Passage of Information from Parents to Offspring

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Explain the meanings of the terms haploid (n) and diploid (2n)
  2. Explain what is meant by homologous pairs of chromosomes
  3. Explain the need for a reduction division during meiosis in the production of gametes
  4. Describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle
  5. Interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis
  6. Explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes
  7. Explain that the random fusion of gametes at fertilisation produces genetically different individuals

1. Haploid and Diploid Cells

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Cells can contain different numbers of chromosome sets, and we use special terms to describe this.

Diploid cells (2n) contain two complete sets of chromosomes. The "2n" means two sets. In diploid cells, chromosomes come in matching pairs - one chromosome from your mother and one matching chromosome from your father. Nearly all the cells in your body are diploid. For example, humans have 46 chromosomes in total, organized as 23 pairs, so our diploid number is 2n = 46.

Haploid cells (n) contain only one complete set of chromosomes. The "n" means one set - exactly half the number found in diploid cells. In humans, haploid cells contain 23 chromosomes with no matching pairs. These haploid cells are your sex cells (gametes) - the egg cells in females and sperm cells in males.

The key point to remember is that haploid cells have half the chromosome number of diploid cells. If you know the diploid number for any organism, simply divide by 2 to find the haploid number. For example, dogs have 78 chromosomes in their diploid cells, so their haploid cells contain 78 ÷ 2 = 39 chromosomes.

These terms describe the number of chromosome sets, not the total number of chromosomes. Different species have different numbers of chromosomes, but the relationship between diploid and haploid stays the same: haploid is always half of diploid.

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