Diploid vs Haploid

Inside the nucleus of a cell are thread-like structures called chromosomes, which carry the genes (the instructions for the body). The number of chromosomes a cell has tells you whether it is diploid or haploid.

A diploid cell contains two complete sets of chromosomes. The chromosomes come in matching pairs — one chromosome of each pair came originally from the mother and one from the father.

In humans, a diploid cell has 46 chromosomes. These are arranged as 23 pairs (because 23 × 2 = 46). We call 46 the diploid number for humans.

Almost all of the cells in your body are diploid. These ordinary cells are called body cells (sometimes called somatic cells). Examples include:

• skin cells
• muscle cells
• liver cells
• nerve cells (neurones)

So whenever you are talking about a normal working cell of the body, it is diploid (46 chromosomes).

Exam tip. The word "diploid" starts with "di-", which means two — two sets of chromosomes. Link "diploid" to "double" to remember it is the bigger number.

A haploid cell contains only one complete set of chromosomes — exactly half the number found in a diploid cell.

In humans, a haploid cell has 23 chromosomes (one from each of the 23 pairs). We call 23 the haploid number for humans.

The only haploid cells in the human body are the gametes — the sex cells used in reproduction:

• the egg cell (ovum) made by the female
• the sperm cell made by the male

Each gamete carries just one set of 23 chromosomes, so it has half the genetic information of a body cell. This is essential for sexual reproduction, as you will see in the next section.

Notice the relationship the specification asks you to know: the diploid number is always double the haploid number.

$\text{diploid number} = 2 \times \text{haploid number}$

For humans: $46 = 2 \times 23$.

Exam tip. The word "haploid" starts with "ha-" — link it to "half". A haploid gamete has half the chromosomes of a body cell.

This is the most important idea in the topic, and a favourite exam question: why must gametes be haploid?

During sexual reproduction a sperm cell fuses with an egg cell. This fusion is called fertilisation, and it produces a single cell called a zygote, which will grow into a new individual.

When the two gametes fuse, their chromosomes are added together:

• the haploid sperm brings 23 chromosomes
• the haploid egg brings 23 chromosomes
• the zygote therefore receives 23 + 23 = 46 chromosomes

So the zygote is diploid (46) — the correct full number for a human body cell. The zygote then divides over and over to build a whole new body of diploid cells.

Why this matters: if the gametes were not haploid — if both egg and sperm were diploid (46) — fertilisation would give 46 + 46 = 92 chromosomes. The number would then double every generation (92, then 184, and so on), which would not work. By being haploid, gametes make sure the chromosome number stays constant from one generation to the next.

Exam tip. The full-mark answer is: gametes are haploid so that when two gametes fuse at fertilisation, the diploid number is restored — and so the chromosome number stays the same from generation to generation.

It helps to lay the two types of cell side by side. Notice that the diploid number is always exactly double the haploid number.

Feature	Diploid cell	Haploid cell
Sets of chromosomes	Two sets	One set
Number in humans	46	23
Chromosomes in pairs?	Yes — 23 pairs	No — single chromosomes
Which cells?	Body cells (skin, muscle, liver, nerve)	Gametes (egg, sperm)
Made by	Ordinary cell division (mitosis)	Cell division that halves the number

Reading across the table, the key message is simple: body cells are diploid (46) and gametes are haploid (23), and the diploid number is 2 × the haploid number.

Exam tip. Questions often give you the diploid number of a different organism and ask for the haploid number (or the other way round). Just halve the diploid number, or double the haploid number. For example, if a plant body cell has 14 chromosomes, its gametes have 7.