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 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

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.

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:

A cross between a heterozygous plant with a short plant

• 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

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%