Genes, inheritance and selection Study Guide

The theory of evolution by natural selection is a cornerstone of modern biology, explaining how life on Earth has changed over vast periods. This fundamental idea was independently developed by two naturalists: Charles Darwin and Alfred Russel Wallace. Their work, though separated by time and geography, converged on the same groundbreaking concept. They both observed that within any population of organisms, there is natural variation. Individuals with characteristics best suited to their environment are more likely to survive, reproduce, and pass on these advantageous traits to their offspring. Over many generations, this process leads to a gradual change in the inherited characteristics of a population, resulting in evolution. Darwin is widely credited for his extensive research and publication of 'On the Origin of Species' in 1859, which meticulously detailed the evidence for natural selection. Wallace, working in the Malay Archipelago, independently arrived at similar conclusions and sent his findings to Darwin, which prompted Darwin to publish his own work. Their combined contributions provided a robust mechanism for how evolution occurs, moving biological thought away from static views of species. The impact of their work on modern biology is immense; it underpins fields such as genetics, ecology, conservation, and medicine, helping us understand antibiotic resistance, the diversity of life, and our own place in the natural world.

In humans, biological sex is determined by a specific pair of chromosomes called the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During reproduction, each parent contributes one sex chromosome to their offspring. The mother, having two X chromosomes, can only pass on an X chromosome to her egg cells. The father, having both an X and a Y chromosome, can pass on either an X chromosome or a Y chromosome to his sperm cells. This means that the father's sperm ultimately determines the sex of the child.

A genetic cross, often represented using a Punnett square, can illustrate the probabilities of inheriting these sex chromosomes. When a sperm carrying an X chromosome fertilises an egg (which always carries an X chromosome), the resulting zygote will be XX, developing into a female. If a sperm carrying a Y chromosome fertilises an egg, the zygote will be XY, developing into a male. Therefore, there is an approximately 50% chance of a child being female and a 50% chance of a child being male with each pregnancy. This demonstrates that the male parent's gametes are responsible for determining the sex of the offspring, as they are the only parent capable of contributing a Y chromosome.