Monohybrid Inheritance (SQA National 5 Biology): Revision Note

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

• To construct a Punnett square, these steps can be followed:

  1. 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

     • Add them to the Punnett square around the outside

  2. 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

  • 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

Crossing the offspring from the first cross

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:

Predicting phenotype ratios

• Predicted phenotype ratios (such as 3:1 in a monohybrid cross) show the expected outcomes of a genetic cross in theory

• However, the actual results in offspring often differ from these predictions due to several factors:

  • Chance (random fertilisation):

    • Fertilisation is a random process, so the actual combinations of alleles that occur can vary from predicted ratios, especially when sample sizes are small

  • Small sample size:

    • With few offspring, random variation can cause ratios to differ noticeably from the expected values

    • Larger numbers of offspring usually produce ratios closer to predictions

  • Environmental effects:

    • The environment can influence how genes are expressed, meaning the observed phenotype may not exactly match the genotype

• Predicted ratios are theoretical probabilities, and random chance, small sample sizes and environmental influences can all cause real-life results to differ