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Ploidy refers to the number of sets of chromosomes in the nucleus of a cell. A gamete usually carries one full set of chromosomes1. The total number of chromosomes in a gamete is the gametic or haploid number and is referred to as n. When gametes combine to form a zygote, the haploid number is doubled (2n) and becomes the zygotic or diploid number. For example, humans have 23 chromosomes2. In a gamete, n = 23, but when combined to form a zygote, 2n = 46.  The number of actual chromosomes does not affect the ploidy level of a cell. A species with ten different chromosomes could be diploid just as well as tetraploid. Rather the number of total chromosomes in a cell, ploidy refers to the number of copies of a specific chromosome present

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"Haploid vs diploid" by Ehamberg - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Haploid_vs_diploid.svg#mediaviewer/File:Haploid_vs_diploid.svg

The ploidy level refers to the number of sets of chromosomes in this zygotic number, so in the case of humans, our ploidy level is 2n or diploid.  Other ploidy levels include triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n), etc.  Cells with three or more sets of chromosomes can also be referred to generally as polyploid.  Different ploidy levels may have varying effects on a species fitness.  For example, triploid individuals are generally sterile3.  During the process of meiosis, the cell is unable to split the odd number of chromosomes evenly between two gametes and is unable to reproduce successfully. 

References

1Ploidy, what is it? Mitosis vs. meiosis. Auburn University. Retrieved March 10, 2015 from http://www.auburn.edu/academic/classes/zy/vert_embryo/html/PloidyandMeiosis.html.

2How many chromosomes do people have? Published March 10, 2015. Genetics Home Reference, National Institute of Health. Retrieved March 10, 2015 from http://ghr.nlm.nih.gov/handbook/basics/howmanychromosomes.

3Allen, S.A. and Wattendorf, R.J. 1987. Triploid grass carp management: status and management implications. Fisheries 12: 20-24.

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