Inheritance
SEAB syllabus topic 14: genes and alleles, monohybrid crosses, codominance and the ABO blood groups, sex determination, mutation, variation and natural selection, with the why, the traps and the exam checks.
A monohybrid cross between two heterozygous parents gives offspring in the ratio 3:1.
The ABO blood group is controlled by three alleles, and IA and IB are codominant.
Natural selection favours the organisms that are best fitted to the environment.
Gene and allele
distinguish between the terms gene and allele
A gene is a length of DNA on a chromosome that codes for one polypeptide, and an allele is one of the different forms of that gene.
- Gene
- Gene: a length of DNA on a chromosome that codes for one polypeptide and is a unit of inheritance, such as the gene for stem height in pea plants
- Allele
- Allele: one of the different forms of the same gene, found at the same position on the two chromosomes of a homologous pair
- Locus
- Locus: the position of a gene on a chromosome, so the two alleles of a gene in a body cell lie at the same locus on a pair of homologous chromosomes
- Homologous chromosomes
- Homologous chromosomes: a pair of chromosomes, one from each parent, that carry the same genes in the same order, although they may carry different alleles supports K325-14(a)
- Diploid
- Diploid: having two sets of chromosomes in each cell, so each autosomal gene is present as two alleles supports K325-14(a)
The big picture
- This chapter covers the inheritance terms, monohybrid crosses, the ABO blood groups, sex determination, mutation, variation and natural selection.
- Inheritance is the passing of characteristics from parents to offspring, and it happens because parents pass alleles to their offspring in the gametes.
Gene and allele compared
| Feature | Gene | Allele |
|---|---|---|
| Meaning | A gene is a length of DNA that codes for one polypeptide. | An allele is one of the alternative forms of a gene. |
| In a diploid body cell | A body cell has two copies of each autosomal gene, one on each chromosome of a homologous pair. | The two copies may be the same allele or two different alleles. |
| Example in pea plants | The gene for stem height decides whether a pea plant is tall or dwarf. | The tall allele T and the dwarf allele t are the two alleles of this gene. |
| Example in humans | The gene for the ABO blood group is found on one pair of chromosomes. | The ABO gene has three alleles, IA, IB and IO. |
Depth: why and how
- Why does a body cell have two alleles of each autosomal gene? One chromosome of each homologous pair comes from the mother and the other comes from the father, so an offspring inherits one allele from each parent.
- Why can two people have the same gene for blood group but different blood groups? They have the same gene, but they can have different alleles of it, and different combinations of alleles give different phenotypes.
- What is the difference between a gene and an allele in one sentence? A gene is the instruction for a characteristic such as stem height, and an allele is one version of that instruction, such as tall or dwarf.
- Cross-link to Molecular Genetics: a gene is a sequence of nucleotides in a DNA molecule, and different alleles of a gene have slightly different sequences of nucleotides.
- Cross-link to Reproduction: meiosis separates the two chromosomes of a homologous pair, so each gamete receives only one allele of each gene.
- New alleles are formed by mutation, which is a change in the sequence of nucleotides of a gene. supports K325-14(a)
Common mistakes
A gene and an allele are the same thing.
A gene is a length of DNA that codes for one polypeptide, and an allele is one of the alternative forms of that gene.
An allele is a type of chromosome.
An allele is a form of a gene, and it is found at a locus on a chromosome.
A person has only one allele for each gene.
A diploid body cell has two alleles of each autosomal gene, one from each parent.
A gene is a length of DNA that codes for one polypeptide, and the alleles are the alternative forms of that gene found at the same locus.
Exam check
Q1 Which statement describes an allele?
- A A section of a chromosome that carries many genes
- B One of the alternative forms of a gene
- C A type of cell division that halves the chromosome number
- D A characteristic that can be seen in an organism
Show answer
MCQ answer key B (One of the alternative forms of a gene): an allele is one of the alternative forms of a gene, and the other options describe a chromosome region, meiosis and a phenotype.
