Down Syndrome results from trisomy 21, in which an individual has three copies of chromosome 21 in their genome. The cause for the extra chromosome is a nondisjunction event, resulting in an uneven splitting of the genome during meiosis.

Nondisjunction mainly occurs during meiosis I, particularly during anaphase I when homologous chromosomes are separated. When both chromosomes are pulled to the same pole the result of meiosis I is two cells, one with 22 chromosomes and one with 24. Meiosis II is used to segregate the sister chromatids of these cells, but does not change the amount of genetic material. When a gamete with 24 chromosomes fuses with a normal gamete with 23 chromosomes, the result is trisomy. When the trisomy particularly affects chromosome 21, the result is Down Syndrome.

Meiosis allows for the creation of genetically different haploid cells from one original germ cell. Following anaphase I, homologous chromosomes are separated from one another, resulting in a halving of the genetic material (haploid). As a result, the two cells are haploid following meiosis I. The separation of genetic material in anaphase II involves the splitting of chromatids, not homologous chromosomes. This does not affect the number of chromosomes in each cell, meaning all cells remain haploid.

Parent: diploid (XX)

Meiosis I: haploid, full chromosome (X)(X)

Meiosis II: haploid, single chromatid (/)(\)(/)(\)

Note that crossing over can only occur when the cell is diploid in meiosis I, specifically during prophase I.

Reduction of ploidy implies that the cell is losing a duplicate copy of each chromosome. In meiosis I, the cell segregates homologous chromosomes into two unique daughter cells. These daughter cells now technically only contain one copy of each chromosome. The parent cell contained two copies of each chromosome (diploid), while the daughter cells contain only one copy of each chromosome (haploid). This results in the reduction of ploidy after meiosis I.

After meiosis II, each cell contains only one chromatid. This chromatid, however, contains the code for a full chromosome, meaning that each daughter cell contains the genetic material for one chromosome. There is no reduction of ploidy after meiosis II, since both parent and daughter cells carry only one copy of each chromosome.

After mitosis, each cell contains one chromatid for each homologous chromosome. As such, the cells each contain DNA for two copies of each chromosome. Since both parent and daughter cells are diploid, there is no reduction of ploidy.

Mitosis is used by somatic cells throughout the body. The goal of mitosis is to replace older cells with newer, healthier cells. In order for this replacement to be effective, the daughter cells must be identical to the parent cell. Somatic cells, or "body cells," are diploid, meaning that they carry two copies of each allele. Each round of mitosis produces two daughter cells after one division.

Meiosis only takes place in the gonads and is used to produce gametes. Gametes fuse to form a diploid zygote, but each individual gamete carries only half of the genetic information to form this zygote; as such, all gametes are haploid and carry only one copy of each allele. Gametes are not identical to the parent cell for this reason (the parent cell is diploid). Genetic variation (crossing over) can also occur during meiosis to enhance genetic diversity. Each round of meiosis produces four daughter cells after two divisions.

Meiosis involves two cell divisions. During the first division, pairs of homologous chromosomes align at the center of the cell and are separated into two daughter cells. During the second division, single chromosomes align at the cell center (as they would during mitosis) and sister chromatids are separated into the daughter cells.

When homologous chromosomes align during the first division there are a total of four chromatids in each set, forming a tetrad. The alignment of chromosomes at the equatorial plate takes place during metaphase. Since we are looking at the alignment of chromosomes during the first meiotic division, the correct answer will be metaphase I.

In prophase I, a process called synapsis involves the pairing of chromosomes. Chromosome pairs are referred to as a tetrad, homologous pair, or as bivalents.

There are two events that occur in prophase I that do not occur in any other stage: chiasmata (crossing over) and synapsis (pairing of the chromosomes). Note that chiasmata does not occur during prophase of mitosis, but synapsis does occur.

The separation of sister chromatids is the only item of the answer choices that occurs in both mitosis and meiosis. Prophase II and metaphase II only occur in meiosis, as does recombination between homologous chromosomes. Recombination between sister chromatids does not occur (they are identical).

Since the cell has 44 autosomes and two sex chromosomes it must be a somatic cell (2n=46). Two X chromosomes corresponds to a female so it must a somatic cell from a female. Sperm, and ova are the male and female gametes, which are haploid.

Meiosis is the form of cell division that results in gametes and so meiosis takes place in both ovaries and testes, which are the primary sex organs. Somatic cells undergo mitosis for cell division, which yields identical daughter cells that are diploid.