How is information passed from parents to offspring, and how do we predict the features of the young?
Describe chromosomes, genes and DNA, explain dominant and recessive alleles, and use a genetic diagram to predict the offspring of a simple cross
A focused N(A)-Level answer on inheritance. Chromosomes, genes and DNA, dominant and recessive alleles, and using a simple genetic diagram (Punnett square) to predict offspring.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
SEAB wants you to describe chromosomes, genes and DNA, to explain dominant and recessive alleles, and to use a simple genetic diagram to predict the offspring of a cross. The central idea is that features are passed from parents to offspring through genes carried on chromosomes.
The answer
Chromosomes, genes and DNA
Inside the nucleus of a cell are thread-like structures called chromosomes. Chromosomes are made of a chemical called DNA. A gene is a short section of DNA that carries the instructions for one feature, such as eye colour or plant height. Offspring inherit one copy of each gene from each parent.
Alleles
Most genes come in different versions called alleles. For example, the gene for height in pea plants has a tall allele and a short allele. An organism has two alleles for each gene, one from each parent. These two alleles may be the same or different.
Dominant and recessive
When the two alleles are different, one may hide the other:
- a dominant allele shows its effect even if only one copy is present. We write it with a capital letter, for example for tall.
- a recessive allele only shows its effect when two copies are present, because a dominant allele would otherwise mask it. We write it with a small letter, for example for short.
So a plant with or is tall, but only a plant with is short.
Genetic diagrams
A genetic diagram (Punnett square) predicts the likely offspring of a cross. You write the alleles each parent can pass on (the gametes), then combine them in all possible ways. The diagram gives the expected ratio of offspring, not an exact count.
Examples in context
Example 1. Why two brown-eyed parents can have a blue-eyed child. If brown is dominant and both parents carry a hidden recessive blue allele (so each is ), their child could inherit a blue allele from each parent and be , showing blue eyes. The recessive feature appears when two recessive alleles come together.
Example 2. How plant breeders use crosses. A grower wanting a feature controlled by a recessive allele must breed plants until offspring inherit two copies of that allele. Genetic diagrams help predict how likely each cross is to produce the wanted plants, saving time and space.
Try this
- Cue. Define a gene. A section of DNA that carries the instructions for a particular feature.
- Cue. State the genotype (alleles) of a short pea plant, where short is recessive. (two recessive alleles).
- Cue. A plant is crossed with a plant. State the ratio of tall to short offspring. .
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksIn pea plants, tall (T) is dominant to short (t). A plant with genes is crossed with a short plant (). (a) Draw a genetic diagram to show the offspring. (b) State the ratio of tall to short offspring.Show worked answer →
(a) Parent gametes: the plant gives or ; the plant gives or .
Combining them: , , , .
(b) Two are tall () and two are short (), so the ratio of tall to short is .
What markers reward: correct gametes from each parent, the four offspring combinations, and the ratio of tall to short.
Original3 marks(a) Define a gene. (b) Explain the difference between a dominant and a recessive allele.Show worked answer →
(a) A gene is a section of DNA that carries the instructions for a particular feature (characteristic).
(b) A dominant allele shows its effect even if only one copy is present. A recessive allele only shows its effect when two copies are present (no dominant allele is there to mask it).
What markers reward: a gene defined as a section of DNA coding for a feature, and the dominant allele showing with one copy while a recessive needs two.
Related dot points
- Describe food chains and food webs, explain the roles of producers, consumers and decomposers, and account for energy loss along a chain
A focused N(A)-Level answer on ecology. Producers, consumers and decomposers, food chains and food webs, why energy is lost at each step, and what happens when a chain is disturbed.
- Describe how human activities such as pollution and deforestation affect the environment, and outline ways to conserve resources and reduce harm
A focused N(A)-Level answer on human impact. The effects of pollution and deforestation, the importance of biodiversity, and practical ways to conserve resources and reduce harm.
- Identify the parts of animal and plant cells and their functions, compare the two cell types, and describe how cells are organised into tissues, organs and systems
A focused N(A)-Level answer on cells. The parts of animal and plant cells and their jobs, the differences between them, and how cells build up into tissues, organs and systems.
- Describe enzymes as biological catalysts, explain how they speed up reactions, and describe the effect of temperature and pH on enzyme activity
A focused N(A)-Level answer on enzymes. Enzymes as biological catalysts, why each enzyme is specific, and how temperature and pH change enzyme activity including denaturing.