In biology, the term “ploidy” refers to the number of chromosome sets per cell. Haploid cells have half of the number of chromosomes as parent cells, meaning that they only carry a single copy of each gene. Haploid cells are formed during meiosis and, in humans, produce gametes, which mature into sperm and egg cells.

Diploid cells contain two copies of each chromosome, therefore containing twice as many (2n) chromosomes as a haploid cell (n), which contain only one copy of each chromosome. In humans, diploid somatic cells contain 46 chromosomes, or 23 pairs of chromosomes.

Trisomy 21, or Down syndrome results when a human individual inherits three copies of chromosome 21, instead of the normal two copies (one maternal and one paternal). This is caused by nondisjunction, or the improper segregation of chromosomes during cell division. In the case of trisomy 21, nondisjunction leads to a failure of chromosome 21 segregation during meiosis (most of the time during anaphase I). This results in either an egg or sperm carrying two copies of chromosome 21 instead of one. The zygote formed by fertilization develops into an individual that has three copies of chromosome 21. Trisomy 21 manifests in physical growth delays, intellectual disabilities, and distinctive facial features. 

Nondisjunction is the improper segregation of chromosomes during meiosis or mitosis. The molecular causes behind nondisjunction are (1) increased rate of nondisjunction in female cells, (2) failure to properly pass the spindle assembly checkpoint, and (3) weakening of the cohesion complex at the centromere due to age.

Meiosis is a process that produces gametes in sexual reproduction. Therefore, organisms that undergo sexual reproduction or feature sexual life stages undergo meiosis. Mammals produce haploid sperm and eggs through meiosis, which fuse to form a diploid zygote. In mosses, reproduction is called the alternation of generations, meaning that generations alternate between haploid and diploid forms. In this system, meiosis produces haploid spores during the sporophyte generation, which germinate to form gametophyte precursors. In flowering plants, cells in male and female sexual organs undergo meiosis to form precursor sex cells, called spores. Prokaryotes, including bacteria, perform asexual reproduction that does not include the process of meiosis. Types of asexual reproduction include fission, budding, and fragmentation. In the case of bacteria, cells reproduce by binary fission, or cellular division without mitotic spindles.  

During crossing over in prophase I of meiosis, there is a physical exchange of genetic material between homologous chromosomes. This exchange occurs at the sites of double stranded breaks, where recombinase enzymes facilitate the invasion of the other chromatid. The original strand and invading strand anneal following this invasion. When this happens between two chromosomes, the strands form a tetrahedral arrangement called a Holliday junction. Other recombinase enzymes move the junction down the strands, furthering recombination.

During prophase I of meiosis, homologous chromosomes exchange genetic material in a process called crossing over. Double stranded breaks and recombinase enzymes facilitate chromatid invasion and annealing. The tetrahedral structure formed through strand invasion between two chromosomes is called the Holliday junction.

In humans, and many other mammals, sex determination follows the XX/XY system. This system has XX (two X-chromosomes) conferring the female gender and XY (one X-chromosome and one Y-chromosome) determining the male gender. The SRY gene on the Y-chromosome begins development into maleness. In XX females, one X-chromosome is inactivated, forming a Barr body.

Both Klinefelter’s syndrome and Turner syndrome are examples of sex chromosome disorders. In Klinefelter’s syndrome, a male individual is XXY, containing one extra chromosome. Physical symptoms include reduced muscle tone, less body hair, and sometimes breast tissue enlargement. Developmental symptoms include reading and language impairment. The cause of Klinefelter’s syndrome is nondisjunction of either gamete, resulting in either a sperm with both an X-chromosome and a Y-chromosome or an egg with two X-chromosomes. Turner syndrome is when a female is XO, meaning that the individual is missing one X-chromosome. There are a number of symptoms, including characteristic facial features and nonverbal learning disabilities. Turner syndrome results from paternal nondisjunction, leading to a sperm cell without a sex chromosome.

Sex chromosomes undergo crossing over during prophase I of meiosis, but only in a small region of homology.