Form 5 · Paper 2 essay guide

Inheritance, Paper 2 essay guide

How Inheritance appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Explaining genes, alleles and monohybrid inheritance: Define gene, allele, genotype, phenotype, dominant and recessive precisely. → State Mendel's law of segregation in terms of gamete formation. → Complete a Punnett square for a monohybrid cross between two heterozygotes. → Give the resulting genotype ratio (1:2:1) and phenotype ratio (3:1). → Use correct capital and lower-case letter notation for a named allele pair.
Explaining dihybrid inheritance and comparing it with monohybrid inheritance: State Mendel's law of independent assortment. → List the four gamete types produced by a double heterozygote. → Complete a 4 x 4 Punnett square for a dihybrid cross. → Explain how the 16 boxes group into the 9:3:3:1 phenotype ratio. → Compare the scale of a dihybrid cross with a monohybrid cross.
Explaining codominance, multiple alleles and incomplete dominance: Define codominance and give the ABO blood group system as an example. → Explain multiple alleles using the three ABO alleles, I^A, I^B and i. → Define incomplete dominance and give an example producing a blended phenotype. → Distinguish codominance from incomplete dominance by whether the heterozygote blends or shows both traits separately.
Explaining inheritance in humans: sex determination, sex-linked conditions and pedigrees: Explain how the X and Y chromosomes determine the sex of a child. → Explain why a sex-linked recessive condition is more common in males than females. → Read a pedigree diagram to identify whether a condition is dominant or recessive, and autosomal or sex-linked. → Calculate the probability that a child of two carrier parents shows a recessive condition.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	State what the question asks and define the key genetic term it uses, such as 'genotype', 'codominance' or 'sex-linked', in one sentence.
Point 1	State the genotypes of the parents or individuals involved, using consistent capital and lower-case letters for each allele.
Point 2	Work through the cross or pedigree systematically, such as listing gamete types or reading the pattern of affected individuals.
Point 3	Give the resulting ratio or genotype clearly, supported by a Punnett square or pedigree reasoning.
Conclusion	State what the result shows about the trait's inheritance pattern, linking back to the term defined in the introduction.

Key terms to include

gene
allele
genotype
phenotype
dominant
recessive
monohybrid inheritance
dihybrid inheritance
test cross
codominance
multiple alleles
incomplete dominance
sex-linked inheritance
pedigree diagram
carrier

A model essay outline

Genes and alleles, A gene is a section of DNA that codes for a characteristic; alleles are its different versions, one from each parent.
Dominant and recessive, A dominant allele shows in the phenotype whenever present; a recessive allele shows only when both alleles are recessive.
Genotype and phenotype, The genotype is the alleles an organism has; the phenotype is the characteristic that shows.
Monohybrid inheritance, The inheritance of a single characteristic, worked out with a Punnett square to predict ratios.
Dihybrid inheritance, The inheritance of two characteristics at once, giving a 9:3:3:1 ratio in a typical cross.

Frequently asked questions

What is the difference between genotype and phenotype?

The genotype is the pair of alleles an organism has for a characteristic, written with letters such as TT, Tt or tt. The phenotype is the characteristic that is actually shown, such as being tall or short. Two organisms can have different genotypes (TT and Tt) but the same phenotype (both tall), because T is dominant.

How do you predict the offspring of a monohybrid cross?

Write the genotypes of the two parents, work out the gametes each can produce, and combine them in a Punnett square. The square shows all the possible offspring genotypes and their proportions. From these you read off the phenotype ratio (often 3:1) and the genotype ratio (often 1:2:1) for a cross between two heterozygotes.

How is the sex of a baby determined?

Sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). All eggs carry an X, while sperm carry either an X or a Y. If an X-carrying sperm fertilises the egg the baby is female (XX); if a Y-carrying sperm does, the baby is male (XY). There is an equal chance of each.