Single Gene Inheritance
What is monohybrid inheritance?
- 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
- Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lowercase
Monohybrid inheritance 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
Homozygous dominant X homozygous recessive
- 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
F1 generation monohybrid inheritance Punnett square diagram
A pure-breeding genetic cross in pea plants
- This shows that there is a 100% chance that all the offspring will be tall
Heterozygous X heterozygous
- Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together
F2 generation monohybrid inheritance Punnett square diagram
A genetic cross diagram (F2 Generation)
- 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
Heterozygous X homozygous recessive
- 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:
Monohybrid inheritance Punnett square diagram
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
How to construct 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 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
Examiner Tip
You should always write the dominant allele first, followed by the recessive allele.
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.
For example, C and c are not very different from each other, whereas A and a are!