Q2 On which structures are the two alleles of a gene found in a body cell?
- A Two chromosomes that carry different genes
- B One chromatid of a single chromosome
- C The two nuclei of the cell
- D A pair of homologous chromosomes
Show answer
MCQ answer key D (A pair of homologous chromosomes): the two alleles lie at the same locus, one on each chromosome of a homologous pair.
Q3 How many alleles of an autosomal gene are present in a normal human body cell?
- A Two
- B One
- C Three
- D Four
Show answer
MCQ answer key A (Two): a body cell is diploid, so it has two alleles of each autosomal gene, one from each parent.
Structured Pea plants have a gene for stem height.
- Distinguish between a gene and an allele. [2]
- State why a body cell has two alleles of this gene. [1]
- State the term for the position of a gene on a chromosome. [1]
Total: 4 marks
Show mark scheme
(a) 2 marks
- Mark scheme: a gene is a length of DNA that codes for one polypeptide, or for a protein (accept: one characteristic)
- Mark scheme: an allele is one of the alternative forms of a gene
(b) 1 mark
- Mark scheme: one allele is inherited from each parent, or one allele is on each chromosome of a homologous pair
(c) 1 mark
- Mark scheme: locus
Genetics vocabulary
explain the terms dominant, recessive, codominant, homozygous, heterozygous, phenotype and genotype
The genotype is the combination of alleles that an organism has for a gene, and the phenotype is the characteristic that is expressed as a result of the genotype and the environment.
- Genotype
- Genotype: the combination of alleles that an organism has for a gene, such as Tt
- Phenotype
- Phenotype: the characteristic of an organism that can be observed or measured, such as tall stems
- Dominant allele
- Dominant allele: an allele that is expressed in the phenotype whether it is present in one copy or in two copies, so it masks the recessive allele in a heterozygote
- Recessive allele
- Recessive allele: an allele that is expressed only when two copies are present, because it is masked by a dominant allele in a heterozygote
- Homozygous
- Homozygous: having two identical alleles of a gene, such as TT or tt
- Heterozygous
- Heterozygous: having two different alleles of a gene, such as Tt
- Codominant alleles
- Codominant alleles: two alleles that are both fully expressed in the phenotype of the heterozygote, such as IA and IB in the ABO blood group
- Incomplete dominance
- Incomplete dominance: a situation in which the heterozygote has a phenotype between the phenotypes of the two homozygotes, such as pink flowers from a red allele and a white allele supports K325-14(b)
- Pure-bred
- Pure-bred: an organism that is homozygous for the gene, so it breeds true when it is crossed with a similar organism supports K325-14(b)
Genotype and phenotype in pea plants
| Genotype | Description | Phenotype |
|---|---|---|
| TT | TT has two identical dominant alleles, so it is homozygous dominant. | A plant with the genotype TT is tall. |
| Tt | Tt has one dominant allele and one recessive allele, so it is heterozygous. | A plant with the genotype Tt is tall, because the dominant allele T is expressed and masks t. |
| tt | tt has two identical recessive alleles, so it is homozygous recessive. | A plant with the genotype tt is dwarf, because there is no dominant allele to mask t. |
Write homozygous dominant for AA, homozygous recessive for aa and heterozygous for Aa, and never write heterozygous dominant, because a heterozygote has one dominant allele and one recessive allele.
Different genotypes, one phenotype
The genotypes TT and Tt both give a tall plant, so the genotype cannot be worked out from the phenotype alone.
Environment also matters
The phenotype depends on the genotype and on the environment, so a plant with the genotype TT grown in poor light may be shorter than a TT plant grown in good light.
Depth: why and how
- Why does a heterozygote Tt look tall? The dominant allele T is expressed and masks the recessive allele t, so the plant has the tall phenotype.
- Why does a recessive allele need two copies to be expressed? In a heterozygote the dominant allele is expressed, so the recessive allele has no visible effect unless there is no dominant allele in the genotype.
