Haploid chromosome number
The n number of an organism (in particular a cell) represents a Haploid count of chromosomes. Cells have a natural number of chromosomes pairs, the number of which is not limited. Being Haploid (one pair) or Diploid (two pairs) or multi-ploid (any number of pairs) is usually regulated by speciation in animals. Plants tend to have species directly related which may have a different ploid and different n numbers. In the case of humans we have 23 homologous chromosomes.
The discussion of chromosomes is needed to explain the n number in more detail. Chromosomes are the gathering of genetic material known as DNA or deoxyribonucleic acid. Each chromosome is made of one continuous macro molecule. The DNA that is contained in these are expressed in the organism through phenotype and cellular interactivity. Each group of chromosomes is contained in every cell in an organism other then sporophytes and gametophytes or sperm and eggs. In any cell the ploidicity or ploidy number of that cell represents how many complete sets of the chromosomes that are meant to be present.
For example, look at four genes, each on different chromosomes. There is one for eye color, one for hair. We have two chromosomes, one for hair one for eyes. In the human case we have 23 homologous pairs in our normal cells. This means we are diploid, as there are in fact 46 chromosomes in each cell nucleus. The n for humans is n=23 because we have to divide by 2 since we are diploid.
A classic consequence of having the wrong ploidy number is found in suffers of Down's Syndrome which have an extra chromosome 21 thereby making them trisomic for 21. This is also an example of aneuploidy, having a chromosome count that is different for the species. This can happen for instance during a non-disjunction event during meiosis.
Haploid animals are very rare, and require their own discussion. Plant reproduction does not negate the ability to easily be haploid because of the many forms it takes, including multiple in one organism. Haploid is defined as having only a single set of chromosomes. In this case any haploid organism will not have multiple copies of the genes that program the same information. In animals all reproductive cells are haploid. Sperm and eggs are formed with half the diploid chromosomes so that when they combine they will be diploid.
Most animals we find are diploid, although it is no less common in plants. Diploid means they have two copies of each chromosome. In this case we may have 46 chromosomes, but they represent two copies of 23. So n=23.
In plants we find that they may be triploid or more. No matter how many copies we find the underlying n number is divided by the number of copies that represent the same information. If we have a 5-ploid animal with 115 chromosomes that represents still only 23 sets of homologous chromosomes (same info) then we have an n of 23 (115/5).
Lecture Notes from:
- Dr. Chris Randall, Professor of Biology, Sam Houston State University
- Dr. Tamara Cook, Professor of Biology, Sam Houston State University