Chromosome number

Ploidy is the number of sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes.

Cells are described according to the number of sets present (the ploidy level): monoploid (1 set), diploid (2 sets), triploid (3 sets), tetraploid (4 sets), pentaploid (5 sets), hexaploid (6 sets), heptaploid or septaploid (7 sets), etc. The generic term polyploid is used to describe cells with three or more sets of chromosomes (triploid or a higher ploidy).

Humans are diploid organisms, carrying two complete sets of chromosomes: one set of 23 chromosomes from their father and one set of 23 chromosomes from their mother. The two sets combined provide a full complement of 46 chromosomes. This total number of chromosomes is called the chromosome number. When a species has a varying chromosome number, e.g. a diploid and tetraploid form, the chromosome number is called diploid number in the diploid form, and tetraploid number in the tetraploid form.

The number of chromosomes found in a single complete set of chromosomes is called the monoploid number (x). The haploid number (n) is unique to gametes (sperm or egg cells), and refers to the total number of chromosomes found in a gamete, which under normal conditions is half the total number of chromosomes in a somatic cell.

The haploid number for humans (half of 46) is 23; and the monoploid number equals 46 divided by the ploidy level of 2, which is also 23. When a human germ cell undergoes meiosis the two sets of 23 chromosomes are split in half to form gametes. After fusion of a male and a female gamete (fertilization) both containing 1 set of 23 chromosomes, the resulting zygote has 46 chromosomes: 2 sets of 23 chromosomes (22 autosomes, and 1 allosome).

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