- Why is dominant not the same as common? Dominance describes how two alleles interact in the phenotype of one individual, and it says nothing about how common the allele is in a population.
- Cross-link to Molecular Genetics: a dominant allele usually codes for a polypeptide that works, and a recessive allele often codes for a polypeptide that does not work, which is why one dominant allele is enough to give the dominant phenotype.
- The allele for polydactyly, which gives extra fingers or toes, is dominant but rare, which shows that a dominant allele is not always the common one. supports K325-14(b)
Common mistakes
A dominant allele is the stronger allele and is the commonest one in a population.
A dominant allele is expressed even when only one copy is present, and it may be rare in a population.
Tt is heterozygous dominant.
Tt is heterozygous, and the dominant allele T is expressed in the phenotype.
The genotype is what an organism looks like.
The genotype is the combination of alleles, and the phenotype is the characteristic that can be observed.
Homozygous means that the two alleles are different.
Homozygous means that the two alleles are identical, and heterozygous means that they are different.
A dominant allele is expressed in the homozygous and the heterozygous genotype, but a recessive allele is expressed only in the homozygous recessive genotype.
Exam check
Q1 A pea plant has the genotype Tt. Which term describes this genotype?
- A Homozygous dominant
- B Homozygous recessive
- C Heterozygous
- D Codominant
Show answer
MCQ answer key C (Heterozygous): Tt has two different alleles, so it is heterozygous.
Q2 Which statement about a recessive allele is correct?
- A It is expressed only when two copies are present
- B It is expressed only when one copy is present
- C It is destroyed during meiosis
- D It is expressed in the heterozygote but not in the homozygote
Show answer
MCQ answer key A (It is expressed only when two copies are present): a recessive allele is masked by a dominant allele, so it is expressed only when there are two copies.
Q3 A plant with the genotype Tt is tall, and a plant with the genotype tt is dwarf. What does this show?
- A t is dominant to T
- B T and t are codominant
- C T and t are on different chromosomes
- D T is dominant to t
Show answer
MCQ answer key D (T is dominant to t): the heterozygote Tt shows the tall phenotype, so the tall allele T masks t and is dominant.
Structured In pea plants the allele for tall stems (T) is dominant to the allele for dwarf stems (t).
- State the genotype of a heterozygous tall plant. [1]
- Explain why a plant with the genotype Tt is tall. [2]
- State the phenotype of a plant with the genotype tt, and the term that describes this genotype. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: Tt
(b) 2 marks
- Mark scheme: T is the dominant allele, so it is expressed even though only one copy is present
- Mark scheme: it masks the recessive allele t
(c) 2 marks
- Mark scheme: dwarf
- Mark scheme: homozygous recessive
Monohybrid crosses: 3:1 and 1:1
predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation
In a monohybrid cross, one gene with two alleles is followed from the parents to the offspring, and the offspring are expected in the ratio 3:1 when two heterozygous parents are crossed and in the ratio 1:1 when a heterozygous parent is crossed with a homozygous recessive parent.
- Monohybrid cross
- Monohybrid cross: a cross that follows the inheritance of one gene, which has two alleles, from the parents to the offspring
- Parental generation (P)
- Parental generation (P): the two parents that are crossed at the start of a genetic cross supports K325-14(c)
- F1 generation
- F1 generation: the first filial generation, which is the offspring of the parental generation
- F2 generation
- F2 generation: the second filial generation, which is the offspring produced when the F1 individuals are crossed with each other
- Gamete
- Gamete: a sex cell that carries one allele of each gene supports K325-14(c)
From pure-bred parents to the F2 generation
- 1The parents are two pure-bred pea plants, a tall plant with the genotype TT and a dwarf plant with the genotype tt.
- 2The tall parent makes gametes that all carry T, and the dwarf parent makes gametes that all carry t.
