A monohybrid cross is a genetics problem used to predict the possible offspring from parents that differ in one inherited trait. It is one of the simplest applications of Mendelian genetics and is usually solved with a Punnett square.
To solve a monohybrid cross, you identify the parents’ genotypes, determine which alleles each parent can pass to an offspring, place those alleles into a Punnett square, and then interpret the resulting genotypes and phenotypes.
What is a monohybrid cross?
A monohybrid cross examines the inheritance of a single gene or, more precisely in basic Mendelian problems, a single genetic characteristic controlled by a pair of alleles.
An allele is one version of a gene. For example, suppose a fictional plant trait is controlled by two alleles:
- A = dominant allele
- a = recessive allele
An organism receives one allele from each parent, so its genotype for this gene contains two alleles.
The three possible genotypes are:
- AA — homozygous dominant
- Aa — heterozygous
- aa — homozygous recessive
Homozygous means the two alleles are the same. Heterozygous means they are different.
In a simple dominant-recessive model, an organism with at least one dominant allele (AA or Aa) shows the dominant phenotype. Only aa shows the recessive phenotype.
The key idea behind a Punnett square
A Punnett square organizes the allele combinations that can occur when two parents reproduce.
Each parent contributes only one allele for the gene being studied to each offspring. The possible allele contributed by one parent is called a gamete in this context.
For example, a parent with genotype Aa can produce gametes carrying either A or a. A parent with genotype AA can produce only A gametes, while a parent with genotype aa can produce only a gametes.
How to solve a monohybrid cross step by step
Step 1: Identify the alleles
First, determine which allele represents the dominant version of the trait and which represents the recessive version.
For a basic problem, a capital letter is commonly used for the dominant allele and the corresponding lowercase letter for the recessive allele. For example:
A = dominant allele
a = recessive allele
The letter choice itself has no biological significance; it is simply a notation system.
Step 2: Determine the parents’ genotypes
Next, identify the genotype of each parent.
Suppose both parents are heterozygous:
Parent 1: Aa
Parent 2: Aa
This is often called a heterozygous × heterozygous monohybrid cross.
Do not confuse genotype with phenotype. The genotype describes the alleles an organism carries, whereas the phenotype is the observable characteristic produced by the genotype and its environment.
Step 3: Determine each parent’s possible gametes
Each offspring receives one allele from each parent.
An Aa parent can therefore contribute either A or a.
For this cross:
Parent 1 gametes: A, a
Parent 2 gametes: A, a
Because each parent has two different alleles, each has two possible types of gametes.
Step 4: Set up the Punnett square
Write the possible gametes from one parent across the top of a two-by-two grid. Write the possible gametes from the other parent down the left side.
Then combine the allele at the top of each column with the allele at the left of each row.
For Aa × Aa, the combinations are:
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Each of the four boxes represents one possible allele combination. The boxes are not four guaranteed offspring; they represent the relative probabilities of the possible outcomes under the assumptions of the model.
Step 5: Find the genotype probabilities
Count each genotype in the completed square.
There is:
- 1 AA
- 2 Aa
- 1 aa
So the expected genotype ratio is:
1 AA : 2 Aa : 1 aa
Expressed as probabilities:
- AA = 25%
- Aa = 50%
- aa = 25%
These are probability expectations, not a prediction that every four offspring will necessarily contain exactly one AA, two Aa, and one aa.
Step 6: Determine the phenotype probabilities
Now translate the genotypes into phenotypes.
If A is completely dominant over a, both AA and Aa produce the dominant phenotype. Only aa produces the recessive phenotype.
Therefore:
- AA = dominant phenotype
- Aa = dominant phenotype
- Aa = dominant phenotype
- aa = recessive phenotype
The expected phenotype ratio is therefore:
3 dominant : 1 recessive
The corresponding probabilities are:
- Dominant phenotype = 75%
- Recessive phenotype = 25%
A worked example
Suppose a fictional plant species has purple flowers controlled by a dominant allele P, while white flowers are produced by the recessive allele p. Two heterozygous purple-flowered plants are crossed.
