By the end of this lesson
- Construct a monohybrid cross.
- Separate genotype and phenotype ratios.
- Interpret probabilities correctly.
BIG QUESTION
How can we predict offspring genotypes without predicting an individual child?
A genetic cross gives probabilities for each independent fertilisation event.
Begin with defined parental genotypes
State what each allele symbol represents and write the parental genotypes. Do not infer an unknown genotype without evidence.
A homozygous parent produces one gamete type at the locus; a heterozygote produces two.
Combine gametes systematically
Place gametes on the edges of a Punnett gridA diagram used to show all possible combinations of gametes in a genetic cross and the probability of each offspring genotype. or use a branching method. Each box combines one allele from each parent.
The boxes show equally likely combinations only when the gametes listed are equally likely and fertilisation is random.
State genotype, phenotype and probability
Count genotype combinations before applying the dominance relationship to obtain phenotypes.
A 1:1 or 3:1 ratio is an expectation across many offspring, not a guaranteed pattern in a small family.
Cross workflow
Each line earns a different kind of evidence.
| Step | Action | Purpose |
|---|---|---|
| Alleles | Define symbols. | Removes ambiguity. |
| Parents | Write genotypes. | Sets the cross. |
| Gametes | One allele per gamete. | Applies segregation. |
| Offspring | Combine gametes. | Lists possible genotypes. |
| Interpretation | Count and convert to phenotype. | States probability/ratio. |
Language that earns precision
- monohybrid cross
- Inheritance model following one gene locus.
- Punnett grid
- Diagram combining possible parental gametes.
- genotypic ratio
- Relative frequencies of offspring allele combinations.
- phenotypic ratio
- Relative frequencies of observable outcomes.
Common misconceptions
Claim: A Punnett grid predicts the exact next child.
Correction: It gives probabilities for each fertilisation event.
Claim: A heterozygous gamete is Aa.
Correction: A haploid gamete carries one allele, A or a.
Claim: Four boxes mean a parent will have four children.
Correction: Boxes represent possible combinations, not family size.
Match the answer to the command word
Allele key → parents → gametes → grid → ratio/probability.
Each gamete must contain one allele for the locus.
Recall, apply, then justify
For the cross Aa × Aa, state the expected genotypic and phenotypic ratios when A is completely dominant.
Hint
Combine A/a with A/a, then group phenotypes.
Suggested marking points
- The possible genotypes are AA, Aa, Aa and aa.
- The genotypic ratio is 1 AA : 2 Aa : 1 aa.
- AA and Aa show the dominant phenotype.
- The phenotypic ratio is 3 dominant : 1 recessive.
Explain why two recessive-phenotype parents cannot have a dominant-phenotype child in this simple model.
Hint
Write parental and gamete alleles.
Suggested marking points
- Each recessive-phenotype parent has genotype aa.
- Each parent produces only a gametes.
- All offspring are aa and lack a dominant allele.
A dominant-phenotype individual is crossed with aa and half the offspring are recessive. Infer the unknown genotype.
Planning hint
Use the recessive offspring as evidence of a gamete.
What a strong design includes
- A recessive offspring has genotype aa.
- The known aa parent always contributes a.
- The unknown parent must also contribute a.
- The unknown parent shows the dominant phenotype, so it also carries A.
- Its genotype is therefore Aa.
- An Aa × aa cross predicts a 1:1 phenotype ratio.