Why can brothers and sisters resemble one another without being genetically identical? Why can two parents showing the same phenotype sometimes produce a child with a different phenotype? The answer lies in the way alleles are separated into gametes and recombined during fertilisation.
In this topic, we will build the language needed to describe inheritance, then use genetic diagrams to predict monohybrid inheritance. We will also examine codominance and the ABO blood group system, before applying the same inheritance reasoning to sex determination in humans.
Start with five questions on genes, alleles and basic genetic vocabulary before moving into genetic crosses.
- 5 diagnostic questions
- Focus on essential inheritance vocabulary
- Instant explanation for every answer option
From chromosomes to alleles in the offspring
Most body cells are diploid: they contain pairs of homologous chromosomes, with one chromosome of each pair inherited from the mother and the other from the father. The two chromosomes of a homologous pair carry genes for the same characteristics at corresponding positions, although the alleles present may be different.
Suppose an organism has genotype Aa for one gene. The two alleles do not normally remain together when gametes are formed. During meiosis, homologous chromosomes are separated, so each gamete receives only one allele — either A or a.
Parent with two alleles → meiosis separates the alleles into gametes → fertilisation combines one maternal gamete with one paternal gamete → offspring receives a new pair of alleles.
The diagram shows the key idea behind a Punnett square. Each heterozygous parent has two alleles, A and a, but these alleles separate during meiosis. Each individual gamete therefore carries only A or a. At fertilisation, one gamete from each parent combines, producing the possible offspring genotypes AA, Aa, Aa and aa.
This is why an offspring normally receives one allele of a gene from its mother and one from its father. Which two alleles come together depends on which gametes take part in fertilisation.
The essential language of inheritance
Genetics becomes much easier once the vocabulary is used precisely. Several words describe quite different ideas, even though they are often confused in examination answers.
| Term | Meaning | Example |
|---|---|---|
| Gene | A length of DNA containing information for making a particular polypeptide | A gene involved in determining a particular characteristic |
| Allele | An alternative form of a gene | T and t |
| Genotype | The combination of alleles possessed by an organism for the gene being considered | TT, Tt or tt |
| Phenotype | The observable characteristic of an organism | Tall or short |
| Dominant allele | An allele that is expressed in the phenotype when present in a heterozygous genotype | T in Tt |
| Recessive allele | An allele expressed in the phenotype only when the dominant allele is absent in a complete-dominance example | t in tt |
| Homozygous | Having two identical alleles of a gene | TT or tt |
| Heterozygous | Having two different alleles of a gene | Tt |
Genotype and phenotype are not the same thing
A genotype is written using allele symbols. A phenotype describes the characteristic that is expressed. If T represents a dominant allele for tall plants and t represents a recessive allele for short plants, then both TT and Tt give the tall phenotype, whereas tt gives the short phenotype.
TT → homozygous dominant → tall
Tt → heterozygous → tall
tt → homozygous recessive → short
This is why seeing a dominant phenotype does not always tell you the exact genotype. A tall plant in this example could be TT or Tt. However, a plant showing the recessive phenotype must be tt.
How to construct a monohybrid genetic diagram
A monohybrid cross follows the inheritance of one gene. A clear genetic diagram should show how the alleles in the parents enter their gametes and how these gametes may combine at fertilisation.
A reliable examination sequence is:
- State the parental phenotypes where relevant.
- State the parental genotypes using correctly defined allele symbols.
- Show the possible gametes produced by each parent.
- Combine the gametes using a genetic diagram or Punnett square.
- State the possible offspring genotypes.
- Convert these genotypes into phenotypes.
- Give the required ratio or probability and label what the numbers represent.
Worked example: heterozygous × heterozygous
Let T be the dominant allele for tall plants and t be the recessive allele for short plants.
Cross two heterozygous tall plants:
Each parent produces two possible types of gamete: T and t.
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
The expected genotype ratio is:
Because TT and Tt are both tall, the expected phenotype ratio is:
Notice that 3 : 1 is a phenotype ratio. Writing only “3 : 1” without stating what the numbers represent can make an otherwise correct answer incomplete.
Worked example: heterozygous × homozygous recessive
Now cross a heterozygous tall plant with a short plant:
The heterozygous parent produces T and t gametes, while the homozygous recessive parent produces only t gametes.