- 3All the F1 offspring are Tt, so they are all heterozygous and they are all tall, because T is dominant.
- 4The F1 plants are crossed with each other, and each F1 plant makes gametes that carry T and gametes that carry t in equal numbers.
- 5The F2 generation has the genotypes 1 TT : 2 Tt : 1 tt, so the phenotypes are 3 tall : 1 dwarf.
| Gametes | T | T |
|---|---|---|
| t | Tt Tall | Tt Tall |
| t | Tt Tall | Tt Tall |
In cross 1, the cross TT × tt, the offspring genotypes are all Tt and the offspring phenotypes are all Tall.
| Gametes | T | t |
|---|---|---|
| T | TT Tall | Tt Tall |
| t | Tt Tall | tt Dwarf |
In cross 2, the cross Tt × Tt (F1 × F1), the offspring genotypes are in the ratio 1 TT : 2 Tt : 1 tt and the offspring phenotypes are in the ratio 3 Tall : 1 Dwarf.
Choose a letter whose capital and lower-case forms look different, such as T and t, and avoid letters such as C and c or S and s, which are easy to confuse when they are written by hand.
Use F1 for the offspring of the parental (P) cross and F2 for the offspring of two F1 individuals, and label the results as the offspring when the parents are not pure-bred P parents, as in a test cross or a family problem.
All six monohybrid combinations
| Cross | Punnett boxes (out of 4) | Genotype ratio | Phenotype ratio | In words |
|---|---|---|---|---|
| AA × AA | 4 AA | all AA | all Tall | In the cross AA × AA, the offspring genotypes are all AA and the offspring phenotypes are all Tall. |
| aa × aa | 4 aa | all aa | all Dwarf | In the cross aa × aa, the offspring genotypes are all aa and the offspring phenotypes are all Dwarf. |
| AA × aa | 4 Aa | all Aa | all Tall | In the cross AA × aa, the offspring genotypes are all Aa and the offspring phenotypes are all Tall. |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 1 AA : 2 Aa : 1 aa | 3 Tall : 1 Dwarf | In the cross Aa × Aa, the offspring genotypes are in the ratio 1 AA : 2 Aa : 1 aa and the offspring phenotypes are in the ratio 3 Tall : 1 Dwarf. |
| Aa × aa | 2 Aa : 2 aa | 1 Aa : 1 aa | 1 Tall : 1 Dwarf | In the cross Aa × aa, the offspring genotypes are in the ratio 1 Aa : 1 aa and the offspring phenotypes are in the ratio 1 Tall : 1 Dwarf. |
| Aa × AA | 2 AA : 2 Aa | 1 AA : 1 Aa | all Tall | In the cross Aa × AA, the offspring genotypes are in the ratio 1 AA : 1 Aa and the offspring phenotypes are all Tall. |
Depth: why and how
- Why do all the F1 plants look tall when one parent was dwarf? Each F1 plant received T from the tall parent and t from the dwarf parent, and T is dominant, so the recessive dwarf phenotype is masked.
- Why does the dwarf phenotype appear again in the F2 generation? Two heterozygous F1 plants can each pass on t, and a tt offspring forms when two t gametes fuse, so the recessive allele was hidden but not lost.
- Why is the 3:1 ratio a ratio of phenotypes and not of genotypes? The genotype ratio is 1 TT : 2 Tt : 1 tt, and TT and Tt both give the tall phenotype, which makes 3 tall : 1 dwarf.
- Why is the ratio 1:1 when Aa is crossed with aa? Half of the gametes of the heterozygous parent carry A and half carry a, and all the gametes of the other parent carry a.
- Cross-link to Reproduction: alleles are separated when meiosis makes the gametes, and the alleles are brought together again, at random, at fertilisation.
- Gregor Mendel, a monk who worked in the 1800s, crossed pea plants and counted the offspring, and his results showed that characteristics are passed on by inherited factors that we now call genes. supports K325-14(c)
Common mistakes
The 3:1 ratio means that a family of four plants has exactly three tall plants.
