Monohybrid Inheritance: Genetic Diagrams (Edexcel IGCSE Biology)
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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
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
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
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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