Monohybrid cross
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of key inheritance terms and genetic crosses from GCSE. You can test your knowledge on these below.
What is a genotype?
The genetic constitution of an organism (the alleles it possesses).
What is a phenotype?
The observable characteristics of an organism resulting from the interaction of its genotype with the environment.
What is a dominant allele?
An allele that is expressed in the phenotype even if only one copy is present (heterozygous condition).
Topic Explainer Video
Check out this @JoeDoesBiology video that explains monohybrid cross or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is a Monohybrid Cross?
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A monohybrid cross involves the study of inheritance of a single gene with two different alleles.
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It predicts the genotypes and phenotypes of offspring from two parents.
Key Concepts:
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Homozygous: Both alleles for a gene are the same (e.g., TT or tt).
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Heterozygous: The two alleles for a gene are different (e.g., Tt).
Setting up a Punnett Square
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Identify parental genotypes.
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Determine the gametes each parent can produce, these are often circled to show they are gametes (produced by meiosis)
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Complete a Punnett square to find all possible offspring combinations.
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Calculate phenotype ratios and probabilities.
Example
Tall (T) is dominant to dwarf (t)
Parents: Tt × Tt
|
T (from Parent 1) |
t (from Parent 1) |
|
|
T (Parent 2) |
TT |
Tt |
|
t (Parent 2) |
Tt |
tt |
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Genotypic Ratio: 1 TT : 2 Tt : 1 tt
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Phenotypic Ratio: 3 Tall : 1 Dwarf
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Probability of tall offspring: ¾ or 75%
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Probability of dwarf offspring: ¼ or 25%
No answer provided.
Key Terms
- Genotype: The combination of alleles an organism has for a particular gene.
- Phenotype: The observable traits of an organism, determined by both genotype and its interaction with the environment.
- Homozygous: Two identical alleles (TT or tt).
- Heterozygous: Two different alleles (Tt).
- Dominant: Allele that is always expressed if present.
- Recessive: Allele that is only expressed if there is no dominant allele present.
No answer provided.
Exam Tip
Clearly show working steps in Punnett squares, including which parent is which and the offspring phenotypes of each genotype.
A pea plant heterozygous for purple flowers (Pp) is crossed with another heterozygous plant (Pp).
Use a genetic diagram to predict the genotype and phenotype ratios of the offspring.
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Parental genotypes: Pp × Pp
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Gametes: P and p from each parent
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Punnett square:
|
P |
p |
|
|
P |
PP |
Pp |
|
p |
Pp |
pp |
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Genotypic ratio: 1 PP : 2 Pp : 1 pp
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Phenotypic ratio: 3 purple : 1 white
Bullet Points for Exam Success:
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Genotypes listed clearly.
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Gametes circled/identified.
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Punnett square correctly completed.
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Clear phenotype ratio at the end.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!