Learning Objective

In this lesson we will learn about the three modes of biological inheritance – dominance, incomplete dominance and codominance.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe the three modes of biological inheritance:
  • Dominance.

  • Incomplete dominance.
  • Codominance.
  • Define ‘dominant’ and ‘recessive’ in terms of alleles and phenotypes.
  • Write genotypes for dominance, incomplete dominance and codominance.
  • Predict phenotypes of homozygotes and heterozygotes for dominance, incomplete dominance and codominance.

 

 dominance incomplete dominance and codominance worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.

 

Introduction

  • The phenotype of homozygous individuals corresponds to the type of allele that makes up the genotype.
  • For example, if a plant contains two alleles for red flower colour, it will have red flowers; if a plant contains two alleles for white flower colour, it will have white flowers.
  • The phenotype of heterozygous individuals will depend on the way that particular gene is inherited.
  • There are three main modes of inheritance for autosomal genes – dominance, incomplete dominance and codominance.
  • For example, if a plant contains one allele for red flower colour and one allele for white flower colour, there are three possible ways its phenotype is determined.
  • If inheritance is by dominance, the plant will have either red flowers or white flowers, depending on whether red or white is the dominant colour.
  • If inheritance is by incomplete dominance, the plant will have pink flowers.
  • If inheritance is by codominance, the plant will have flowers that are partly red and partly white.

 
dominance incomplete dominance codominance genotype phenotype

The phenotype of heterozygotes depends on how a particular gene is inherited.

 

Dominant and Recessive Inheritance (Dominance)

  • The most common mode of biological inheritance is dominance, also known as dominant and recessive inheritance.
  • When dominance occurs, only one allele determines the phenotype of heterozygotes. In other words, one allele will be dominant over the other allele.
  • The allele that determines the phenotype of heterozygotes is called the dominant allele and the phenotype it codes for is called the dominant phenotype.
  • The allele that does not determine the phenotype of heterozygotes is called the recessive allele and the phenotype it codes for is called the recessive phenotype.
  • For example, if a plant contains one allele for red flower colour and one allele for white flower colour, the phenotype is determined by whichever is the dominant allele.
  • If the allele for red flower colour is dominant, the flowers will be red – red flower colour is the dominant phenotype.
  • If the allele for white flower colour is dominant, the flowers will be white – white flower colour is the dominant phenotype.
  • Dominant alleles are represented by uppercase letters and recessive alleles are represented by lowercase letters.
  • For example, if the allele for red flower colour is dominant:
  • RR would represent individuals that have two dominant alleles – the homozygous dominant genotype.
  • rr would represent individuals that have two recessive alleles – the homozygous recessive genotype.
  • Rr would represent individuals that have one dominant allele and one recessive allele – the heterozygous genotype.
  • The dominant (uppercase) allele is always written before the recessive (lowercase) allele.

 
dominant recessive alleles phenotypes

A dominant allele determines the phenotype of heterozygotes.

 

  • When dominance occurs:
  • Homozygous dominant individuals will have the dominant phenotype.
  • For example, RR plants will have red flowers.
  • Heterozygous individuals will have the dominant phenotype.
  • For example, Rr plants will have red flowers.
  • Homozygous recessive individuals will have the recessive phenotype.
  • For example, rr plants will have white flowers.

 
dominance genotype phenotype

When inheritance is by dominance, heterozygous individuals will have the dominant phenotype.

 

Incomplete Dominance and Codominance

  • When incomplete dominance or codominance occurs, both alleles determine the phenotype of heterozygotes. In other words, neither allele is dominant over the other.
  • Alleles are written with different letters and both are in uppercase.
  • For example, if neither red flower colour nor white flower colour is dominant:
  • RR would represent individuals that have two alleles for red flower colour – a homozygous genotype.
  • WW would represent individuals that have two alleles for white flower colour – a homozygous genotype.
  • RW would represent individuals that have one allele for red flower colour and one allele for white flower colour – the heterozygous genotype.
  • Either allele could be written before the other, as long as it is consistent.

 

Incomplete Dominance

  • When inheritance is by incomplete dominance, heterozygous individuals will have an intermediate phenotype – a ‘blending’ of the phenotypes encoded by the two alleles.
  • For example, RW plants will have pink flowers (a blending of red and white).

 
incomplete dominance genotype phenotype

When inheritance is by incomplete dominance, heterozygous individuals will have an intermediate phenotype.

 

Codominance

  • When inheritance is by codominance, heterozygous individuals will have two phenotypes – a combination of the phenotypes encoded by the two alleles.
  • For example, RW plants will have flowers that are both red and white.

 
codominance genotype phenotype

When inheritance is by codominance, heterozygous individuals will have a combination of phenotypes.

 

Summary

  • There are three main modes of inheritance – dominance (dominant and recessive inheritance), incomplete dominance and codominance.
  • These vary by how alleles contribute to the phenotype, in particular, how the phenotype of heterozygotes is determined.
  • When dominance occurs, one allele (the dominant allele) is dominant over the other (the recessive allele).
  • Heterozygotes will have the dominant phenotype.
  • When incomplete dominance or codominance occurs, neither allele is dominant over the other.
  • For incomplete dominance, heterozygotes will have an intermediate phenotype.
  • For codominance, heterozygotes will have a combination of phenotypes.
  • For dominant and recessive inheritance, dominant alleles are represented by uppercase letters and recessive alleles are represented by lowercase letters.
  • For incomplete dominance and codominance, alleles are represented by different uppercase letters.

 
dominant recessive alleles genotype

(Image: GraphicsRF, Adobe Stock)

(Header image: htrnr, Adobe Stock)

 

 dominance incomplete dominance and codominance worksheet  year 10 biology pdf workbook

Click images to preview the worksheet for this lesson and the Year 10 Biology Workbook.