Monohybrid Inheritance: Genetic Diagrams (Edexcel IGCSE Biology: Double Science)
Revision Note
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LáraExpertise
Biology Lead
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 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
Exam Tip
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.
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