Chromosome numbers
Understanding what follows needs two symbols, n and 2n. This is where they come from.
For a given species the number of chromosomes in an ordinary body cell is a constant. It is peculiar to that species.
Example
A human body cell contains 46 chromosomes. These consist of 23 pairs, the two members of each pair carrying the same genetic information.
Homologous chromosome pairs
A pair of chromosomes carrying the same genetic information is called a homologous pair. Of each homologous pair, one chromosome is inherited by the offspring from the mother and the other from the father.
n and 2n
A child inherits 23 chromosomes from the father (n) and 23 from the mother (n), making 46 chromosomes in all (2n).
Two ways it happens
There are two main methods by which cell division takes place.
Mitosis
Keeps the chromosome number constant — 2n to 2n.
Meiosis
Halves the chromosome number — 2n to n.
The next section takes them one at a time; both symbols are now in hand.