The 3:1 ratio is an expected ratio, and the number of tall plants in a small family may be different from three.
The F2 generation is the offspring of the two pure-bred parents.
The F1 generation is the offspring of the parental generation, and the F2 generation is the offspring of two F1 individuals.
Aa × Aa gives offspring in the ratio 1:1.
Aa × Aa gives 3 dominant : 1 recessive, and a 1:1 ratio comes from Aa × aa.
A recessive allele is lost when it is masked in the F1 generation.
The recessive allele is still present in the F1 heterozygotes, and it can be expressed again in the F2 generation.
Two heterozygous parents are expected to give 3 dominant : 1 recessive, and a heterozygous parent crossed with a homozygous recessive parent is expected to give 1 dominant : 1 recessive.
Exam check
Q1 Two heterozygous parents (Aa × Aa) are crossed. What is the expected ratio of the dominant phenotype to the recessive phenotype?
- A 1 : 1
- B 1 : 2 : 1
- C 3 : 1
- D all dominant
Show answer
MCQ answer key C (3 : 1): Aa × Aa gives 1 AA : 2 Aa : 1 aa, and AA and Aa both show the dominant phenotype, so the phenotype ratio is 3 : 1.
Q2 A pure-bred tall plant (TT) is crossed with a pure-bred dwarf plant (tt). What are the genotype and phenotype of the F1 plants?
- A All TT and all tall
- B All Tt and all tall
- C 1 Tt : 1 tt, with half tall and half dwarf
- D All tt and all dwarf
Show answer
MCQ answer key B (All Tt and all tall): all the gametes of the tall parent carry T and all those of the dwarf parent carry t, so all the F1 plants are Tt and are tall.
Q3 Which cross is expected to give the dominant phenotype and the recessive phenotype in the ratio 1 : 1?
- A Aa × Aa
- B AA × aa
- C AA × Aa
- D Aa × aa
Show answer
MCQ answer key D (Aa × aa): Aa × aa gives 1 Aa : 1 aa, so half of the offspring show the dominant phenotype and half show the recessive phenotype.
Structured A tall pea plant (TT) is crossed with a dwarf pea plant (tt), and two of the F1 plants are then crossed with each other.
- State the genotype of all the F1 plants. [1]
- State the gametes that an F1 plant makes. [1]
- State the expected ratio of tall plants to dwarf plants in the F2 generation, and explain why some dwarf plants appear. [3]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: Tt
(b) 1 mark
- Mark scheme: T and t
(c) 3 marks
- Mark scheme: 3 tall : 1 dwarf
- Mark scheme: the dwarf plants have the genotype tt
- Mark scheme: both F1 parents pass on the recessive allele t in their gametes
Why observed ratios differ from expected ratios
explain why observed ratios often differ from expected ratios, especially when there are small numbers of progeny
The observed ratio of the offspring often differs from the expected ratio, because fertilisation is random and a small number of offspring is affected more by chance.
- Expected ratio
- Expected ratio: the ratio that a genetic diagram predicts from the probability of each fertilisation
- Observed ratio
- Observed ratio: the ratio that is found when the offspring are actually counted
- Random fertilisation
- Random fertilisation: a situation in which any male gamete has an equal chance of fusing with any female gamete, so which allele is passed on at each fertilisation depends on chance
- Probability
- Probability: the chance that an event happens, such as a chance of 1 in 4, which is 25% supports K325-14(d)
Expected and observed results
Random fertilisation
Any sperm can fertilise any egg, so which alleles come together is a matter of chance, and the ratio in the gametes is not the ratio in the zygotes of a small family.
Small numbers
When only a few offspring are counted, one unusual fertilisation changes the ratio a lot, but when many offspring are counted, the chance effects tend to cancel out.
Independent events
Each fertilisation is a separate event, so earlier offspring have no effect on the alleles of later offspring.
