Revision Notes

17.2.1 Monohybrid Inheritance

Inheritance Definitions

  • A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)
  • Alleles are variations of the same gene


  • As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene
  • One of the alleles is inherited from the mother and the other from the father
  • This means that the alleles do not have to ‘say’ the same thing
  • For example, an individual has two copies of the gene for eye colour but one allele could code for brown eyes and one allele could code for blue eyes
  • The observable characteristics of an organism (seen just by looking – like eye colour, or found – like blood type) is called the phenotype
  • The combination of alleles that control each characteristic is called the genotype
  • Alleles can be dominant or recessive
  • A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype
  • A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype.
  • If there is only one recessive allele, it will remain hidden and the dominant characteristic will show
  • If the two alleles of a gene are the same, we describe the individual as being homozygous (homo = same)
  • An individual could be homozygous dominant (having two copies of the dominant allele), or homozygous recessive (having two copies of the recessive allele)
  • If the two alleles of a gene are different, we describe the individual as being heterozygous (hetero = different)
  • When completing genetic diagrams, alleles are abbreviated to single letters
  • The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case


Alleles, IGCSE & GCSE Biology revision notesAlleles of a gene can carry the same instructions or different instructions. You can only inherit two alleles for each gene, and they can be the same or different


  • We cannot always tell the genotype of an individual for a particular characteristic just by looking at the phenotype – a phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
  • If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents – we describe them as being ‘pure breeding’ as they will always produce offspring with the same characteristics
  • A heterozygous individual can pass on different alleles for the same characteristic each time it is bred with any other individual and can therefore produce offspring with a different genotype and phenotype than the parents – as such, heterozygous individuals are not pure breeding

Genetic Diagrams

  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene (mono = one)
  • This can be determined using a genetic diagram known as a Punnett square
  • A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
  • From this the ratio of these combinations can be worked out
  • Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case



  • The height of pea plants is controlled by a single gene that has two alleles: tall and short
  • The tall allele is dominant and is shown as T
  • The small allele is recessive and is shown as t

‘Show the possible allele combinations of the offspring produced when a pure breeding short plant is bred with a pure breeding tall plant’

  • The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic

F1 genetic cross, IGCSE & GCSE Biology revision notesA pure-breeding genetic cross in pea plants


  • This shows that there is a 100% chance that all the offspring will be tall


‘Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together’

F2 genetic cross, IGCSE & GCSE Biology revision notesA genetic cross diagram (F2 Generation)


  • All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are:
  • There is more variation in this cross, with a 3:1 ratio of tall : short, meaning each offspring has a 75% chance of being tall and a 25% chance of being short
  • The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed


‘Show the results of crossing a heterozygous plant with a short plant’

  • The heterozygous plant will be tall with the genotype Tt
  • The short plant is showing the recessive phenotype and so must be homozygous recessive – tt
  • The results of this cross are as follows:

Heterozygous pure recessive cross, IGCSE & GCSE Biology revision notesA cross between a heterozygous plant with a short plant


  • In this cross, there is a 1:1 ratio of tall to short, meaning a 50% chance of the offspring being tall and a 50% chance of the offspring being short


How to construct Punnett squares

  • Determine the parental genotypes
  • Select a letter that has a clearly different lower case, for example: Aa, Bb, Dd
  • Split the alleles for each parent and add them to the Punnett square around the outside
  • Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring
  • You may be asked to comment on the ratio of different allele combinations in the offspring, calculate a percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring
  • Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses

Exam Tip

You should always write the dominant allele first, followed by the recessive allele.

If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lower case so the examiner is not left in any doubt as to which is dominant and which is recessive.

For example, C and c are not very different from each other, whereas A and a are!

Extended Only

Identifying an Unknown Genotype

  • Breeders can use a test cross to find out the genotype of an organism showing the dominant phenotype
  • This involves crossing the unknown individual with an individual showing the recessive phenotype – if the individual is showing the recessive phenotype, then its genotype must be homozygous recessive
  • By looking at the ratio of phenotypes in the offspring, we can tell whether the unknown individual is homozygous dominant or heterozygous


‘A plant breeder has a tall plant of unknown genotype. How can they find out whether it is homozygous dominant or heterozygous?’

  • The short plant is showing the recessive phenotype and so must be homozygous recessive – tt

Identifying an unknown genotype, IGCSE & GCSE Biology revision notesDetermining genotypes from offspring


  • If the tall plant is homozygous dominant, all offspring produced will be tall
  • If the tall plant is heterozygous, half the offspring will be tall and the other half will be short

Family Pedigrees

  • Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
  • This can be used to work out the probability that someone in the family will inherit the genetic disorder

Family pedigree chart, IGCSE & GCSE Biology revision notesA family pedigree chart


  • Males are indicated by the square shape and females are represented by circles
  • Affected individuals are red and unaffected are blue
  • Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)
  • The family pedigree above shows:
    • both males and females are affected
    • every generation has affected individuals
    • That there is one family group that has no affected parents or children
    • the other two families have one affected parent and affected children as well

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now