So, what is “ploidy?”

Can you define all of the following terms?

{Haploid . Diploid . Monoploid . Aneuploid . Polyploid . Allopolyploid . etc}

You probably won’t need to be able to define all of the terms in your classes… not for a while anyway… but once you study genetics for a while, you’ll find that these terms pop up pretty often. It’s going to be helpful to you to recognize what they mean.

That being said, the word “ploidy” describes the number of COMPLETE SETS of chromosomes in a cell. There are lots of terms derived from that word, and they all have to do with numbers of chromosomes – but they don’t all describe the number of COMPLETE SETS. It’s strange, yes, but you’ve just gotta know it.

Haploid vs. Monoploid

Most students will learn the meaning of “haploid” (one complete set of chromosomes) as the alternative to diploid (2 sets). This isn’t usually a problem until one encounters the term “monoploid,” which isn’t the same as haploid.

You see, “haploid” refers to the number of complete sets, and is represented by the letter “n.” (“n” is not a variable. You’ll see this further down, when we talk about diploid) The monoploid number (represented by x) IS A VARIABLE that represents the number of chromosomes in a complete set.

So, if I have a haploid cell of a Buhbushka Toad that only holds 6 chromosomes, then I would say that the cell is 1n (or just n) and that x=6.

Diploidy

The prefix “di-” means two, so “diploid” means two complete sets of chromosomes. It’s usually denoted by “2n.”

Let’s look at that toad cell again. If x=6 in a 1n cell, then that would be double in a 2n cell. So, in the diploid form of the Buhbushka Toad, x=12.

Haploid and Diploid are related that way. Haploid is HALF of Diploid, and Diploid is DOUBLE of Haploid.

Polyploidy

A polyploid cell has MANY sets of chromosomes. If you’re already thinking that it’s denoted by 3n, 4n, 5n, etc, then you’re incorrect. Sorry. After the diploid level, the ploidy level of a cell is denoted by 3x, 4x, 5x, etc.

There are also some terms that you should be familiar with concerning ploidy.

• Aneuploid – describes changes in the number of a PORTION of the chromosome set rather than changes in the number of sets. For instance, if half of a set of chromosomes disappeared, leaving part of the genome diploid and part of the genome haploid. (From Greek words that mean “not good” because this usually results in deformations in humans)
• Autopolyploid – recognizes that an organism is polyploid, but adds “auto-”, denoting that all extra sets of chromosomes come from the same species
• Allopolyploid – also recognizes that an organism is polyploid, but with the caveat that some of the sets of chromosomes come from a different species (not usually seen in humans)