Haploid or n number of chromosomes in the body cells may be normal or abnormal. In lower plants and animals, haploid condition is normal in the sense, here the individuals have only one set of chromosomes in the body cells, while diploid condition is represented only in the zygote.
This is of normal occurrence. Haploidy is of interest when it occurs in diploid individuals. Here normally the body cells have 2 sets of chromosomes. Reduction division takes place and haploid gametes are formed. These gametes however have no morphological expression i.e., they cannot directly develop into new individuals. They fuse with another gamete resulting in the formation of diploid individual.
Rarely however haploid individuals are produced as during parthenogenesis when the egg directly develops into a new individual without fertilization. Haploids are of two kinds euhaploids and aneuhaploids. While the former are derived from euploids the latter are derived from aneuploids.
Origin and production of Haploids: In some insects haploid individuals are produced naturally due to parthenogenesis. In plants also parthenogenetic development of egg results in haploid individuals. Such haploids have been seen in cotton, tomato etc.
In rare instances, pollen tube, or microspores may develop into new individuals. Such individuals are called androgenic haploids as they are produced from the male tissue. Occasionally synergids, antipodals etc may also develop into haploid embryos.
Haploids can artificially be obtained by tissue culture of the anther, pollen, X ray treatment, colchicine treatment, distance hybridization etc. Anther culture has yielded excellent haploids in Nicotiana and Potato. Similarly interspecific crosses may also result in haploid individuals.
According to Kasha (1970), in a cross between Hordeum vulgare X H. bulbosum; chromosomes of H. bublosum are eliminated in early zygotic division. Embryosacs of these can be cultured to obtain haploid individuals.