Monohybrid Inheritance: Genetic Diagrams (Edexcel IGCSE Biology)

Revision Note

Lára Marie McIvor

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Monohybrid Crosses

  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene

  • 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 lowercase

Constructing Punnett squares

  • Determine the parental genotypes

  • Select a letter that has a clearly different lowercase, for example, Aa, Bb, Dd (avoid letters such as Cc or Ss)

  • 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 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

Example of monohybrid inheritance: Pea plants

  • 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

A pure-breed short plant is bred with a pure-breed tall plant

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

F1 genetic cross, IGCSE & GCSE Biology revision notes

A pure-breeding genetic cross in pea plants. It shows that all offspring will be have the tall phenotype.

Crossing the offspring from the first cross

F2 genetic cross, IGCSE & GCSE Biology revision notes

A genetic cross diagram (F2 generation). It shows a ratio of 3 tall : 1 short  for any offspring.

Interpreting the results

  • All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short)

  • There is more variation in the second cross, with a 3:1 ratio of tall : short

  • The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed

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 notes

A cross between a heterozygous plant with a short plant

Examiner Tips and Tricks

You will NOT be expected to explain the polygenic inheritance of characteristics using a genetic diagram, you just need to be aware that many characteristics are controlled by groups of genes and that this is known as polygenic inheritance.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.

Predicting Probabilities of Outcomes from Monohybrid Crosses

  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene

  • This can be determined using a genetic diagram known as a Punnett square

Punnett squares with pea plants

  • 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

Crossing two heterozygous plants

F2 genetic cross, IGCSE & GCSE Biology revision notes

A genetic cross diagram crossing two heterozygous pea plants. It shows a ratio of 3 tall : 1 short  for any offspring.

Interpreting the results

  • The possible combinations of offspring bred from two parents with Tt genotype are: TT (tall), Tt (tall), tt (short)

    • The Tt parent plants would have been produced from an F1 cross of two homozygous plants: TT and tt (pure-breeding parents)

  • The ratio for the offspring is 3:1 of tall : short

Calculating probabilities from Punnett squares

  • 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

  • However, you can also make predictions of the offsprings’ characteristics by calculating the probabilities of the different phenotypes that could occur

    • For example, in the genetic cross above, two plants with the genotype Tt (heterozygous) were bred together

    • The possible combinations of offspring bred from these two parent plants are: TT (tall), Tt (tall), tt (short

    • The offspring genotypes showed a 3:1 ratio of tall : short

    • Using this ratio, we can calculate the probabilities of the offspring phenotypes

    • The probability of an offspring being tall is 75%

    • The probability of an offspring being short is 25%

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.