In most vertebrates, sperm cells join with egg cells to form a zygote. Each cell produces a haploid complement of chromosomes in order to form the zygote. The result is a new organism with a full set of maternal chromosomes and a full set of paternal chromosomes.

When a sperm joins with an egg, only the nucleus of the sperm enters to egg to form the zygote. The nuclear envelope is then altered, allowing the paternal DNA to intermix with the maternal DNA in the zygote nucleus. The sperm does not contribute a nucleus (only genetic material), cytoplasm, or diploid copies of any chromosome.

Reductional division of cells occurs during meiosis. This means that the number of chromosomes in the cells undergoing meiosis is reduced by half, as compared to the parental cells. The cells that arise from meiosis are genetically unique from their parental cells, since they do not retain the same genetic information.

The cells that result from meiosis are haploid, carrying only half of the normal DNA present in somatic cells.

Crossing over, or recombination, is a phenomenon in which small parts of homologous chromosomes move from one copy of the chromosome to the other. This process helps promote genetic diversity by providing slightly different copies of chromosomes for offspring.

Inducing genetic mutations would be a way to increase diversity, but it is not something that actually happens during meiosis. Condensation of chromosomes and separation of sister chromatids occur during both mitosis and meiosis, and do not promote genetic diversity.

In meiosis I the cell separates homologous chromosomes. In meiosis II the cell separates sister chromatids. In general, meiosis I and II are similar processes that go through the same steps (prophase, metaphase, etc.) with only a few key difference. During metaphase I, homologous chromosomes line up in tetrads. During metaphase II, chromosomes line up singularly. Crossing over can only occur during the formation of tetrads, and cannot occur during meiosis II.

Germ cells are diploid stem cells that give rise to the gametes of organisms that reproduce sexually. These cells can undergo both meiosis and mitosis. Mitosis is used to duplicate the germ cell, while meiosis is used to generate gametes.

Somatic cells are found in all other regions of the body, and are only capable of mitosis.

Crossing over occurs during prophase I of meiosis I. Crossing over is the physical exchange of chromosome parts, resulting in recombinant chromosomes and increased genetic variability. In order for this to occur, there is a requirement that the two homologous chromosomes be aligned next to one another, which occurs in prophase I of meiosis during tetrad formation.

Prophase I is the correct answer. During oogenesis in mammals, meiosis I occurs during the prenatal age. When the germ cells reach prophase I, the cell cycle is arrested, and the cells are frozen in prophase until puberty.

During puberty, the female will begin to ovulate. This means that one egg cell will progress from prophase I to metaphase I and complete meiosis on a cyclical basis, known as the menstrual cycle.

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.