Haploidi

Haploidi, or haploidy, is the state of possessing a single complete set of chromosomes (n) in a cell or organism, in contrast to a diploid set (2n). In haploid cells there is only one allele at each gene locus, so the phenotype directly reflects the genotype. Haploidi is common in the gametes of many animals and plants, and it also characterizes the dominant life stage of several fungi, algae, and mosses as part of their life cycles.

Biology and life cycles: In most animals and many plants, meiosis produces haploid gametes that fuse during fertilization to form a diploid zygote. In plants with alternation of generations, the haploid gametophyte and the diploid sporophyte alternate; meiosis in the sporophyte yields haploid spores, which germinate into haploid gametophytes. Some fungi spend most of their life cycle as haploid, joining to form a diploid zygote only briefly before returning to haploid growth.

Genetic consequences: Because there is only one chromosome set, mutations are not masked by a second allele. This makes haploids especially useful for loss-of-function studies but also means deleterious mutations can have immediate fitness effects.

Applications: Haploid organisms or cells are widely used in genetics and plant breeding. Yeast studies exploit haploids for rapid genotype-phenotype analysis. In plant breeding, haploid induction followed by chromosome doubling can create homozygous lines quickly.