Monohybrid Inheritance: Genetic Diagrams (Edexcel IGCSE Biology: Double Science)

• 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 lower case

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 breeding short plant is bred with a pure breeding tall plant

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

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

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

• 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:

A cross between a heterozygous plant with a short plant

Constructing Punnett squares

• Determine the parental genotypes
• Select a letter that has a clearly different lower case, for example, Aa, Bb, Dd
• 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