By Shirley Day (article contributor)
Every individual in any species, plant or animal, starts as a haploid egg with a complete set of chromosomes, (n). Bacteria stay like this and reproduce by simple mitosis.
Some species became multi-celled; the cells were able to make organs to provide what we call a body, or a plant. But each individual in these species still started as a single haploid cell with a complete set of chromosomes. As soon as the zygote is formed, millions of new individuals are returned to the haploid state by meiosis and await, in the ovary, their chance for the body, (2n) part of their life cycle.
The method used to produce body cells is the adding of a second complete and matching set of chromosomes, (2n). Thus there is a doubling of the DNA, not a joining of two halves. (No cell can have only ‘half’ of its DNA).
The second set is made, by meiosis, in adapted ovaries (testes in animals) and consists of a pared down cell (sperm or pollen grain) which contains only a nucleus with a single matching set.
Fertilization is actually the arrival in the egg of the second set; when this happens, the two nuclei undergo meiosis to provide two sets in each of the new cells; these are somatic cells. The individual has now begun its 2n part of the life cycle. (Recombination is a method of arranging the chromosomes for this procedure). But the new haploid individuals are being stored in the ovary, as above.
I wonder if any Biologists will be able to confirm this description.
This form of Reproduction is therefore not the mixing of two different lots of DNA, but the exact matching of two complete sets of chromosomes; deletions, mutations, etc. are harmful and cause impairment.
Humans are really bacteria (a haploid egg with one complete set of chromosomes) with the n part of the life cycle followed by a 2n part.