Not every zygote is counted
Some zygotes may not survive to be counted, which can also make the observed ratio differ from the expected ratio. supports K325-14(d)
Example: the same cross with more offspring
| Number counted | Expected | Observed | In words (example results, not from a real experiment) |
|---|---|---|---|
| 4 offspring | 3 tall : 1 dwarf | 2 tall : 2 dwarf | With 4 offspring, the observed result of 2 tall : 2 dwarf differs from the expected result of 3 tall : 1 dwarf by 1 plant, which is 25% of the offspring. |
| 20 offspring | 15 tall : 5 dwarf | 13 tall : 7 dwarf | With 20 offspring, the observed result of 13 tall : 7 dwarf differs from the expected result of 15 tall : 5 dwarf by 2 plants, which is 10% of the offspring. |
| 1000 offspring | 750 tall : 250 dwarf | 748 tall : 252 dwarf | With 1000 offspring, the observed result of 748 tall : 252 dwarf differs from the expected result of 750 tall : 250 dwarf by 2 plants, which is 0.2% of the offspring. |
Depth: why and how
- Why is the expected ratio only a prediction? The genetic diagram gives the probability of each genotype at a fertilisation, and probability does not fix the result for a small number of fertilisations.
- Why do larger numbers of offspring give results closer to the expected ratio? Chance effects tend to cancel out over many fertilisations, so the observed ratio moves closer to the expected ratio.
- What does random fertilisation mean? Any sperm has an equal chance of fusing with any egg, so which alleles are passed on at each fertilisation depends on chance.
- Why does tossing a coin help to explain a genetic cross? Each toss has two outcomes with equal probability, like the two alleles in the gametes of a heterozygous parent, and ten tosses may not give exactly five heads.
- Cross-link to Reproduction: sexual reproduction involves the fusion of the nuclei of male and female gametes, and which gametes fuse is a matter of chance, which is one reason why the offspring of the same parents differ from each other.
- In a family with two children, the chance that both are girls is 1 in 4, so a family does not need to have one child of each sex. supports K325-14(d)
Common mistakes
The results must match the expected ratio if the cross is done correctly.
The results often differ from the expected ratio because fertilisation is random, especially when the number of offspring is small.
If the first child of a couple is a boy, the next child must be a girl.
Each fertilisation is independent, so the chance of a girl is still 1 in 2 for each child.
A ratio of 2:2 shows that the parents cannot be heterozygous.
A small number of offspring can give a ratio such as 2:2 by chance, even when the parents are heterozygous, so a larger number of offspring is needed to decide.
A larger number of offspring changes the expected ratio.
The expected ratio stays the same, and a larger number of offspring makes the observed ratio closer to it.
Observed ratios often differ from expected ratios because fertilisation is random, and the differences are larger when there are only a few progeny.
Exam check
Q1 Which statement explains why observed ratios often differ from expected ratios?
- A Recessive alleles change into dominant alleles during fertilisation
- B Fertilisation is random, so chance affects which gametes fuse
- C Meiosis makes gametes with different numbers of chromosomes
- D Dominant alleles are lost when the gametes fuse
Show answer
MCQ answer key B (Fertilisation is random, so chance affects which gametes fuse): the expected ratio is a probability, and random fertilisation means that the observed numbers can differ from it by chance.
Q2 Which change would make an observed ratio more likely to be close to the expected ratio?
- A Counting a larger number of offspring
- B Counting fewer offspring
- C Counting only the offspring with the dominant phenotype
- D Using parents of different species
Show answer
MCQ answer key A (Counting a larger number of offspring): with a larger number of offspring, the effects of chance tend to cancel out.
Q3 Two heterozygous parents produce 80 offspring. How many offspring are expected to show the recessive phenotype?
- A 10
- B 40
- C 20
- D 60
Show answer
MCQ answer key C (20): the recessive phenotype is expected in 1 out of every 4 offspring, and 80 divided by 4 is 20.
