Genetics questions can look complicated because they use letters, ratios and unfamiliar vocabulary all at once. But most simple inheritance problems follow the same chain of reasoning: identify the alleles, work out which alleles enter the gametes, combine those gametes during fertilisation, and then interpret the possible offspring.
Once you understand that sequence, a Punnett square is no longer something to memorise. It becomes a compact way of showing what meiosis and random fertilisation are doing.
Try these five questions before reading the main explanation. They test the ideas you will need when constructing genetic diagrams.
- 5 diagnostic questions
- Key genetics vocabulary and inheritance ideas
- Instant explanation for every answer option
The Genetics Vocabulary You Need First
Before drawing a genetic diagram, it helps to separate several terms that students often mix up. Each describes a different part of the inheritance story.
| Term | Meaning | Example |
|---|---|---|
| Gene | A section of DNA carrying information that can influence a characteristic | A gene involved in pea plant height |
| Allele | An alternative form of a gene | T and t |
| Genotype | The combination of alleles possessed by an organism | TT, Tt or tt |
| Phenotype | The characteristic expressed by an organism | Tall plant or short plant |
| Homozygous | Having two identical alleles for a gene | TT or tt |
| Heterozygous | Having two different alleles for a gene | Tt |
Dominant and recessive alleles
Suppose a gene affecting plant height has two alleles. We will use T for the allele producing the dominant tall phenotype and t for the recessive allele producing the short phenotype when two copies are present.
| Genotype | Description | Phenotype |
|---|---|---|
| TT | Homozygous dominant | Tall |
| Tt | Heterozygous | Tall |
| tt | Homozygous recessive | Short |
Notice that the word dominant does not mean that an allele is stronger, better or more common. It simply describes what happens when two different alleles occur together in a heterozygous genotype.
If Tt produces the tall phenotype, then T is dominant to t.
Useful rule: When representing complete dominance, use the same letter for the two alleles of the same gene. The dominant allele is written as a capital letter and the recessive allele as the corresponding lowercase letter.
For example, use T and t, not T and S.
Why Each Gamete Carries Only One Allele
A diploid body cell contains pairs of homologous chromosomes. For a particular gene, this means that the organism normally possesses two alleles — one inherited from each parent.
During meiosis, homologous chromosomes separate. The alleles carried on those chromosomes therefore separate as well.
Each haploid gamete receives only one allele for the gene being considered.
For a heterozygous organism with genotype Tt:
Diploid parent: Tt
Possible gametes: T or t
The gamete does not contain Tt. The pair of alleles is restored only when two gametes fuse during fertilisation.
How to Construct a Monohybrid Genetic Diagram
Consider two heterozygous tall pea plants:
Here, T is the dominant allele for tall plants and t is the recessive allele for short plants.
Step 1: State the parental phenotypes
Both parents are tall because each possesses at least one dominant T allele.
Step 2: State the parental genotypes
Step 3: Work out the possible gametes
Each heterozygous parent can produce gametes carrying either T or t.
Parent 2 gametes: T, t
Step 4: Combine the gametes
A Punnett square shows the possible combinations formed when one gamete from each parent fuses.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Step 5: State the genotype ratio
The four equally likely combinations are TT, Tt, Tt and tt.
Step 6: Convert the genotypes into phenotypes
TT and Tt are both tall. Only tt is short.
Do not stop at the Punnett square. Read exactly what the question asks for. A question may require the offspring genotypes, the genotype ratio, the phenotype ratio or the probability of one particular outcome.
Three Monohybrid Crosses Worth Recognising
You should understand how the ratios arise rather than memorising them blindly. However, several crosses occur often enough that recognising their expected outcomes is useful.
| Cross | Possible offspring genotypes | Expected phenotype outcome |
|---|---|---|
| TT × tt | All Tt | All dominant phenotype |
| Tt × Tt | 1 TT : 2 Tt : 1 tt | 3 dominant : 1 recessive |
| Tt × tt | 1 Tt : 1 tt | 1 dominant : 1 recessive |
The last cross is particularly useful. If an organism showing the dominant phenotype produces an offspring showing the recessive phenotype, the dominant-looking parent must have carried the recessive allele.
For example, if a tall plant produces a short offspring, that tall parent cannot be TT. It must be Tt, because the short offspring needs to receive a t allele from each parent.
Expected Ratios Are Probabilities, Not Guarantees
A Punnett square gives the probability of each fertilisation outcome. It does not predict exactly what must happen in every small group of offspring.
