Side by side
| Haploid | Diploid | |
|---|---|---|
| Chromosome sets | One set of chromosomes | Two sets, paired as homologous chromosomes |
| Shorthand | n | 2n |
| Human example | Egg and sperm cells, each carrying one set of 23 chromosomes | Somatic cells, carrying 23 homologous pairs |
| Where homologs come from | There is no pairing, because only one copy of each chromosome is present | One copy of each homologous chromosome is inherited from each genetic contributor |
| Produced by | Meiosis, which forms haploid nuclei from a diploid cell | Fertilization, which unites two haploid gametes into a zygote, and then mitosis |
| Number of alleles per gene | One copy of each gene, so no dominance relationship exists within the cell | Two copies, which may or may not encode the same version of the characteristic |
| Why the distinction exists | Halving the sets prevents the chromosome number doubling at every fertilization | Restores the full complement after two haploid gametes unite |
Sets, not numbers
The definitions are about chromosome sets rather than chromosome counts, and that is what makes them portable between species. Cells containing one set of chromosomes are haploid; cells containing two sets are diploid. In a diploid somatic cell the nucleus holds two copies of each chromosome, called homologous chromosomes, which are matched pairs carrying the same genes in identical locations along their lengths. One copy of each pair is inherited from each genetic contributor. Sexual reproduction needs both states: gametes must be haploid so that when they unite the zygote is diploid rather than doubling its chromosome sets with every generation.
Where students go wrong
The first slip is treating haploid as meaning half the DNA at all times. A human gamete carries one set of 23 chromosomes, but a chromosome that has just been replicated consists of two sister chromatids, so DNA quantity and ploidy level are separate bookkeeping systems and they change at different moments. The second slip is assuming diploid is the normal state for all life; plants alternate between haploid and diploid generations and use mitosis in both, and many unicellular organisms spend most of their life haploid. The third is confusing homologous chromosomes with sister chromatids, which are copies of a single chromosome.
Where this leads next
The division that converts diploid to haploid is meiosis, and the clearest way to see why it needs two divisions on one round of replication is the mitosis and meiosis comparison. From the other direction, the genetics pages depend on this distinction entirely: a Punnett square only makes sense because a diploid organism carries two alleles per gene and segregates one into each haploid gamete, which is Mendel's law of segregation restated in cellular terms. The genotype and phenotype page picks up what having two copies actually means for a visible trait.
Common mistakes
- Saying humans are haploid because gametes have 23 chromosomes. The gamete is haploid; the organism's somatic cells are diploid with 23 pairs.
- Equating ploidy with DNA quantity. A replicated chromosome has two sister chromatids and the cell is still diploid.
- Treating homologous chromosomes as identical copies. They carry the same genes in the same order but can carry different alleles.
- Assuming haploid cells are always gametes. Plants grow haploid gametophytes by mitosis, and many unicellular organisms are haploid for most of their life.
Related concepts
- Mitosis vs meiosis: Mitosis is a single nuclear division that produces two nuclei genetically identical to the original and at the same ploidy level.
- Cell cycle: The eukaryotic cell cycle has four phases: G₁, S and G₂, which together make up interphase, followed by M phase, the mitotic phase.
- Genotype vs phenotype: A genotype is an organism's underlying genetic makeup, including alleles that are not expressed.