Structured A student crossed two heterozygous tall pea plants (Tt × Tt) and counted 8 offspring, of which 7 were tall and 1 was dwarf.
- State the expected ratio of tall plants to dwarf plants. [1]
- Calculate the number of dwarf plants that were expected from the 8 offspring. [1]
- Explain why the observed numbers differ from the expected numbers. [2]
- Suggest how the student could improve the reliability of the results. [1]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: 3 : 1
(b) 1 mark
- Mark scheme: 8 × 1/4 = 2
(c) 2 marks
- Mark scheme: fertilisation is random, so chance affects which gametes fuse
- Mark scheme: only a small number of offspring was counted, so chance has a large effect
(d) 1 mark
- Mark scheme: count a larger number of offspring, or repeat the cross and combine the results
Genetic diagrams and problem solving
use genetic diagrams to solve problems involving monohybrid inheritance
A genetic diagram shows the genotypes of the parents, the gametes that they make, the possible genotypes of the offspring and the ratio of the phenotypes, and it can be used to predict results or to work out the genotype of a parent.
- Punnett square
- Punnett square: a grid that shows all the possible combinations of the gametes of two parents, with the gametes of one parent across the top and the gametes of the other parent down the side
- Test cross
- Test cross: a cross between an individual that shows the dominant phenotype but has an unknown genotype and a homozygous recessive individual
- Genetic diagram
- Genetic diagram: a diagram that shows how alleles pass from the parents through the gametes to the offspring
Five steps to a genetic diagram
- 1Choose a letter for the gene, using a capital letter for the dominant allele and the same letter in lower case for the recessive allele.
- 2Write the phenotypes and genotypes of the two parents.
- 3Write the gametes that each parent makes, with only one allele in each gamete.
- 4Show the fertilisation between the gametes, using a Punnett square or lines, to give the genotypes of the offspring.
- 5Write the phenotype for each genotype of the offspring and state the ratio of the phenotypes.
Send the PAGER: Pick a letter for each allele, Adult genotypes are written for both parents, Gametes are worked out for each parent, Each possible fertilisation is shown in a Punnett square, Ratio of the offspring genotypes and phenotypes is stated.
The A stands for adult, which means the parents, and the letter chosen in the first step uses a capital letter for the dominant allele and a lower-case letter for the recessive allele.The test cross
| Gametes | T | t |
|---|---|---|
| t | Tt Tall | tt Dwarf |
| t | Tt Tall | tt Dwarf |
In the test cross Tt × tt (unknown is heterozygous), the offspring genotypes are in the ratio 1 Tt : 1 tt and the offspring phenotypes are in the ratio 1 Tall : 1 Dwarf.
| Gametes | T | T |
|---|---|---|
| t | Tt Tall | Tt Tall |
| t | Tt Tall | Tt Tall |
In the test cross TT × tt (unknown is homozygous dominant), the offspring genotypes are all Tt and the offspring phenotypes are all Tall.
- If all the offspring of a test cross show the dominant phenotype, the unknown individual is likely to be homozygous dominant.
- If the offspring of a test cross are in the ratio 1 dominant phenotype : 1 recessive phenotype, the unknown individual is heterozygous.
- If the unknown individual shows the recessive phenotype, it must be homozygous recessive, so a test cross is needed only for an individual that shows the dominant phenotype.
- A homozygous recessive individual is used as the second parent because all its gametes carry the recessive allele, so the phenotype of each offspring shows which allele it received from the unknown parent.
Working backwards from the offspring
Two dominant parents, a recessive child
If two parents with the dominant phenotype have a child with the recessive phenotype, the gene is on an autosome and both parents are heterozygous.
Offspring in the ratio 1:1
If one parent has the recessive phenotype and the offspring are in the ratio 1 dominant phenotype : 1 recessive phenotype, the other parent is heterozygous.
