Haploid

In the syllabus and the exam

Haploid describes the chromosome state of gametes, produced specifically by meiosis, and the syllabus expects candidates to state clearly that a haploid cell contains only one chromosome from each homologous pair (written as n), exactly half the number found in a diploid body cell (2n), rather than simply 'fewer' chromosomes without a precise relationship. Exam questions frequently combine haploid with fertilisation, asking candidates to explain that when a haploid sperm cell fuses with a haploid egg cell, the resulting zygote becomes diploid again, restoring the normal chromosome number for the new individual's body cells, which will then be produced by mitosis throughout growth and development. A common error is stating that haploid cells are produced by mitosis, when mitosis actually preserves the existing chromosome number (diploid cells producing diploid cells) rather than halving it; only meiosis produces haploid cells from diploid ones. Another frequent mistake is applying 'haploid' loosely to any cell that seems 'smaller' or 'simpler', rather than specifically to a cell containing exactly one set of chromosomes, a distinction candidates should be ready to justify with reference to the actual chromosome number (n) rather than the cell's size or appearance. Candidates should also be able to explain why haploid cells such as gametes cannot themselves divide by mitosis to produce more gametes directly; new gametes are instead produced only through further rounds of meiosis starting from diploid cells in the reproductive organs. This distinction matters directly in Paper 2 mark schemes, where stating 'haploid' without specifying n, or confusing it with a diploid body cell, is one of the most consistently penalised errors in genetics-linked structured answers.

Example

A human egg cell is haploid, containing only 23 chromosomes, one from each homologous pair found in a normal body cell.

Related terms

DiploidMeiosisChromosome

Source:SRC-DSKP-EN