For a Tt × Tt cross, each fertilisation has:
- a ¼ probability of producing TT,
- a ½ probability of producing Tt,
- a ¼ probability of producing tt.
This corresponds to an expected phenotype ratio of 3 tall : 1 short. However, four actual offspring do not have to consist of exactly three tall and one short individual.
Why? Fertilisation is a random process. Each offspring represents a separate chance event. Random variation can therefore cause the observed ratio to differ from the expected ratio.
As the number of offspring becomes larger, the observed proportions are generally more likely to approach the expected probabilities.
A ratio does not remember previous offspring
Suppose three successive offspring happen to show the dominant phenotype. The probability for the next fertilisation does not change simply because of what happened previously. Each fertilisation is a new event.
Codominance: When Both Alleles Are Expressed
Not every pair of alleles follows a simple dominant-recessive relationship. In codominance, both alleles in a heterozygous individual are expressed in the phenotype.
The human ABO blood group system provides an important example.
The three ABO alleles
There are three common alleles in the population: IA, IB and i.
- IA and IB are codominant to each other.
- IA is dominant to i.
- IB is dominant to i.
Although three alleles exist in the population, one person still possesses only two alleles for the ABO gene because one is inherited from each parent.
| Blood group phenotype | Possible genotype(s) |
|---|---|
| A | IAIA or IAi |
| B | IBIB or IBi |
| AB | IAIB |
| O | ii |
A person with genotype IAIB expresses both alleles, producing blood group AB. Neither allele masks the other.
Example: IAi × IBi
The first parent can produce IA or i gametes. The second parent can produce IB or i gametes.
| IB | i | |
|---|---|---|
| IA | IAIB AB | IAi A |
| i | IBi B | ii O |
In this particular cross, each of the four blood group phenotypes has an expected probability of ¼.
Sex Determination in Humans
The same idea of separating chromosomes into gametes can be used to understand the standard XX–XY model of sex determination in humans.
- Females usually have two X chromosomes: XX.
- Males usually have one X and one Y chromosome: XY.
During meiosis, an XX female produces eggs carrying an X chromosome. An XY male produces sperm carrying either an X chromosome or a Y chromosome.
| X sperm | Y sperm | |
|---|---|---|
| X egg | XX | XY |
| X egg | XX | XY |
The expected probability is therefore approximately:
In this model, the egg always contributes an X chromosome, whereas the fertilising sperm contributes either X or Y. Therefore, it is the chromosome carried by the sperm that determines whether the zygote is XX or XY.
Important probability idea: A 1 : 1 expected ratio does not mean that every family must contain equal numbers of sons and daughters. Each fertilisation is an independent event.
F₁ and F₂ Generations
Genetics questions may also describe generations using the symbols F₁ and F₂.
| Term | Meaning |
|---|---|
| Parental generation | The original parents used in the cross |
| F₁ generation | The first filial generation produced from the parental cross |
| F₂ generation | The generation produced when suitable F₁ individuals reproduce |
For example, crossing homozygous tall plants with homozygous short plants:
If two of these heterozygous F₁ plants are crossed:
Common Mistakes
Mistake 1: Saying a dominant allele is “stronger”.
Dominance describes expression in a heterozygous individual. It does not
mean that one allele is physically stronger, healthier or more common.
Mistake 2: Writing different letters for the two alleles of one gene.
If T is the dominant allele, use t for the recessive allele. Writing T
and S can incorrectly suggest two different genes.
Mistake 3: Putting two alleles into one gamete.
A Tt parent produces T gametes and t gametes. A normal gamete does not
contain Tt for that gene.
Mistake 4: Confusing genotype ratio with phenotype ratio.
In Tt × Tt, the genotype ratio is 1 TT : 2 Tt : 1 tt, while the phenotype
ratio is 3 dominant : 1 recessive.
Mistake 5: Treating a 3 : 1 ratio as a guaranteed result.
A Punnett square gives expected probabilities. Small samples can differ
considerably from the expected ratio.
Mistake 6: Assuming an organism with the dominant phenotype must be homozygous dominant.
A tall plant could be TT or Tt. The phenotype alone may not reveal which
genotype it possesses.
Mistake 7: Calling IA dominant over IB.
IA and IB are codominant. Both are expressed in an
individual with genotype IAIB.
Mistake 8: Saying the mother determines whether the zygote is XX or XY.