The parental cross is:
Pp × Pp
Each parent can contribute either P or p.
The Punnett square gives:
| P | p | |
|---|---|---|
| P | PP | Pp |
| p | Pp | pp |
The offspring have three possible genotypes:
PP, Pp, and pp
Their expected proportions are 25% PP, 50% Pp, and 25% pp.
Because P is dominant, PP and Pp produce purple flowers, while pp produces white flowers. Thus, the expected phenotype probabilities are 75% purple and 25% white.
How different parental crosses change the answer
The method stays the same even when the parents have different genotypes. What changes is the set of gametes each parent can produce.
Homozygous dominant × homozygous recessive
Consider:
AA × aa
The AA parent can produce only A gametes, and the aa parent can produce only a gametes.
Every offspring must therefore be:
Aa
The expected genotype is 100% Aa. Under complete dominance, the expected phenotype is 100% dominant.
Homozygous dominant × heterozygous
For:
AA × Aa
The AA parent contributes only A. The Aa parent can contribute A or a.
The offspring are expected to be:
- 50% AA
- 50% Aa
All have at least one dominant allele, so the expected phenotype is 100% dominant.
Heterozygous × homozygous recessive
For:
Aa × aa
The Aa parent can contribute A or a, while the aa parent contributes only a.
The offspring are expected to be:
- 50% Aa
- 50% aa
With complete dominance, this produces an expected phenotype ratio of 1 dominant : 1 recessive, or 50% for each phenotype.
Homozygous recessive × homozygous recessive
For:
aa × aa
Both parents can contribute only a.
Every offspring is therefore aa, giving an expected 100% recessive phenotype under the simple dominant-recessive model.
Genotype versus phenotype: the distinction that matters most
Many monohybrid-cross mistakes happen when genotype and phenotype are treated as though they mean the same thing.
A genotype describes the allele combination, such as AA, Aa, or aa.
A phenotype describes the observable trait associated with that genotype under the conditions being considered.
For a completely dominant allele, AA and Aa have the same dominant phenotype even though their genotypes differ. This is why the genotype ratio and phenotype ratio can be different.
For the cross Aa × Aa, the genotype ratio is 1:2:1, whereas the phenotype ratio is 3:1.
Common mistakes to avoid
One frequent error is putting both alleles from a parent into a single gamete. A gamete receives only one allele for the gene in a basic monohybrid cross.
Another mistake is assuming that a dominant allele is always more common in a population. Dominant describes how an allele affects phenotype in a particular genetic relationship; it does not mean the allele is more frequent, stronger, or more evolutionarily successful.
It is also important not to interpret the four Punnett-square boxes as four actual offspring. They represent possible allele combinations and their expected proportions. Probability does not guarantee that a small family will match the theoretical percentages exactly.
Finally, the familiar 3:1 phenotype ratio applies only under the assumptions of a simple single-gene, complete-dominance model with the relevant parental genotypes. Not every real-world trait follows this pattern. Incomplete dominance, codominance, multiple alleles, gene interactions, sex linkage, and environmental effects can produce different inheritance patterns.
A quick method for solving any basic monohybrid cross
When faced with a new problem, work through it in this order:
Identify the alleles → write the parental genotypes → determine each parent’s gametes → construct the Punnett square → combine the alleles → count genotypes → translate genotypes into phenotypes.
The most important question at each stage is simple: What allele can this parent contribute? Once those possible gametes are identified, the rest of the Punnett square follows directly.
For the classic Aa × Aa cross, the final result is 1 AA : 2 Aa : 1 aa for genotype and 3 dominant : 1 recessive for phenotype, assuming complete dominance. That distinction—and the fact that Punnett squares describe probabilities rather than guaranteed outcomes—is the foundation for correctly interpreting a monohybrid cross.
