Genetics study guide

In humans, sex is determined at the moment of fertilisation by the combination of sex chromosomes inherited from the parents. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During meiosis, gametes (sperm and egg cells) are produced. Females produce egg cells, each carrying a single X chromosome. Males produce sperm cells, half of which carry an X chromosome and half carry a Y chromosome. When fertilisation occurs, an egg (always X) fuses with either an X-carrying sperm or a Y-carrying sperm. If an X-carrying sperm fertilises the egg, the resulting offspring will be XX (female). If a Y-carrying sperm fertilises the egg, the offspring will be XY (male). This mechanism results in an approximately 50% chance of the offspring being male and a 50% chance of being female.

Sexual reproduction involves the fusion of male and female gametes, typically from two parents, to form a zygote with a unique combination of genetic material. A primary benefit of this process is the significant genetic variation it introduces into a population. This variation increases the chances that some individuals will possess traits enabling them to survive and reproduce in changing or challenging environments, such as during disease outbreaks or climatic shifts. However, sexual reproduction also has limitations. A key constraint is the requirement for organisms to find and attract a mate, which can be time-consuming, energy-intensive, and expose individuals to predation. Competition for mates can also be fierce, and successful reproduction is not always guaranteed. Furthermore, the process is generally slower than asexual reproduction, and fewer offspring may be produced at one time.