AQA A Level Biology

Revision Notes

7.1.2 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: 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
  • In this example a heterozygous male is crossed with heterozygous female

Parental phenotype:   black coat x black coat

Parental genotype:     Bb                   Bb

Parental gametes:      B or b              B or b

Monohybrid punnett square with heterozygotes 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


  • 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 (eg. IAIA)
  • There will be more possible phenotypes and so the predicted ratios will be different

Worked example: Codominance

  • The gene for blood type has three alleles:
    • A, a dominant allele produces blood type A
    • B, a dominant allele produces blood type B
    • O, 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 punnett square with codominance 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


  • Sex-linked genes are only present on one sex chromosome and not the other
  • This means the sex of an individual affects what alleles they pass on to their offspring through their gametes
  • If the gene is on the X chromosome males (XY) will only have one copy of the gene, whereas females (XX) will have two
  • There are three phenotypes for females – normal, carrier and has the disease, whereas males have only two phenotypes – normal or has the disease

Worked example: Sex-linkage

  • Haemophilia is a well known sex-linked disease
  • There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
  • There are two alleles for factor VIII, the dominant F allele which codes for normal factor VIII and the recessive f allele which results in a lack of factor VIII
  • When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can’t clot normally
  • The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia

Parental phenotypes: carrier female x normal male

Parental genotypes:      XFXf                              XFY

Parental gametes:      XF or Xf                        XF or Y

Monohybrid punnett square with sex-linkage table

Monohybrid Crosses_1, downloadable AS & A Level Biology revision notes

Predicted ratio of phenotypes in offspring – 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring: 1 XFXF : 1 XFXf : 1 XFY : 1 XfY

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. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific gender (boys or girls).


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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