Learn about this topic in these articles:

In ploidy
This condition is called haploidy. When two germ cells (e.g., egg and sperm) unite, the diploid condition is restored.
Read More

In fungus: Sexual reproduction
…single set of chromosomes (a haploid state). Plasmogamy, the fusion of two protoplasts (the contents of the two cells), brings together two compatible haploid nuclei. At this point, two nuclear types are present in the same cell, but the nuclei have not yet fused. Karyogamy results in the fusion of…
Read More

In meiosis
…of chromosomes, consisting of two haploid sets (one inherited from each parent). These haploid sets are homologous—i.e., they contain the same kinds of genes, but not necessarily in the same form. In humans, for example, each set of homologous chromosomes contains a gene for blood type, but one set may…
Read More

In heredity: During meiosis
…number of chromosomes, or the haploid number. Thus, a human gamete contains 23 chromosomes, while a Drosophila gamete contains four. Meiosis produces the haploid gametes.
Read More

In reproduction: Binary fission
…each pair of chromosomes (haploid). During the two successive meiotic divisions involved in the production of eggs, a primordial diploid egg cell is converted into a haploid egg and three small haploid polar bodies (minute cells). In this instance the egg receives far more cytoplasm than the polar bodies.
Read More
In reproduction: The evolution of variation control
…life cycle, during which the haploid gamete of each parent can combine to form the diploid zygote. This is also often true in organisms that reproduce asexually, but in this case the asexual reproductive bodies (e.g., spores) are small and hence are effectively dispersed.
Read More