Biology · Unit 4

DNA and Genetics

Genes, inheritance and DNA

Genetics is the part of biology where a small number of rules generate an enormous amount of consequence. Two alleles, a dominance relationship and a bit of probability explain eye colour, blood groups and a large fraction of inherited disease.

The unit builds from DNA's structure through chromosomes, genes and alleles, into genetic crosses you can actually work, and then into what happens when copying goes wrong. Punnett squares look like arithmetic but they are really about populations of possible outcomes, which is why the same square can be right while any single child is an exception.

This unit breaks down into 15 short steps and 90 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 15
Questions: 90
Difficulty: 1-3

What this unit covers

Cell Division and Mutations
Inheritance and Traits
Chromosomes, Genes and Alleles
Genetic Crosses
DNA Structure

Where this fits

Comes after The Cell. It is the entry point to Molecular Biology, Evolution and Ecology, and Biotechnology.

Where people slip

Dominant does not mean common, strong or better. It only describes which allele's effect shows when both are present - plenty of dominant traits are rare, and plenty of recessive ones are near-universal.

How the unit is structured

DNA and Genetics runs as 15 short steps that unlock in order. 11 are practice rounds and 4 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 15 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 90 in the unit altogether.

Cell Division and Mutations

Fill the blankLevel 2
1. Genes located close together on the same chromosome tend to be inherited together and are described as ____.
- linkedcorrect
- dominant
- recessive
- codominant
Linked genes lie on the same chromosome and do not assort independently, so they are usually passed on as a group unless separated by crossing over.
Match the pairsLevel 2
2. Match each type of gene mutation to what it does to the DNA sequence.
Answer: Substitution = One base is swapped for a different base; Insertion = An extra base is added to the sequence; Deletion = A base is removed from the sequence; Frameshift = Every codon after the change is shifted
Substitution swaps a base, insertion adds one, deletion removes one, and a frameshift shifts the whole reading frame.
Multiple choiceLevel 2
3. Down syndrome is most often caused by having three copies of which chromosome (trisomy 21)?
- Chromosome 21correct
- Chromosome 23
- The X chromosome
- Chromosome 13
Down syndrome is usually caused by trisomy 21, an extra copy of chromosome 21 from an error in cell division.
Odd one outLevel 2
4. Three of these are large-scale structural chromosome mutations. Tap the odd one out that is a small-scale gene (point) mutation instead.
- Base substitutioncorrect
- Deletion of a chromosome segment
- Inversion
- Translocation
A base substitution changes a single nucleotide within a gene, whereas segment deletion, inversion and translocation are large chromosome mutations.
Choose all that applyLevel 3
5. Which of these are true about meiosis? Select all that apply.
- It produces sex cells (gametes)correct
- It creates four daughter cellscorrect
- The daughter cells have half the number of chromosomescorrect
- It makes two identical copies of a body cell
Meiosis makes four gametes, each with half the chromosome number. Making two identical body cells is the job of mitosis.
Guess the numberLevel 3
6. A person with Down syndrome (trisomy 21) has one extra chromosome. How many chromosomes are in each of their body cells?
Answer: 47 chromosomes
A typical cell has 46 chromosomes, and the extra copy of chromosome 21 makes 47 in Down syndrome.

Chromosomes, Genes and Alleles

Guess the numberLevel 2
7. How many pairs of chromosomes are in a typical human body cell?
Answer: 23 pairs
Human body cells contain 46 chromosomes arranged as 23 pairs, one set inherited from each parent.
Multiple choiceLevel 2
8. What is a gene?
- A section of DNA that codes for a particular traitcorrect
- A protein that gives a cell its shape
- The energy-making part of a cell
- A jelly that fills the cell
A gene is a segment of DNA that carries the instructions for a specific trait, such as eye colour.
Spell itLevel 2
9. Spell the word for a picture of all of an organism's chromosomes arranged in pairs by size.
Answer: karyotype
A karyotype is an organised image of a cell's full set of chromosomes, used to spot conditions like trisomy 21.
Type the answerLevel 2
10. What is the name of the thread-like structure of tightly coiled DNA that carries genes? (one word)
Answer: Chromosome
A chromosome is DNA wound tightly around proteins, and each one carries many genes.
Put in orderLevel 3
11. Put these genetic structures in order from smallest to largest.
Answer: Nucleotide -> Gene -> Chromosome -> Genome
Nucleotides build up into genes, genes sit along chromosomes, and all the chromosomes together make up the genome.
True or falseLevel 3
12. Because sperm mitochondria are normally destroyed after fertilisation, a woman with a mitochondrial disorder will pass it on to all of her children.
Answer: True
Mitochondrial DNA is maternally inherited, so an affected mother transmits the condition to every child, both sons and daughters.

