Monohybrid Inheritance (Cambridge (CIE) O Level Biology): Revision Note

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

• The dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case

Example:

• 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

Two pure breeding pea plants genetic cross diagram

A pure-breeding genetic cross in pea plants showing a 100% heterozygous outcome from the cross

• 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’

Heterozygous pea plants genetic cross diagram

A genetic cross diagram (F2 Generation) with a 3:1 possible outcome of offspring phenotypes

• 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:

Punnett square between heterozygous and homozygous recessive plants

A 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

• The predicted genotypes that Punnett squares 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, especially when there are small numbers of offspring 

How to construct Punnett squares

• Determine the parental genotypes

• Select a letter that has a clearly different lower case, for example: Aa, Bb, Dd (not Cc, Oo, 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 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