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
The expected genotype ratio is:
The expected phenotype ratio is therefore:
Genetic ratios describe probability, not a fixed sequence
A Punnett square shows the probability of each possible genetic combination. It does not mean that every group of four offspring from a Tt × Tt cross must contain exactly three tall individuals and one short individual.
The allele combination in one offspring does not force the next offspring to have a different genotype. Chance operates again each time fertilisation occurs.
This explains why observed results may differ from an expected ratio. For example, a cross with an expected 3 tall : 1 short ratio might produce 35 tall plants and 15 short plants in one investigation.
Such a result does not automatically mean the genetic model is wrong. With a relatively small number of offspring, chance variation can produce noticeable differences from the expected ratio. As the number of offspring becomes larger, observed proportions are generally more likely to approach the expected probabilities.
Codominance: when both alleles are expressed
Dominant-recessive inheritance is not the only possible relationship between alleles. In codominance, both alleles present in a heterozygous individual are expressed in the phenotype.
The human ABO blood group system provides an important example. Three alleles are involved:
- Iᴬ
- Iᴮ
- Iᴼ
Iᴬ and Iᴮ are codominant to each other. If both occur together in genotype IᴬIᴮ, both are expressed and the person has blood group AB. Both Iᴬ and Iᴮ are dominant to Iᴼ.
| Blood group phenotype | Possible genotype(s) |
|---|---|
| A | IᴬIᴬ or IᴬIᴼ |
| B | IᴮIᴮ or IᴮIᴼ |
| AB | IᴬIᴮ |
| O | IᴼIᴼ |
Why blood group A does not reveal a person’s exact genotype
A person with blood group A may be homozygous IᴬIᴬ or heterozygous IᴬIᴼ. The phenotype alone therefore does not always identify the genotype.
In contrast, a person with blood group O must have genotype IᴼIᴼ, while a person with blood group AB must have genotype IᴬIᴮ.
Worked example: two heterozygous parents
Consider one parent with genotype IᴬIᴼ and another with genotype IᴮIᴼ.
| Iᴬ | Iᴼ | |
|---|---|---|
| Iᴮ | IᴬIᴮ | IᴮIᴼ |
| Iᴼ | IᴬIᴼ | IᴼIᴼ |
The four possible phenotypes are therefore AB, B, A and O, each with an expected probability of ¼ in this cross.
Sex determination in humans
Humans normally have 23 pairs of chromosomes in diploid body cells. One pair consists of the sex chromosomes. In the simplified XX/XY model used at O-Level:
- Females normally have XX sex chromosomes.
- Males normally have XY sex chromosomes.
During meiosis, an XX parent produces eggs carrying an X chromosome. An XY parent produces sperm carrying either an X chromosome or a Y chromosome.
| X sperm | Y sperm | |
|---|---|---|
| X egg | XX | XY |
Fertilisation by an X-bearing sperm produces an XX zygote, while fertilisation by a Y-bearing sperm produces an XY zygote. In this simplified inheritance model, each fertilisation therefore has an expected probability of ½ for XX and ½ for XY.
The egg contributes X in either case. The sperm contributes either X or Y, so the chromosome carried by the fertilising sperm determines whether the zygote is XX or XY in this model.
As with every probability in genetics, previous pregnancies do not change the probability for the next fertilisation. A family that already has several sons does not become genetically “due” for a daughter.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying that a gene is simply “a characteristic” | A characteristic is a phenotype. A gene is genetic information carried in DNA. | Define a gene as a length of DNA containing information for making a particular polypeptide. |
| Using “gene” and “allele” as if they mean the same thing | Alleles are alternative forms of the same gene. | Remember that a gene can exist in different alternative forms called alleles. |
| Calling Tt homozygous | The two alleles are different. | Tt is heterozygous. TT and tt are homozygous. |
| Assuming a dominant phenotype must have a homozygous dominant genotype | A heterozygous genotype also shows the dominant phenotype under complete dominance. | Remember that TT and Tt may have the same phenotype. |
| Writing the gametes from Tt as “Tt” | The alleles separate during meiosis. | Write the possible gametes separately as T and t. |
| Writing only “3 : 1” as the answer to a cross | The examiner needs to know whether this is a genotype ratio or phenotype ratio and what each number represents. | Write, for example, “3 tall : 1 short”. |
| Expecting every four offspring from a Tt × Tt cross to contain exactly three dominant and one recessive phenotype | The 3 : 1 ratio represents probabilities across many fertilisation events. | Treat every fertilisation as an independent chance event. |
| Calling Iᴬ dominant over Iᴮ | Iᴬ and Iᴮ are codominant and are both expressed in IᴬIᴮ. | State that Iᴬ and Iᴮ are codominant, while both are dominant to Iᴼ. |
| Assuming blood group A always means genotype IᴬIᴬ | Blood group A may also result from IᴬIᴼ. | Give both possible genotypes unless further information allows one to be eliminated. |
| Saying that the mother determines whether an XX or XY zygote forms | Eggs carry X, whereas sperm may carry X or Y. | State that the sex chromosome carried by the fertilising sperm determines XX or XY in the simplified model. |
What You Should Be Able to Do
Distinguish clearly between gene, allele, genotype, phenotype, dominant, recessive, homozygous and heterozygous.
