17.4 Monohybrid inheritance
- Syllabus
- 0610–2026–2027
- Topic
- 17.4
- Level
- —
Learning objectives
17.4.1Inheritance as the transmission• Describe inheritance as the transmission of genetic information from generation to generation17.4.2Genotype as the genetic make-up• Describe genotype as the genetic make-up of an organism and in terms of the alleles present17.4.3Phenotype as the observable features• Describe phenotype as the observable features of an organism17.4.4Homozygous as having two identical• Describe homozygous as having two identical alleles of a particular gene17.4.5Two identical homozygous individuals• State that two identical homozygous individuals that breed together will be pure-breeding17.4.6Heterozygous as having two different• Describe heterozygous as having two different alleles of a particular gene17.4.7Heterozygous individual will not be• State that a heterozygous individual will not be pure-breeding17.4.8Dominant allele as an allele that is• Describe a dominant allele as an allele that is expressed if it is present in the genotype17.4.9Recessive allele as an allele that is• Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype17.4.10Interpret pedigree diagrams• Interpret pedigree diagrams for the inheritance of a given characteristic17.4.11Genetic diagrams to predict• Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1: 1 and 3: 1 ratios17.4.12Punnett squares in crosses which• Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes17.4.13To use a test cross to identify• Explain how to use a test cross to identify an unknown genotype17.4.14Codominance as a situation in which• Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype17.4.15Inheritance of ABO blood groups• Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are IA, IB and Io17.4.16Sex-linked characteristic as a feature• Describe a sex-linked characteristic as a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other17.4.17Red-green colour blindness• Describe red-green colour blindness as an example of sex linkage17.4.18Genetic diagrams to predict• Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios
Define inheritance across generations
Inheritance is the transmission of genetic information from generation to generation.
Parents pass alleles to offspring in their gametes. At fertilisation, the offspring receives one allele of each gene from each parent, creating its genotype.
Inheritance refers to genetic information passed between generations, not features acquired during an organism's lifetime.
Read genotype as an allele combination
Genotype is an organism's genetic make-up, described in terms of the alleles it has for a gene.
If a gene has alleles T and t, possible diploid genotypes are TT, Tt and tt. The two symbols represent the two alleles, one inherited from each parent.
Genotype names the allele combination, not the visible feature. Tt is a genotype; tall is a phenotype.
Distinguish phenotype from genotype
Phenotype means the observable features of an organism.
Phenotype can result from the genotype and, for some features, environmental effects. In a simple dominant–recessive example, TT and Tt may both give the tall phenotype while tt gives dwarf.
A phenotype is what can be observed or measured; it is not the pair of allele symbols carried by the organism.
Recognise a homozygous genotype
An organism is homozygous for a gene when it has two identical alleles of that gene.
With alleles B and b, BB is homozygous dominant and bb is homozygous recessive. Both contain a matching pair.
Homozygous does not always mean dominant: both BB and bb are homozygous.
Explain why identical homozygotes are pure-breeding
Two identical homozygous individuals that breed together are pure-breeding for that gene because each parent can pass on only one allele type.
BB × BB produces only BB offspring; bb × bb produces only bb offspring. Under the same conditions, the relevant phenotype therefore remains consistent across offspring.
Pure-breeding refers to the specified gene or characteristic. It does not mean the individuals are genetically identical at every gene.
Recognise a heterozygous genotype
An organism is heterozygous for a gene when it has two different alleles of that gene.
For alleles T and t, the heterozygous genotype is Tt. It can make gametes carrying T and gametes carrying t.
Tt is one genotype containing two different alleles; it is not a mixture of the two parental phenotypes.
Explain why heterozygotes are not pure-breeding
A heterozygous individual is not pure-breeding because it can pass either of its two different alleles to its offspring.
A Tt parent makes T and t gametes. When suitable parents are crossed, this can produce offspring with different genotypes and may reveal more than one phenotype.
Showing a dominant phenotype does not prove an individual is pure-breeding: both TT and Tt can show it, but only TT is homozygous dominant.
Define a dominant allele by expression
A dominant allele is expressed in the phenotype whenever it is present in the genotype.
If T is dominant, both TT and Tt express the dominant phenotype. One copy of T is sufficient for expression in this simple monohybrid model.
Dominant does not mean common, stronger or better. It describes expression in a heterozygous genotype.
Define a recessive allele by its condition
A recessive allele is expressed only when no dominant allele of the same gene is present.
If t is recessive to T, the recessive phenotype appears in tt. A Tt individual carries t but shows the dominant phenotype.
A recessive allele is not absent from a heterozygote; it is present but its phenotype is not expressed while the dominant allele is present.
Interpret a pedigree systematically
A pedigree traces a characteristic through a family. Squares represent males, circles represent females, a horizontal line joins parents, and vertical lines lead to offspring. Shading is defined by the diagram's key.
- Read the key before assigning a phenotype.
