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.

During crossing over, double stranded breaks allow for chromatid invasion and genetic recombination. These double stranded breaks cannot be maintained, as they may result in genome rearrangement. The invading chromatid strand anneals within this double stranded breaks, thus repairing them. 

During meiosis in animal cells, haploid gametes are formed. Genetic diversity is critically important in gamete formation to ensure different genetic combinations are made during reproduction. Meiosis maintains genetic diversity through crossing over and independent assortment. Crossing over occurs during prophase I and is the physical exchange of genetic material between homologous chromosomes. Crossing over occurs at the chiasma and is facilitated by double stranded breaks and recombinase enzymes. Independent assortment is the separate segregation of homologs during meiosis; namely that each homolog segregates free of other pairs. The independent nature of this segregation allows a greater variety of genetic recombinations in gametes. Both crossing over and independent assortment allow for the production of genetically diverse gametes and therefore genetically diverse organisms.

The processes of mitosis and meiosis have many differences between them. These differences include the genetic recombination event called crossing over unique to meiosis, the fact that only mitotic daughter cells are genetically identical to parent cells, and the paring of homologous chromosome pairs during metaphase I of meiosis. One characteristic that is common to both processes is that they occur in all animals.

During prophase I of meiosis, a crossing over event allows for genetic recombination. Crossing over ensures the formation of gametes with different genetic combinations. The process involves the swapping of genetic material from one homologous chromosome pair to another, and is facilitated by double stranded breaks in the DNA helix and recombinase enzymes. The structure formed during crossing over is called the “Holliday junction”.

During meiosis, the “chiasma” is the point of chromatid contact that precedes crossing over. The crossing over event then occurs at this point. Synapsis refers to the pairing of homologs during prophase I of meiosis.

During the anaphase I stage of meiosis, homologous chromosome pairs separate to opposite poles of the cell, and the cell elongates. The sister chromatids remain attached at the centromeres. This maintenance of sister chromatids is the key difference between meiosis and mitosis. In mitosis, anaphase features the separation of sister chromatids, which is what will happen during anaphase II of meiosis. Note however, that the sister chromatids in meiosis are not identical, due to crossing over in prophase I. 

Meiosis is the process that forms haploid gametes, or sex cells, that will go on to form zygotes after fertilization. Meiosis is performed by germ line cells, which, in humans, are located in the gonads (ovaries of females and in the testicles of males).