In the standard XX–XY model, eggs carry X, while sperm may carry X or Y.
What You Should Be Able to Do
Distinguish between gene, allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive.
Determine which allele or chromosome can enter each type of gamete produced during meiosis.
Show parental genotypes, gametes, fertilisation outcomes and offspring genotypes clearly.
Determine genotype ratios, phenotype ratios and probabilities from a monohybrid cross.
Explain why actual offspring numbers may differ from an expected genetic ratio because fertilisation is random.
Use IA, IB and i correctly when predicting ABO blood group inheritance.
Understand that more than two alleles may exist in a population even though each individual possesses only two.
Use the XX–XY chromosome system to predict the approximately 1 : 1 expected outcome.
These questions combine allele notation, genetic crosses, probability, codominance and sex determination.
- 10 mastery questions
- Genetic crosses, ratios and probability
- Instant explanation for every answer option
Frequently Asked Questions About Monohybrid Crosses
Once the basic genetic diagram is clear, most remaining questions come down to interpreting what the letters and ratios actually mean.
What exactly is a monohybrid cross?
A monohybrid cross follows the inheritance of one characteristic. At O-Level, this commonly involves a gene with two alleles and predicting how those alleles may be inherited by the offspring.
What is the difference between a gene and an allele?
A gene is a section of DNA carrying information that can influence a particular characteristic. An allele is an alternative form of that gene. For example, T and t can represent two different alleles of the same gene.
Why should dominant and recessive alleles use the same letter?
They are alternative forms of the same gene, so using the same letter makes that relationship clear. A capital letter is normally used for the dominant allele and the corresponding lowercase letter for the recessive allele, such as T and t.
Why does Tt show the dominant phenotype?
In a simple complete-dominance relationship, one copy of the dominant allele is sufficient for the dominant phenotype to be expressed. Therefore, both TT and Tt show the dominant phenotype.
Why does a recessive phenotype tell us the genotype immediately?
In a simple dominant-recessive system, the recessive phenotype appears only when no dominant allele is present. An organism showing the recessive phenotype must therefore be homozygous recessive, such as tt.
Why are the genotype and phenotype ratios different in Tt × Tt?
The genotype ratio distinguishes TT, Tt and tt, giving 1 : 2 : 1. However, TT and Tt produce the same dominant phenotype. Combining these two groups gives the phenotype ratio 3 dominant : 1 recessive.
Does a 3 : 1 ratio mean every four offspring must include exactly three dominant and one recessive individual?
No. The ratio describes expected probabilities. Because fertilisation is random, four actual offspring could show several different combinations. Larger sample sizes are generally more likely to approach the expected ratio.
How can I work out the genotype of an organism showing the dominant phenotype?
The dominant phenotype alone may represent either a homozygous dominant or heterozygous genotype. Additional information about its parents or offspring may allow you to decide between them.
For example, if an organism produces a recessive offspring, it must possess and pass on a recessive allele.
What is codominance?
Codominance occurs when both alleles in a heterozygous individual are expressed. In the ABO blood group system, IA and IB are codominant, so a person with genotype IAIB has blood group AB.
How can the ABO system have three alleles if a person has only two?
Multiple alleles refers to the number of alternative alleles that exist within a population. An individual is diploid and normally inherits only two alleles for that gene — one from each parent.
The ABO system has IA, IB and i in the population, but each individual has only two of them.
Why does the sperm determine whether a zygote is XX or XY?
In the standard XX–XY model, eggs from an XX female contain an X chromosome. Sperm from an XY male may contain either X or Y.
An X-bearing sperm produces an XX zygote, while a Y-bearing sperm produces an XY zygote.
Are all human characteristics inherited as simple monohybrid traits?
No. Monohybrid crosses are useful models for learning basic inheritance, but many human characteristics are influenced by several genes, environmental factors or more complicated patterns of gene expression.
Simple Mendelian ratios should therefore be applied only when the inheritance pattern in the question supports them.
Monohybrid crosses become much easier once you stop treating the Punnett square as a collection of boxes to memorise. Start with the parental genotypes, separate their alleles into gametes, combine those gametes during fertilisation, and only then translate the offspring genotypes into phenotypes.
That same reasoning carries you from simple dominant-recessive inheritance to ABO blood groups and the inheritance of sex chromosomes. The notation may change, but the central idea remains the same: alleles separate during gamete formation and are brought together again at fertilisation.