Sexual Reproduction

Meiosis

Sexual reproduction involves two parents and results in offspring that are genetically unique. This process begins with meiosis, which takes place in the gonads (the testes and ovaries). During meiosis, specialized sex cells called gametes—sperm and eggs—are formed. Each gamete contains only half of the parent's genetic information, ensuring that every sex cell has a one-of-a-kind combination of genes.

Fertilization

The next stage is fertilization, where a sperm and an egg fuse together to combine their DNA (aka conception). This creates a single cell known as a zygote, which is the very first body cell of the new organism. In order for the zygote to contain the correct number of  chromosomes, each gamete needs to have half as many chromosomes as the zygote. In humans, each gamete should have 23 chromosomes, so that the zygote contains 46.

Human chromosomal disorders often occur due to an error during meiosis called nondisjunction. This happens when homologous chromosomes or sister chromatids fail to separate properly, resulting in gametes (sperm or egg) with an abnormal number of chromosomes. If one of these gametes is involved in fertilization, the resulting zygote will have a chromosomal mutation.

A well-known example is Down syndrome, also called Trisomy 21. In this condition, an individual has three copies of chromosome 21 instead of the usual two, leading to specific physical traits and developmental delays. Other examples include Klinefelter syndrome, where a male has an extra X chromosome (XXY), and Turner syndrome, where a female is missing an X chromosome (XO). These disorders demonstrate that having the correct "dosage" of genetic information is essential for typical human development.