Monohybrid Crosses (Edexcel A Level Biology (A) SNAB): Revision Note
Monohybrid Crosses
Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
This means there is an equal chance of the zygote inheriting either allele from their parent
Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
These diagrams include a characteristic table called a Punnett square
The predicted genotypes that genetic diagrams produce are all based on chance
There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions
Genetic crosses can be shown by a Punnet square
Worked Example
Worked example: Genetic diagram
One of the genes for the coat colour of horses has the following two alleles:
B, a dominant allele produces a black coat when present
b, a recessive allele produces a chestnut coat when present in a homozygous individual
In this example a heterozygous male is crossed with heterozygous female
Parental phenotype: Black coat x Black coat
Parental genotypes: Bb x Bb
Parental gametes: B or b x B or b
Monohybrid Punnett square with Heterozygotes Table
Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb
Examiner Tips and Tricks
You need to be able to interpret genetic diagrams so take some time to get familiar with different sorts of diagrams such as pedigree charts and Punnet squares.
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