Monohybrid Inheritance

Before you learn about monohybrid inheritance you should ensure you are familiar with this vocabulary, and know all about sources of variation.

Monohybrid inheritance is the inheritance of a trait coded for by just one gene, that only has a pair of alleles. 
There are different types of monohybrid inheritance, as you can see below in the Table of Contents: 

Complete Dominance

Alleles involved in complete dominance are either dominant or recessive. 
  • Dominant alleles will "dominate" the phenotype and be expressed if even one dominant allele is present. 
  • Therefore, an organism expressing the dominant phenotype could either be:
    • homozygous dominant (DD, TT, GG etc) or 
    • heterozygous (Dd, Tt, Gg)
  • Recessive alleles "recede" into the background if a dominant allele is present.
  • For the recessive phenotype to be expressed, an individual must possess both recessive alleles, and NO dominant allele.
    • homozygous recessive (dd, tt, gg)

Prestructural - watch these tutorials, to become familiar with the terms F1 & F2 generation, genotype, phenotype, dominant and recessive.

Prestructural - next, complete this interactive tutorial to practice completing Punnet squares to work out the genotype and phenotype ratio for possible offspring in the F1 generation.

Unistructural - Complete these practice questions; if you are stuck, click on the hint to be guided through punnet squares. You may need some scrap paper to help you work out the answers using your own drawn punnet squares as you go. 

Unistructural - Click through these 5 practice questions as well. Feedback will be given to you immediately. 

Pedigree Charts

Pedigree charts show the pattern of inheritance of a trait across generations in a family. The pedigree charts also include information on gender (males are squares, females are circles) and whether a trait is present or not. 

Unistructural - Complete all 3 rounds of this challenge to show you understand the relationships shown on a pedigree chart.

Multistructural - Complete this doc full of questions from Biology Corner; there is a link to answers at the bottom. 

Relational - Complete these practice questions about the autosomal recessive disease haemophilia in the royal family, which has an increased prevalence in their family due to inbreeding. 


Test Crosses

In complete dominance it can be difficult to tell whether an individual is homozygous dominant (AA) or heterozygous (Aa) for the dominant trait, unless it is crossed with an individual KNOWN to be homozygous recessive. 
  • Homozygous individuals are pure-breeders; they can only pass on one type of allele. 
  • A test cross will reveal (by looking at the phenotype of offspring) whether the unknown individual is homozygous dominant: 
    • AA
    • only able to pass on the dominant A allele
    • 100% of offspring will show the dominant phenotype) or 
  • whether the unknown individual is has a heterozygous genotype: 
    • Aa
    • can pass on either A or a alleles to offspring 
    • 50% of offspring or some offspring will show the recessive phenotype. 
                                                              Image from: 

Prestructural - flick through this presentation of 'Harry Potter and the Recessive Allele'

Multistructural - complete this test-cross question.

Relational - complete this more difficult question and punnett square about test crosses. 

Incomplete Dominance

Incomplete dominance is when neither allele is dominant over the other allele, and an intermediate phenotype is formed. 

No allele is more or less dominant than the other (although they may still be represented by capital and lower case letters). 

In the case of incomplete dominance, a heterozygous individual does NOT show the dominant phenotype; they show an in-between phenotype. 
  • Incomplete dominance = in between phenotype = intermediate phenotype
  • Therefore, there are three possible phenotypes:
    • TT will be one phenotype (e.g. red)
    • tt will be another phenotype (e.g. white)
    • Tt will be an intermediate phenotype between the other two (e.g. pink)

Unistructural - complete these online practice questions about incomplete dominance here.


Co-dominance is when neither allele is dominant over the other allele, so both phenotypes are expressed together. 

In the case of co-dominance, a heterozygous individual does NOT show the dominant phenotype; they show a mixture of the two phenotypes, expressed together at the same time. 
  • Co-dominance = "cosy" dominance = in there togetherCo = together.
  • Therefore, there are three possible phenotypes:
    • TT will be one phenotype (e.g. blood group A marker)
    • tt will be another phenotype (e.g. blood group B marker)
    • Tt will have both phenotypes expressed at the same time, (e.g. blood group AB has both A and B markers)

Relational - complete this worksheet on both incomplete and co-dominant monohybrid inheritance.

Relational - complete this past exam question on different types of dominance. Make sure to set out your answer first. 

Multiple Alleles

Multiple allele systems occur when there are more than 2 alleles for one trait (even though an individual can only posses a total of two of them in their own genotype).  

A commonly used example of this is seen in the picture above for co-dominance; blood types. 
  • In the multiple allele system for blood groups:
  • there are three alleles, often expressed like this:  IA, IBand Ii alleles.
    • IA  and IBare co-dominant.
    •  Ii allele is recessive.
  • there are four blood types, or blood groups, created by different combinations of these 3 alleles. 

  • Type A blood. People withIAIA have only A-type markers on their red blood cells and are said to have blood type A.
  • Type B blood. People with an IBIBhave only B-type markers on their red blood cells and have blood type B.
  • Type AB blood. People with IAIB have both A-type and B-type markers on their red blood cells and have blood type AB.
  • Type O blood. People with two Ii alleles (IiIi genotype) have neither A-type nor B-type markers on their cells and are said to have blood type O.
The IA and IB are considered co-dominant to one another because a heterozygous person IAIB has both types of markers on his or her cells, corresponding to type AB blood. 
The IA andIB alleles are both dominant to the recessive Ii allele, because a person where IAor IB paired with Iwill have the cell surface marker (and blood type) specified by the dominant non-Ii allele.

Multistructural - complete the practice question on dog coats first, then have a go at the practice exam question. Scroll down for help.

Lethal Alleles

Lethal alleles are when a mutation creates an allele that codes for a non-functional protein (a nonsense mutation). 
Most lethal alleles are recessive, but some can also be dominant.
Individuals who inherit one of the lethal alleles may have an altered phenotype, and those who are unlucky enough to inherit two die before birth. 

An example is the "creeper" allele in chickens, which causes the legs to be short and stunted.

Creeper is a dominant gene, heterozygous chickens display the creeper phenotype
If two creeper chickens are crossed, one would expect to have (from mendelian genetics) 3/4 of the offspring to be creeper and 1/4 to be normal
Instead the ratio obtained is 2/3 creeper and 1/3 normal.
This occurs because homozygous creeper chickens die.


Information from: 

Examples of Lethal Alleles in Humans:

  • Huntington's Disease - caused by a dominant lethal allele. Ths has stayed in the human gene pool because it doesn't cause an effect in sufferer's until after they've had children, maybe around 40-50. Causes brain degeneration among other symptoms, and death within 5 years of symptoms beginning. 
  • Sickle Cell Anaemia - caused by a recessive lethal allele. Fatal in the homozygous recessive state, but has proven to be an advantage for heterozygous individuals in countries that suffer from malaria. Causes a change in the shape of blood cells, and the bodies ability to transport oxygen around. 
  • Cystic Fibrosis - caused by a recessive lethal allele, homozygous recessive individuals die by age 30. Sticky mucous coats the lungs, causing constant respiratory infections. 
  • Tay-Sachs Disease - caused by a recessive lethal allele, homozygous recessive individuals die by age 4 due to brain deterioration. 

Relational - complete these practice questions from last year's 2015 exam paper. Lethal alleles were in the exam.