DNA Structure

Fill the blankLevel 1
13. In DNA, the base adenine (A) always pairs with ____.
- thyminecorrect
- guanine
- cytosine
Adenine pairs with thymine (A-T) and cytosine pairs with guanine (C-G) through hydrogen bonds.
Multiple choiceLevel 1
14. What does the abbreviation DNA stand for?
- Deoxyribonucleic acidcorrect
- Double nucleus atom
- Dinitro acid
- Deoxyribose amino acid
DNA stands for deoxyribonucleic acid, the molecule that stores genetic information in living things.
Picture questionLevel 1
15. 🧬 This spiral molecule stores the genetic instructions for a living thing. What is it?
- DNAcorrect
- A protein
- A sugar
- A vitamin
The double-helix molecule shown is DNA, which stores genetic information in nearly every cell.
Match the pairsLevel 2
16. Match each DNA term to its correct partner.
Answer: Adenine = Thymine; Guanine = Cytosine; Nucleotide = Building block of DNA; Double helix = Twisted-ladder shape
Bases pair A-T and G-C, nucleotides are the repeating units of DNA, and the whole molecule twists into a double helix.
Guess the numberLevel 3
17. Roughly how many protein-coding genes does the human genome contain? (nearest thousand)
Answer: 20000 genes
The Human Genome Project found humans have only about 20,000 protein-coding genes - far fewer than scientists once expected.
Odd one outLevel 3
18. Which of these is NOT one of the four bases found in DNA?
- Adenine
- Thymine
- Uracilcorrect
- Guanine
Uracil is found in RNA, not DNA. DNA uses only the four bases adenine, thymine, cytosine, and guanine.

Genetic Crosses

Fill the blankLevel 2
19. The 9:3:3:1 dihybrid ratio only appears when the two genes are on different chromosomes and assort ____ of each other.
- independentlycorrect
- dependently
- unequally
- together
Mendel's law of independent assortment states that genes on different chromosomes segregate independently, producing the 9:3:3:1 ratio.
Multiple choiceLevel 2
20. A heterozygous tall pea plant (Tt) is crossed with a short plant (tt). What ratio of tall to short offspring is expected?
- 1:1correct
- 3:1
- 2:1
- 1:2:1
Tt x tt produces half Tt (tall) and half tt (short), giving a 1:1 ratio.
Picture questionLevel 2
21. 🐄 A red bull is crossed with a white cow and the calf is roan - its coat shows separate red hairs AND white hairs mixed together. What is this inheritance pattern?
- Codominancecorrect
- Incomplete dominance
- Complete dominance
- Sex linkage
In roan cattle both coat-colour alleles show fully and separately (red and white hairs), which is codominance rather than a blended colour.
Spell itLevel 2
22. Spell the word for when two alleles are BOTH shown fully in the phenotype, like the A and B in blood type AB.
Answer: codominance
Codominance is when both alleles are expressed together and fully, rather than blending into an in-between form.
Choose all that applyLevel 3
23. For a RrYy x RrYy dihybrid cross (round R and yellow Y are dominant), select ALL statements that are correct.
- The offspring show a 9:3:3:1 phenotype ratiocorrect
- Round yellow is the most common phenotypecorrect
- Wrinkled green is the rarest phenotypecorrect
- Every offspring is round and yellow
- Only two phenotypes are possible
A 9:3:3:1 cross produces all four phenotype combinations, with round-yellow the most common and wrinkled-green the rarest.
Guess the numberLevel 3
24. In a dihybrid cross between two double heterozygotes (AaBb x AaBb), 1 in how many offspring is expected to be homozygous recessive for both genes (aabb)?
Answer: 16 offspring
The 4x4 Punnett square gives 16 equally likely combinations, and only one of them is aabb, so the expected proportion is 1 in 16.

Inheritance and Traits

Fill the blankLevel 1
25. In ____ breeding, humans choose organisms with useful traits and let only those reproduce.
- selectivecorrect
- random
- natural
- asexual
Selective breeding means humans pick which organisms with desired traits get to reproduce, so those traits become more common.
Build the sentenceLevel 2
26. Build the sentence about identical alleles.
Answer: An organism with two identical alleles is homozygous
Homozygous means both alleles for a gene are the same, such as TT or tt.
Fact or fibLevel 2
27. A recessive trait appears only when an organism has two recessive alleles.
Answer: True
A single dominant allele hides a recessive one, so a recessive trait shows only when both alleles are recessive.
Multiple choiceLevel 2
28. Which statement correctly describes genotype and phenotype?
- Genotype is the set of alleles; phenotype is the visible traitcorrect
- Genotype is the visible trait; phenotype is the set of alleles
- Both words describe only the dominant allele
- Genotype and phenotype mean exactly the same thing
Genotype is the combination of alleles an organism carries, while phenotype is the trait you can actually observe.
Choose all that applyLevel 3
29. Select ALL correct statements about X-linked recessive conditions such as haemophilia and red-green colour blindness.
- They are more common in malescorrect
- A male needs only one recessive allele to be affectedcorrect
- The allele is carried on the X chromosomecorrect
- They are more common in females
- The allele is carried on the Y chromosome
These alleles sit on the X chromosome, and because males have only one X a single recessive allele affects them, making the conditions more common in males.
Guess the numberLevel 3
30. In a population at Hardy-Weinberg equilibrium the recessive allele frequency q = 0.1. What percentage of the population is expected to be homozygous recessive (q squared)?
Answer: 1 %
q squared = 0.1 x 0.1 = 0.01, which is 1% of the population homozygous recessive.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 16 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise DNA and Genetics

90 questions across 15 steps. Start with step one and crawl at your own pace.

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Read about DNA and Genetics

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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