Diploid

In the syllabus and the exam

Diploid describes the normal chromosome state of body cells (somatic cells) in most animals and plants, and the syllabus expects candidates to link this term directly to mitosis, which maintains the diploid number across cell generations, and to contrast it against haploid, which describes gametes produced by meiosis. A well-prepared candidate can state that a diploid cell contains two complete sets of chromosomes, one set inherited from each parent, organised into homologous pairs where each pair carries genes for the same characteristics, though possibly different alleles of those genes. A common error is using 'diploid' and 'haploid' as if they refer to a fixed chromosome number that is the same for all organisms, when in fact the diploid number varies between species (for example, humans have a diploid number of 46, while other organisms have different numbers), so candidates should express diploid and haploid in terms of 2n and n respectively rather than a specific fixed value unless the question names the species. Candidates are also expected to explain why fertilisation, which fuses two haploid gametes into one diploid zygote, restores the diploid number that meiosis had earlier halved, connecting diploid directly to the wider cycle of meiosis, gamete formation and fertilisation tested together in the reproduction and genetics chapters. Candidates sometimes need to apply the term outside humans, for example stating the diploid number of a named plant or animal given in a question, so revision should include practising with at least one non-human example rather than relying only on the human figure of 46.

Example

A human liver cell is diploid, containing 46 chromosomes arranged as 23 homologous pairs, with each pair made up of one chromosome from each parent.

Related terms

HaploidChromosomeMitosis

Source:SRC-DSKP-EN