AP Syllabus focus:
‘Monohybrid crosses use Punnett squares to predict genotypic and phenotypic ratios for a single gene.’
Monohybrid crosses connect meiosis, fertilization, and allele segregation to predictable inheritance patterns. By organizing possible gametes in a Punnett square, you can forecast offspring genotypes and phenotypes for one gene.
Core idea: one gene, two alleles
A monohybrid cross tracks inheritance for a single gene with two alternative alleles (often represented by letters such as A and a). Predictions depend on the dominance relationship and the parental genotypes.
Monohybrid cross: A genetic cross that examines inheritance of one gene by predicting offspring allele combinations from parental genotypes.
Alleles, dominance, and notation
Use uppercase for a dominant allele (expressed in heterozygotes) and lowercase for a recessive allele (expressed only when no dominant allele is present).
The two alleles in a diploid organism form a genotype (e.g., AA, Aa, aa), which contributes to an observable phenotype.
Punnett square: A grid-based method that lists all possible gamete combinations from two parents to predict offspring genotypes and expected ratios.
A Punnett square does not “guarantee” outcomes for a small number of offspring; it predicts expected ratios over many fertilization events.
Setting up a Punnett square correctly
Step 1: Identify parental genotypes
Write each parent’s genotype for the gene of interest (e.g., AA, Aa, aa).
If only phenotypes are given, you may be able to infer possible genotypes only when dominance is specified.
Step 2: Determine possible gametes from each parent
Because alleles segregate during gamete formation, each gamete carries one allele for the gene.
This meiosis I figure shows homologous chromosome pairs separating, which is the physical basis for Mendel’s law of segregation. Because homologs carry different alleles, their separation during meiosis produces gametes that contain only one allele per gene, enabling Punnett-square probability predictions. Source
Homozygous parent (AA or aa) produces one gamete type (A only or a only).
Heterozygous parent (Aa) produces two gamete types (A and a), typically in equal proportions.
Step 3: Fill the grid to list zygote genotypes
Place one parent’s gametes along the top and the other parent’s gametes down the side.
Each box represents a possible fertilization outcome (zygote genotype).
Step 4: Convert genotypes to phenotypes and ratios
After counting outcomes:
Genotypic ratio: relative frequencies of each genotype (e.g., 1 AA : 2 Aa : 1 aa).
Phenotypic ratio: relative frequencies of each phenotype (e.g., 3 dominant : 1 recessive when complete dominance applies).
Ratios and probability language (what “predict” means)
A Punnett square lists equally likely combinations when gametes are equally likely and fertilization is random. Expected ratios can be expressed as fractions, percentages, or proportions.
= probability of a specific genotype or phenotype (unitless)
= number of equally likely zygote combinations predicted (unitless)
When expected ratios do not match observed class data, the mismatch may reflect random sampling variation, limited offspring number, or incorrect assumptions about dominance or parental genotypes.
Common monohybrid cross patterns (complete dominance)
Heterozygote × heterozygote (Aa × Aa)
Typical genotypic ratio: 1:2:1
Typical phenotypic ratio: 3:1 (dominant:recessive)
Homozygous dominant × heterozygote (AA × Aa)
Offspring all show the dominant phenotype
Genotypes split between AA and Aa in a predictable ratio
Heterozygote × homozygous recessive (Aa × aa)
Produces both phenotypes in predictable proportions
Especially useful for revealing whether the dominant phenotype parent is heterozygous (conceptually, even when not formally labelled a “test cross”)
Vocabulary that must be used precisely
Homozygous: two identical alleles (AA or aa)
Heterozygous: two different alleles (Aa)
Expected ratio: prediction from the model (Punnett square), not a promise for small samples
FAQ
During meiosis, allele segregation means each gamete receives one allele.
Equal proportions assume:
normal segregation
no meiotic drive
equal gamete viability
Use letters matching the dominant phenotype name (e.g., $P$ for purple), or use gene-based notation.
Avoid value-laden labels; dominance does not mean “stronger” or “more common”.
It may indicate violated assumptions, such as:
non-random fertilisation
reduced viability of a genotype
misidentified parental genotype
Large sample sizes help distinguish chance from bias.
Yes, but you may generate multiple predicted phenotype patterns under different dominance models.
You then compare which model best matches the observed categories and proportions.
State both:
the raw counts (e.g., 47:18)
the simplified ratio (approximate, e.g., $\sim$3:1)
Using “approximately” clarifies that ratios are expectations, not exact outcomes.
Practice Questions
In a plant species, allele is dominant to . Predict the expected phenotypic ratio from a cross between two heterozygous plants. (2 marks)
States parental genotypes are (1)
Gives expected phenotypic ratio dominant : recessive (1)
In mice, black fur () is dominant to white fur (). A black mouse with genotype is crossed with a white mouse. Use a Punnett square approach to determine (i) the expected genotypic ratio and (ii) the expected phenotypic ratio of the offspring, and briefly justify each. (5 marks)
Identifies cross as (1)
Lists gametes from as and (1)
Lists gametes from as only (1)
Correct genotypic ratio (1)
Correct phenotypic ratio black : white with dominance justification (1)
