Cracking the AP Biology Exam


Cell Reproduction


Every organism has a certain number of chromosomes. For example, fruit flies have 8 chromosomes, humans have 46 chromosomes, and dogs have 78 chromosomes. It turns out that most eukaryotic cells in fact have two full sets of chromosomes—one set from each parent. Humans, for example, have two sets of 23 chromosomes, giving us our grand total of 46.

A cell that has both sets of chromosomes is a diploid cell, and the zygotic chromosome number is given as “2n.” That means we have two copies of each chromosome. For example, we would say that for humans the diploid number of chromosomes is 46.

If a cell has only one set of chromosomes, we call it a haploid cell. This kind of cell is given the symbol “n.” For example, we would say that the haploid number of chromosomes for humans is 23.


  • Diploid refers to any cell that has two sets of chromosomes.
  • Haploid refers to any cell that has one set of chromosomes.

Why do we need to know the terms haploid and diploid? Because they are extremely important when it comes to sexual reproduction. As we’ve seen, 46 is the normal diploid number for human beings. We can say, therefore, that human cells have 46 chromosomes. However, this isn’t entirely correct.

Human chromosomes come in pairs called homologues. So while there are 46 of them altogether, there are actually only 23 distinct chromosomes. The homologous chromosomes which make up each pair are similar in size and shape and express similar traits. This is the case in all sexually reproducing organisms. In fact, this is the essence of sexual reproduction: Each parent donates half its chromosomes to its offspring.


Although most cells in the human body are diploid (i.e., filled with pairs of chromosomes), there are special cells that are haploid (i.e., unpaired). These haploid cells are called sex cells, or gametes. Why do we have haploid cells?

As we’ve said, an offspring has one set of chromosomes from each of its parents. A parent, therefore, contributes a gamete with one set that will be paired with the set from the other parent to produce a new diploid cell, or zygote.