- Follow relationships one generation at a time.
- Use parent–offspring combinations to test possible genotypes.
- Mark only genotypes forced by the evidence; keep alternatives when the pedigree does not decide.
Shading does not automatically mean dominant, recessive or diseased. Its meaning and the inheritance model must come from the key and family evidence.
Build a monohybrid genetic diagram
For a monohybrid cross: 1. choose allele symbols and state dominance; 2. write parental phenotypes and genotypes; 3. list each parent's gametes; 4. combine one allele from each parent; 5. translate offspring genotypes into phenotypes; 6. state the phenotypic ratio.
| Cross | Offspring phenotypic ratio |
|---|---|
| Tt × tt | 1 dominant : 1 recessive |
| Tt × Tt | 3 dominant : 1 recessive |
These ratios are expected probabilities across many offspring, not a guarantee for a small family. This Core objective is limited to 1:1 and 3:1 phenotypic ratios.
Use a Punnett square to show possible genotypes
A Punnett square shows every possible combination of parental gamete alleles when a cross can produce more than one genotype.
Put one parent's gametes across the top and the other parent's down the side. Fill each cell by combining its row and column alleles. For Tt × Tt, the cells are TT, Tt, Tt and tt, giving a 1:2:1 genotype ratio.
Each square shows a possible fertilisation, not four actual children. Repeated cells indicate a higher probability of that genotype.
Use a test cross to reveal an unknown genotype
A test cross identifies whether an individual with a dominant phenotype is homozygous dominant or heterozygous by crossing it with a homozygous recessive individual.
| Unknown parent | Cross with tt | Expected offspring |
|---|---|---|
| TT | TT × tt | all Tt, all dominant phenotype |
| Tt | Tt × tt | about 1 dominant : 1 recessive |
A large number of offspring makes the inference more reliable. A small set containing only dominant offspring does not prove the unknown parent is TT.
Explain codominance in a heterozygote
Codominance occurs when both alleles in a heterozygous organism contribute to its phenotype.
If red-hair and white-hair alleles are codominant, a heterozygote can show both red and white hairs. Neither allele is hidden by the other.
Codominance means both allele effects contribute. It is not the same as a dominant allele completely masking a recessive allele.
Map ABO blood-group alleles to phenotypes
The ABO blood-group gene has three alleles: Iᴬ, Iᴮ and Iᵒ. Iᴬ and Iᴮ are codominant with each other, while Iᵒ is recessive to both.
| Phenotype | Possible genotype(s) |
|---|---|
| A | IᴬIᴬ or IᴬIᵒ |
| B | IᴮIᴮ or IᴮIᵒ |
| AB | IᴬIᴮ |
| O | IᵒIᵒ |
For IᴬIᵒ × IᴮIᵒ, the possible offspring are IᴬIᴮ, IᴬIᵒ, IᴮIᵒ and IᵒIᵒ: blood groups AB, A, B and O.
A and B describe phenotypes, while Iᴬ and Iᴮ are alleles. Blood group O requires two Iᵒ alleles.
Explain why sex linkage affects the sexes differently
A sex-linked characteristic is controlled by a gene located on a sex chromosome, making the characteristic more common in one sex than the other.
For an X-linked recessive allele, a male has only one X chromosome, so one recessive allele on that X is expressed. A female usually needs the recessive allele on both X chromosomes to show the characteristic; a heterozygous female is a carrier.
Sex-linked does not mean carried only by males or located automatically on the Y chromosome. The chromosome location must be stated.
Use red-green colour blindness as sex linkage
Red-green colour blindness is an example of sex linkage. The gene is on the X chromosome, and the colour-blindness allele is recessive.
Using Xᴮ for normal colour vision and Xᵇ for colour blindness: XᵇY is a colour-blind male; XᴮXᵇ is a female carrier with normal colour vision; XᵇXᵇ is a colour-blind female.
A father gives his Y chromosome, not his X, to a son. Therefore an X-linked colour-blindness allele cannot pass directly from father to son.
Predict codominant and sex-linked crosses
Use the same genetic-diagram sequence—parental genotypes, gametes, offspring genotypes, phenotypes and ratio—but preserve codominant or sex-chromosome notation throughout.
| Cross type | Essential notation move |
|---|---|
| codominance | keep both allele superscripts, because both may contribute to the heterozygous phenotype |
| sex linkage | attach each allele to X and include Y as a separate male gamete |
Codominance: CᴿCᵂ × CᴿCᵂ gives 1 red : 2 codominant : 1 white. X-linked recessive: XᴮXᵇ × XᴮY gives daughters XᴮXᴮ or XᴮXᵇ and sons XᴮY or XᵇY, so one quarter of all predicted offspring are colour-blind sons.
Calculate phenotype probabilities only after separating genotypes correctly. For sex-linked crosses, a probability among all offspring is different from a probability conditional on the child being male or female.