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

Somatic cells, tissues and individuals can be 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 chromosome sets.

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. The zygotic number is defined as the number of chromosomes in zygotic cells. Human zygotes are diploid, hence with a zygotic number of 46.

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). In most animals, 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).

The common potato (Solanum tuberosum) is an example of a tetraploid organism, carrying four sets of chromosomes. The potato plant inherits two sets of 12 chromosomes from the pollen parent, and two sets of 12 chromosomes from the ovule parent. The four sets combined provide a full complement of 48 chromosomes. The haploid number (half of 48) is 24. The monoploid number equals the chromosome number divided by the ploidy level: 48 chromosomes in total divided by a ploidy level of 4 equals a monoploid number of 12.

The commercial common potato crop is propagated vegetatively (asexual reproduction through mitosis), in which case new individuals are produced from a single parent, without the involvement of gametes and fertilization, and all the offspring are genetically equal to each other and to the parent.

This page was last edited on 20 April 2018, at 22:07.
Reference: under CC BY-SA license.

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