance of the heterozygous phenotype. Here's how you can distinguish them:

Take This Quiz:

Trivia Quiz on Codominance and Incomplete Dominance!

Applying the Concepts with Punnett Squares

Understanding inheritance patterns also means being able to predict the outcome of crosses. Punnett squares help visualize how alleles combine in offspring.

Incomplete Dominance Punnett Square

Parents: RW × RW

(R = red, W = white)

• RR: Red
  
• RW: Pink
  
• WW: White

Phenotype Ratio:

1 Red : 2 Pink : 1 White

Codominance Punnett Square

Parents: BW × BW

(B = black, W = white

• BB: Black
  
• BW: Black and white
  
• WW: White

Phenotype Ratio:

1 Black : 2 Black-and-white : 1 White

Identifying Patterns Based on Descriptions

Recognizing clues in written problems or descriptions can help identify the type of dominance pattern.

Incomplete Dominance Clues

• Words like:
  
  • "Blend"
    
  • "Intermediate"
    
  • "Mix"
    
• Phenotype is a new version not seen in either parent.
  
• Color examples:
  
  • Red + White = Pink
    
  • Black + White = Gray

Codominance Clues

• Words like:
  
  • "Both traits visible"
    
  • "Spotted," "checkered," "striped"
    
• Phenotype shows two features simultaneously.
  
• Color examples:
  
  • Red + White = Red and white patches
    
  • Black + White = Spotted black and white