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 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.

  • The haploid number (n) is half of the total number of chromosomes in a somatic cell
  • The monoploid number (x) is the total number of chromosomes in a single complete set of chromosomes and this does not change whether we are talking about a somatic cell or a gamete

Organisms, like the alien species, that have more than 2 full sets of chromosomes are said to be polyploid. 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.

An earthly example of polyploidy is the crop plant, wheat. Wheat is an allopolyploid species which is specifically hexaploid - 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.

To summarise the above:

In humans: n = 23 and x = 23, and 2n = 2x = 46.
In wheat: n = 21 and x = 7, and 2n = 6x = 42.
And in our alien species: n = 54 and x = 12, and 2n = 9x = 108

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).

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).

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.