IGCSE · Practice questions · Chromosomes, genes and proteins

Reading the code.

Six original Cambridge-style questions on the vocabulary of genetics, a genetic diagram for the inheritance of sex, the route from gene to protein and why different cells make different proteins.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Chromosome, gene, allele, protein.

Chromosome = DNA; gene = length of DNA coding for a protein; allele = alternative form of a gene.
Sex: XX female, XY male; eggs all X, sperm half X and half Y, so 1 : 1.
Extended: gene stays in nucleus → mRNA copy → cytoplasm → ribosome joins amino acids in the order set by the bases.
Extended: diploid = two sets (23 pairs in humans); haploid = one set (23).

Recall

[3 marks]

Define (a) gene and (b) allele, and (c) state what chromosomes are made of.

(a) A length of DNA that codes for a protein. ✓
(b) An alternative form of a gene. ✓
(c) DNA (which contains genetic information in the form of genes). ✓

Analysis

[4 marks]

Draw a genetic diagram to show how sex is inherited in humans, and state the expected ratio of boys to girls.

Parents: female XX x male XY. ✓
Gametes: eggs all X; sperm X or Y (in equal numbers). ✓
Offspring: XX, XX, XY, XY, so 2 female : 2 male. ✓
Ratio 1 boy : 1 girl (a 50% chance of each). ✓

Explain

[2 marks]

Explain why it is the father, not the mother, who determines the sex of a child.

All the mother's eggs carry an X chromosome. ✓
Half the father's sperm carry X and half carry Y, so the sex depends on whether an X or a Y sperm fertilises the egg. ✓

Explain

[5 marks]

Extended. Describe how a cell uses a gene to make a protein.

The gene (DNA) stays in the nucleus. ✓
A copy of the gene is made as messenger RNA (mRNA) in the nucleus. ✓
The mRNA moves out of the nucleus into the cytoplasm. ✓
The mRNA passes through a ribosome, which assembles amino acids into a protein. ✓
The sequence of bases in the mRNA determines the sequence of amino acids in the protein. ✓

Explain

[3 marks]

Extended. Explain how DNA controls the function of a cell. Refer to the shape of protein molecules in your answer.

DNA controls which proteins the cell makes, such as enzymes, membrane carriers and receptors for neurotransmitters. ✓
The sequence of bases in a gene sets the sequence of amino acids, and different sequences give proteins different shapes. ✓
The shape of each protein (for example an enzyme's active site or a carrier's binding site) determines what it does, so the proteins made decide what the cell can do. ✓

Data

[4 marks]

Extended. The table shows the number of chromosomes in some cells.

Cell	Number of chromosomes
human skin cell	46
human sperm	23
dog liver cell	78
dog egg cell	X

(a) State which human cell is haploid. (b) Give the value of X. (c) State how many pairs of chromosomes are in a human skin cell. (d) Explain why a skin cell and a muscle cell from the same person make different proteins.

(a) The sperm (a single set of chromosomes). ✓
(b) 78 ÷ 2 = 39. ✓
(c) 23 pairs. ✓
(d) Both contain the same genes, but each cell only expresses the genes for the proteins it needs; the other genes are not expressed. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.