Ploidy
Ploidy is a biological term that refers to the number of chromosome sets contained within a cell. In eukaryotic organisms, which possess a nucleus, cells typically exhibit either haploid (one set of chromosomes) or diploid (two sets of chromosomes) characteristics. For instance, humans are diploid, having forty-six chromosomes organized into twenty-three pairs, with one set inherited from each parent. Various terms describe different ploidy levels; euploid cells have the correct number of chromosomes, while aneuploid cells have an abnormal chromosome count, which can lead to conditions like Down syndrome or Turner syndrome.
In the context of reproduction, haploid gametes—sperm and egg cells—combine during fertilization to form a diploid zygote. Meiosis is the process that creates these haploid cells, while mitosis maintains the chromosome number in diploid cells. Ploidy also varies across organisms; for example, plants often alternate between haploid and diploid life stages. Some species, especially among plants, can exhibit polyploidy, meaning they have multiple sets of chromosomes, which can lead to the development of new species and traits beneficial for agricultural practices.
Ploidy
Ploidy refers to how many chromosome sets a cell contains. Eukaryotic cells, which have a nucleus, normally have either one or two full sets of chromosomes. The number of chromosomes making up a set differs from species to species. Humans have twenty-three chromosomes that are each different and carry unique genes. Because humans inherit one full set from each parent, they have forty-six chromosomes—or twenty-three pairs—in most cells.
Diploid cells have two sets of chromosomes. Haploid or monoploid cells have only one full set of chromosomes. Many terms exist to describe the "ploidy" of a cell or an organism. The prefix eu- means "true" or "good." Euploid cells have the correct number of chromosomes in a set, while aneuploid cells do not. Polyploids have multiple chromosome sets. The prefixes di-, tri-, tetra-, hexa-, ando cto- can be combined with the root word ploid to indicate two, three, four, six, or eight sets of chromosomes, respectively.
Ploidy in Sexual Reproduction
While most cells of the body are diploid, haploid cells allow reproduction. Haploid cells are the gametes, or sperm and egg cells. Each gamete contains a single set of chromosomes in its nucleus. When a sperm cell fertilizes an egg, it adds its set of chromosomes. The fertilized egg becomes diploid. The ability to produce haploid gametes allows organisms to produce offspring with new and unique gene combinations. Most eukaryotes reproduce in this way. The process that gives rise to haploid gametes is called meiosis, a type of cell division that halves the chromosome number. A diploid fertilized egg divides by mitosis, which keeps the number of chromosomes the same.
Ploidy in Human Disorders and Conditions
Sometimes, cells are aneuploid, or have fewer or more than the normal number of chromosomes in a set. For example, people with Down syndrome have forty-seven chromosomes. Their cells contain two full sets of chromosomes, plus an extra chromosome twenty-one. Down syndrome results when a sperm or egg contributes two copies of chromosome twenty-one. In meiosis, these chromosomes remain together in the same gamete instead of separating to join different gametes. This failure to separate is called nondisjunction. If both chromosomes leave the gamete, the offspring inherits one fewer chromosome. This is the case in Turner syndrome, in which females have only one sex chromosome instead of two. Aside from chromosome twenty-one and the sex chromosomes (X and Y), human aneuploidies involving other chromosomes are rare.
Ploidy in Plants and Other Organisms
In humans and other animals, diploid cells divide by mitosis. In other living things, such as algae and fungi, haploid cells divided by mitosis. Some of these cells are spores that fuse to produce a diploid cell, but this quickly undergoes meiosis to form new haploid cells.
Most plants alternate between haploid and diploid forms. For example, diploid ferns produce haploid spores that fall to the ground and divide to form tiny plants. These produce gametes that fuse, forming a diploid cell that grows into a fern. Green mosses are haploid. These hold reproductive structures that produce sperm and eggs cells through mitosis. Sperm cells travel through moisture, and when one reaches an egg cell, it creates a diploid cell that divides by mitosis. This elongated structure remains attached to the haploid parent until some of the diploid cells undergo meiosis to produce spores. Mosses can release thousands of spores at a time, and each spore can grow into a tiny new plant.
In contrast, most flowering plants are diploid, and the haploid generation remains small and contained within the flowers. Some of these haploid cells are pollen, which fertilizes the egg cells within the flowers. This creates a diploid embryo surrounded by a seed, which can grow into a diploid plant, repeating the cycle. Even though this process seems similar to animal reproduction, haploid cells in flowering plants undergo mitosis.
Ploidy and Food Crops
Many foods come from plant seeds. The seeds of cereal crops—such as wheat, barley, rice, and corn—contain tissue that is neither diploid nor haploid. Cereals are grasses, which are members of the flowering plants. In flowering plants, a pollen grain contains two haploid cells. One haploid cell fertilizes the egg to form the plant embryo. The other fertilizes a special cell with two haploid nuclei, forming a triploid cell. This triploid cell produces the endosperm, a nutrient-rich material that surrounds the embryo within the seed and provides the energy it needs to grow.
People have harvested the energy-rich seeds of grasses for millennia by planting and growing cereal crops. Flour is made by harvesting wheat seeds and grinding the triploid endosperm into a powder.
Polyploidy and Evolution
Sometimes plants produce offspring with more than two full sets of chromosomes. Some polyploid plants are hybrids, resulting from fertilization between two closely related species. Other polyploids contain duplicate chromosomes from the same species. Because the gametes of a newly formed polyploid are not compatible with others of its species, it must self-fertilize to reproduce. Therefore, polyploidy is one way by which new species can form. Species of farmed wheat, for example, are tetraploid or hexaploid because they are hybrids of wild grasses. In addition, farmers may intentionally produce polyploid crops, which may be bigger or lack seeds (such as seedless watermelon).
While polyploidy is more common in plants than in animals, certain species of fish, leeches, and brine shrimp are polyploids that descended from diploid ancestors. Polyploidy has occurred throughout evolution. Scientists believe that two separate duplications of the genome occurred early in the history of vertebrates, the group that includes fishes, frogs, mammals, reptiles, and birds. The evidence for this is that these animals all have four versions of many genes. Although each version is slightly different, having since gained different mutations after the duplications occurred.
Bibliography
Campbell, Neil, and Jane Reece. Biology: AP Edition. 7th ed. New York: Pearson/Benjamin Cummings. 2005. Print.
Madlung, Andreas. "Polyploidy and Its Effect on Evolutionary Success: Old Questions Revisited with New Tools." National Center for Biotechnology Information. US National Library of Medicine National Institutes of Health, Feb. 2013, www.ncbi.nlm.nih.gov/pmc/articles/PMC3554449/. Accessed 25 Nov. 2024.
Muenchrath, Deborah, et al. "Ploidy: Polyploidy, Aneuploidy, and Haploidy." Iowa State University Digital Press, iastate.pressbooks.pub/cropgenetics/chapter/ploidy-polyploidy-aneuploidy-haploidy-2/. Accessed 25 Nov. 2024.