Haploid cells have one pair of chromosomes and are found in male insects and some fungi and algae. Plants can switch from haploid to diploid states, which is useful in breeding crops. Haploid organisms lack a backup gene if one is defective, but have a lower mutation rate. The rarity of haploid organisms is due to their loss of reproductive ability over successive generations, making sexual fitness the main reason for their rarity.
Haploid cells are cells made up of only one pair of chromosomes, which are structures in the cell nucleus that contain genetic information responsible for passing on hereditary traits to offspring. While most adult cells in eukaryotic organisms are diploid, possessing two sets of chromosomes each, adult males in the insect order Hymenoptera of ants, wasps, and bees are composed of haploid cells. Sperm and egg cells in most organisms are also haploid in nature until they come together in the fertilization process and become diploid.
Chromosomal information is used to perpetuate a species, and this information is merged when male and female sex cells are merged so that the genetic information shared by both parents can benefit the offspring by offering them many potentially dominant traits. Some offspring, however, do not require this level of genetic information to function. Male wasps, ants, and bees contain only haploid cells because they grow from unfertilized eggs into adults. Even in some species of fungi and algae, haploid cell structures are the norm.
Plants have the ability to switch from a haploid to a diploid or double haploid state, which is now an important feature of plant breeding to control the genetic traits of crops such as barley, tobacco and canola. Over 250 plant species have now been genetically engineered to be double haploid, using methods of asexual reproduction, or parthenogenesis, to produce offspring without fertilization and shedding unwanted chromosomes. One of the additional benefits of double haploid plants is that their offspring are genetically identical to the parent plants, which eliminates variations in crops along the line when traditional diploid reproduction occurs.
One of the disadvantages of haploid organisms, natural or genetically modified, is that, if a gene turns out to be defective, there is no identical copy of it to perform its function in its place as there is with diploid cells. Every gene in a haploid organism is expressed effectively, either performing its function correctly or negatively affecting the health and vitality of the organism. In examining haploid yeast cultures, the evidence also suggests, however, that the mutation rate in organisms is effectively doubled in diploid cells, as haploid cells have only half as many genes as they can mutate.
Evolutionary biology has for some time studied the underlying cause of the dominance of diploid cells and sexual over asexual reproduction. Research indicates that haploid organisms are rarer not because they are more vulnerable to environmental stresses, but because haploid organisms in successive generations lose the ability to reproduce. Sexual fitness, therefore, is most likely the main reason haploids are rare in nature, even though they are less susceptible to the harmful mutations that diploids can undergo.
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