Two recessive parents
Two parents with the recessive phenotype can have only offspring with the recessive phenotype, because both are homozygous recessive.
Probability from a Punnett square
In the cross Tt × Tt, one of the four boxes is tt, so the probability of a dwarf offspring is 1 in 4, which is 25%, and the probability of a tall offspring is 3 in 4, which is 75%.
- In a family tree, a square stands for a male and a circle stands for a female, and a shaded symbol shows a person who has the condition.
- If two parents who do not have a condition have a child who does, the condition is caused by a recessive allele on an autosome, and both parents are carriers.
Depth: why and how
- Why do we write the gametes with only one allele each? Meiosis separates the two chromosomes of a homologous pair, so each gamete receives one allele of the gene.
- Why can a Punnett square give only probabilities? Each box is one equally likely fertilisation, so the square predicts the chance for each fertilisation, and it does not predict the exact numbers in a family.
- Why is a child with the recessive phenotype so informative when both parents show the dominant phenotype? For a gene on an autosome, the child must be homozygous recessive, so each parent must have passed on a recessive allele, which means that both parents are heterozygous.
- Cross-link to Reproduction: the gametes in the diagram are haploid (n) because meiosis halves the chromosome number, and the zygote is diploid (2n) again after fertilisation.
- If the unknown parent is Tt, the chance that five offspring of a test cross all show the dominant phenotype is 1 in 32, so counting a larger number of offspring makes the result of a test cross more reliable. supports K325-14(e)
Common mistakes
Gametes carry both alleles of a gene.
Each gamete carries only one allele of each gene, because meiosis separates the alleles.
A test cross uses a heterozygous individual as the second parent.
A test cross uses a homozygous recessive individual as the second parent.
In a Punnett square, the offspring are written outside the square.
The gametes of the parents are written outside the square, and the genotypes of the offspring are written inside the square.
An organism that shows the dominant phenotype must be homozygous.
An organism that shows the dominant phenotype can be homozygous dominant or heterozygous, so a test cross is needed to find out which.
Choose the letters, write the parents, work out the gametes, join them in a Punnett square, and state the ratio, and use a test cross with a homozygous recessive individual to find an unknown genotype.
Exam check
Q1 A tall plant of unknown genotype is crossed with a dwarf plant, and all 30 offspring are tall. What is the most likely genotype of the tall parent?
- A Tt
- B tt
- C TT
- D Tt or tt
Show answer
MCQ answer key C (TT): a homozygous dominant parent gives only T gametes, so all the offspring are Tt and tall, and a heterozygous parent would be expected to give some dwarf offspring.
Q2 A test cross between a tall plant of unknown genotype and a dwarf plant gives 12 tall and 11 dwarf offspring. What is the genotype of the tall plant?
- A TT
- B tt
- C TT or tt
- D Tt
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MCQ answer key D (Tt): a ratio of about 1 tall : 1 dwarf shows that half the gametes of the unknown plant carry t, so the plant is heterozygous.
Q3 Which pair of gametes can be made by a parent with the genotype Aa?
- A A and a
- B AA and aa
- C A and A
- D Aa and Aa
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MCQ answer key A (A and a): meiosis separates the two alleles, so each gamete carries one allele, either A or a.
Structured In pea plants the allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). A round-seeded plant of unknown genotype is crossed with a wrinkled-seeded plant, and the offspring are in the ratio 1 round : 1 wrinkled.
- State the genotype of the wrinkled-seeded parent. [1]
- State the genotype of the round-seeded parent. [1]
- State the gametes that the round-seeded parent makes. [1]
- Explain why a wrinkled-seeded plant is used in this kind of cross. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: rr
(b) 1 mark
- Mark scheme: Rr
(c) 1 mark
- Mark scheme: R and r
(d) 2 marks
- Mark scheme: all its gametes carry the recessive allele r
- Mark scheme: so the phenotypes of the offspring show which alleles the unknown parent passed on
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 14 of 14