Ploidy

=Ploidy=


 * Ploidy ** is the number of sets of chromosomes found in a cell.

 Humans are an example of **diploid **organisms, carrying two complete sets of chromosomes. Each complete set is 23 chromosomes, and one complete set from each parent provides a full complement of 46 chromosomes. The 46 chromosomes are arranged into 23 homologous pairs. When humans form gametes, the homologous pairs are split in half, such that each gamete carries only one full set of chromosomes (23). The gamete is **haploid **, and upon fusing with another gamete, will restore the usual diploid number (46) at fertilisation. In humans, we can say that in gametes the haploid number (n) is 23 and in somatic cells 2n = 46.

 The **haploid number (n) ** is a number that is unique to gametes. It is always half the number of chromosomes found in somatic (regular) cells. So, in humans with 2 sets of chromosomes and a total of 46 chromosomes, the haploid number is 46/2 = 23. If an alien species has 9 sets of chromosomes with a total of 108 chromosomes then the haploid number (n) is 108/2 = 54. As you can see, the number of sets of chromosomes is irrelevant in determining haploid number, because the haploid number is always half of whatever the total number of chromosomes in somatic cells is, regardless of how many complete sets that total is made up of.

 The haploid number (n) should not be confused with the **monoploid number (x) ** which is the number of chromosomes found in a //single complete set // of chromosomes. In humans, the monoploid number (x) is 46/2 = 23, because human somatic cells have 2 sets of chromosomes with a total of 46 chromosomes. For humans, the monoploid number is incidentally equivalent to the haploid number (n). In our alien species, however, the haploid number (n) is 108/2 = 54, but the //monoploid //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> number (x) is 108/9 = 12. Thus, the monoploid number tells us that there are 12 chromosomes making up each of the 9 complete sets.


 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">The **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">haploid number (n) **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> is half of the //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">total //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> number of chromosomes in a somatic cell
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">The **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">monoploid number (x) **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> is the total number of chromosomes in a //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">single complete set of chromosomes //<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">and this does not change whether we are talking about a somatic cell or a gamete

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> Organisms, like the alien species, that have more than 2 full sets of chromosomes are said to be **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">polyploid **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">. Polyploidy can be established in one of three ways: where chromosome sets from the same species come together in a single organism, the phenomenon is ** autopolyploidy ** ; where the sets come from different species it is **allopolyploidy**; and where the sets come together in the same organism //and same individual//, due to nuclear division without cell division, the phenomenon is **endopolyploidy.**

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">An earthly example of polyploidy is the crop plant, wheat. Wheat is an allopolyploid species which is specifically **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">hexaploid - **<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;">each somatic cell has 6 full sets of chromosomes, with a total of 42 chromosomes. Thus the haploid number (n) for wheat is 42/2 = 21, while the monoploid number (x) is 42/6 = 7.

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> To summarise the above:

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> In humans: n = 23 and x = 23, and 2n = 2x = 46. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> In wheat: n = 21 and x = 7, and 2n = 6x = 42. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> And in our alien species: n = 54 and x = 12, and 2n = 9x = 108

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> In wheat, the gametes are both haploid (containing half the amount of genetic information) //and// **triploid** (because, in this case, half the amount of genetic information is 3 complete sets of chromosomes).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> **Euploidy** is the state of a cell or organism having an //exact multiple// of the monoploid number (x). So a normal human somatic cell with its 46 chromosomes is euploid because 46 is an exact multiple of the monoploid number for humans, 23. Even an abnormal number of chromosomes, such as 69 or 92 chromosomes, is considered euploid because both these figures are still exact multiples of the monoploid number (providing 3 or 4 complete chromosome sets, respectively).

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 90%;"> **Aneuploidy** is the state of having additional (or fewer) chromosomes than is normal, but the //total// number of chromosomes //not// being an exact multiple of the monoploid number. For example, an additional chromosome 21, the cause of Down's Syndrome, is an aneuploidy because it brings the chromosome total to 47, which is not an exact multiple of the monoploid number 23. Aneuploid syndromes usually end with the suffix //-somy// rather than //-ploid//. For instance, the additional chromosome 21 that causes Down's Syndrome is referred to as //trisomy 21.//