OCR A Level Biology

Revision Notes

6.2.4 Predicting Inheritance: Monohybrid Crosses

Monohybrid Crosses

  • Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
  • Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
  • When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
    • This means there is an equal chance of the zygote inheriting either allele from their parent
  • Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
    • These diagrams include a characteristic table called a Punnett square
  • The predicted genotypes that genetic diagrams produce are all based on chance
    • There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions

Worked Example

Worked example: Genetic diagram

  • One of the genes for the coat colour of horses has the following two alleles:
    • B, a dominant allele produces a black coat when present
    • b, a recessive allele produces a chestnut coat when present in a homozygous individual
  • These two alleles separate during meiosis because they are on different chromosomes of a homologous pair
  • In this example, a heterozygous male is crossed with a heterozygous female

Parental phenotype:   black coat x black coat

Parental genotype:     Bb                   Bb

Parental gametes:      B or b              B or b

Monohybrid Cross with Heterozygotes Punnett Square Table

Monohybrid Punnett Square with Heterozygotes Table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring = 3 black coat : 1 chestnut coat
  • Predicted ratio of genotypes in offspring = 1 BB : 2 Bb : 1 bb

Multiple alleles

  • Many genes have more than two alleles
  • However, a diploid individual will still only inherit two of the possible alleles
  • Human blood groups provide a good example of multiple allelism as there are three different alleles:
    • Alleles IA and IB are codominant, but both are dominant to IO
    • I represents the gene and the superscripts (A, B or O) represent the alleles
    • IA results in the production of antigen A in the blood
    • IB results in the production of antigen B in the blood
    • IO results in no antigens being produced in the blood
  • These three possible alleles can give us the following genotypes and phenotypes:

How Three Alleles (IA , IB and IO) Combine to Give Human Blood Groups Table

Inheritance of Blood Group table, IGCSE & GCSE Biology revision notes


  • When working with codominant alleles the genetic diagrams can be constructed in a similar way, however the genotypes are represented using a capital letter for the gene and superscript letters for the alleles (e.g. IAIA)
  • There will be more possible phenotypes and so the predicted ratios will be different

Worked Example

Worked example: Codominance

  • The gene for blood type has three alleles:
    • IA, a dominant allele produces blood type A
    • IB, a dominant allele produces blood type B
    • IO, two recessive alleles will produce blood type O
  • In this example, a blood type A person is crossed with a blood type B person

Parental phenotype:   Blood type A x Blood type B

Parental genotype:     IAIO                  IBIO

Parental gametes:      IA or IO             IB or IO

Monohybrid Cross with Codominance Punnett Square Table

Monohybrid Punnett Square with Codominance Table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring = 1 Blood type AB : 1 Blood type A : 1 Blood type B : 1 Blood type O
  • Predicted ratio of genotypes in offspring = 1 IAIB : 1 IAIO : 1 IBIO : 1 IOIO

Exam Tip

Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a punnett square, you need to show that you have thought about the possible gametes that can be produced by each parent.

Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross.


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
Already a member?
Go to Top