Explain why each gamete receives only one allele of a gene from a diploid parent.
Show parental genotypes, gametes, fertilisation and offspring genotypes correctly.
Determine genotype ratios, phenotype ratios and probabilities from monohybrid crosses.
Explain why a real set of offspring may not match the exact expected genetic ratio, especially when sample size is small.
Use Iᴬ, Iᴮ and Iᴼ correctly to determine ABO blood group genotypes and inheritance.
Decide when a phenotype uniquely identifies a genotype and when more than one genotype is possible.
Use X and Y chromosomes to explain the expected 1 : 1 probability of XX and XY offspring in the simplified model.
Apply genetic diagrams, probability, codominance and sex determination to ten examination-style questions.
- 10 mastery questions
- Monohybrid crosses and blood groups
- Instant explanation for every answer option
Frequently asked questions
Inheritance questions students often ask
Once the genetic vocabulary is clear, most inheritance problems reduce to one idea: work out which alleles can enter the gametes, then combine them carefully.
What is the difference between a gene and an allele?
A gene is a length of DNA containing information for making a particular polypeptide. An allele is an alternative form of that gene. An organism may therefore possess two different alleles of the same gene.
Why does a gamete contain only one allele of a gene?
Diploid cells contain homologous chromosome pairs. During meiosis, these homologous chromosomes separate, so a gamete receives only one chromosome from each pair and therefore only one allele of the gene being considered.
What is the difference between genotype and phenotype?
Genotype describes the alleles an organism possesses, such as Tt. Phenotype describes the observable characteristic, such as tall. Different genotypes can sometimes produce the same phenotype.
Can two organisms with the same phenotype have different genotypes?
Yes. Under complete dominance, both TT and Tt may produce the same dominant phenotype. This is why a dominant phenotype does not always reveal whether an organism is homozygous or heterozygous.
Why does a Punnett square not always match the actual number of offspring?
A Punnett square gives expected probabilities. Fertilisation involves chance, so the observed numbers in a real sample can differ from the expected ratio. With larger numbers of offspring, observed proportions are generally more likely to approach the expected probabilities.
What is the difference between dominant inheritance and codominance?
In complete dominance, the dominant allele determines the phenotype of a heterozygote and the recessive phenotype is not expressed. In codominance, both different alleles in the heterozygote are expressed. IᴬIᴮ in the ABO blood group system is an example of codominance.
How can two parents who do not show a recessive characteristic have an affected child?
Both parents may be heterozygous carriers. If each parent passes the recessive allele to the same child, that child receives two recessive alleles and shows the recessive phenotype.
Can two parents with blood group A have a child with blood group O?
Yes, if both parents are heterozygous IᴬIᴼ. Each parent can pass the Iᴼ allele, allowing an IᴼIᴼ child with blood group O to be produced.
Does having several sons make a daughter more likely in the next pregnancy?
No. In the simplified XX/XY model used for genetic probability questions, each fertilisation is an independent event. Previous outcomes do not alter the expected probability for the next fertilisation.
Inheritance problems become much less intimidating when you follow the alleles rather than trying to memorise ratios. Start with the parents, work out the gametes they can produce, combine those gametes, and only then translate the resulting genotypes into phenotypes. Once that sequence becomes automatic, monohybrid crosses, blood groups and sex determination all follow the same underlying genetic logic.