Dr Trista has a PhD in Biochemistry and loves to teach college biology and chemistry.
The term polyploidy means to have multiple complete sets of genetic information. What does a 'complete set of genetic information' mean? Think of it as one complete 'book of instructions' on how to make an organism. A polyploid creature has more than two 'books' in its 'library' of genetic material.
Most critters that reproduce sexually have an even number of books: one set from mom and one set from dad. It's important to remember that these 'books' are similar, but they're not identical. Why is this a useful advantage? Well, if a critter inherits one copy of a defective gene, the critter's cells can 'look up' a good copy of the gene in the other 'book.' If the critter only had one 'book' and a defective gene that was critical to survival, that offspring would likely never mature.
It's tempting to think that a polyploid critter would leave extra 'books' on the shelf and not use them, but this isn't the case. Cells use all the genetic information they have available. Because of this, critters that can survive with more 'books' have higher dosage levels of each gene made, usually resulting in larger cells, larger critters, and larger offspring.
Types of Ploidy
Because scientists love specific language, they have created many terms to describe ploidy, or the number of sets of genetic information needed to make a critter. You may see term 'polyploidy' used as a noun, with 'polyploid' as its adjective form, but this 'rule' applies to all the terms for the different types of ploidy.
Here are some of the most common types:
Haploid means a critter has only one set of chromosomes, but can still reproduce. Bacteria and other single-celled organisms are commonly haploid. Occasionally, you'll encounter multicellular haploid critters, which are usually insects or other invertebrates.
Monoploid means a critter which is supposed to have two sets of chromosomes, but accidentally only got one. Monoploids are usually sterile.
Diploid means a critter has two sets of chromosomes; it's the most common type for sexually reproducing critters. Humans, animals, plants, and fungi may be diploid. Diploidy may occur only part of the time, too. For example, yeast cells are usually haploid but can combine sexually to become diploid. Mammals tend to be diploid but may have particular cells or tissue types that are polyploid, such as liver cells or muscle cells.
Triploid means to have three sets of chromosomes, and tetraploid means to have four sets of chromosomes. Theoretically, this naming convention continues ad infinitum.
Aneuploidy, on the other hand, describes an erroneous number of chromosomes, rather than another full 'book.' Think of an aneuploid organism as a 'book' with extra chapters, or worse, missing chapters!
Polyploidy in Plants and Animals
Polyploidy is most commonly observed in the plant kingdom. Thousands of years of selective cultivation and plant breeding have resulted in vigorous food plants that are commonly tetraploid and hexaploid. If you compare diploid and tetraploid varieties of the same type of plant, very often the tetraploid plants grow larger and more vigorously.
Among animals, polyploidy is often observed in bony fish and amphibians. In general, there is a genetic bias for even ploidy numbers in animals. Critters with uneven numbers of 'books,' or with 'books' that contain uneven numbers of chapters, usually cannot produce offspring.
Did you know that farmers have exploited these facts to get seedless varieties of fruits? Seedless watermelons and bananas were created by breeding a tetraploid with a diploid, resulting in triploid offspring. The odd number of chromosomal sets act to prevent seed formation, resulting in shrunken or missing seeds.
Polyploidy in Humans
Since Humans have 23 chromosome pairs, the human 'books' consist of one genetic volume from each parent, each with 23 chromosomal 'chapters.' Some mammalian cells, including those in humans, deliberately induce polyploidy. This occurs when they're in tissues that make lots of proteins, such as muscle and liver. More copies of the master plans means more protein-building factories can be at work at the same time! This is under tight cellular control to prevent cancer.
Cancer cells have many genetic problems, and usually the chromosomal error tracking machinery is broken. It's quite common to see triploid or tetraploid cancer cells in a tumor, or in cancer cells grown in dishes in a laboratory. While the extra genetic information may or may not be advantageous to tumor expansion and migration, the patient generally prefers the cancer gone either way.
Let's review. Polyploidy means to have multiple complete sets of genetic information. Ploidy is a general term that refers to a complete set of genetic information. When more than one set is present, they're similar but not identical. Cells will use all of the genetic material available to them, and polyploid cells tend to be larger.
The most common types of ploidy include haploid, which means a critter has only one set of chromosomes, but can still reproduce; monoploid, which means a critter which is supposed to have two sets of chromosomes, but accidentally only got one; and diploid, which means a critter has two sets of chromosomes. Aneuploidy, on the other hand, describes an erroneous number of chromosomes.
Polyploidy is most commonly observed in the plant kingdom. Thousands of years of selective cultivation and plant breeding have resulted in vigorous food plants that are commonly tetraploid and hexaploid. Among animals, polyploidy is often observed in bony fish and amphibians. In general, there is a genetic bias for even ploidy numbers. Critters with an uneven number of sets usually cannot produce offspring. Human muscle tissue and liver tissue may become polyploid in a controlled manner to make lots of proteins. In human cancers, it's very common to see triploid and tetraploid cells because cancer cells often have errors in cellular reproduction.
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