Genetics is the branch of biology that studies heredity and variation in organisms. It is concerned with the molecular basis of inheritance and the ways in which genes are passed down from one generation to the next. The history of genetics can be traced back to the ancient Greeks, who were the first to speculate about the nature of heredity. In the 19th century, Gregor Mendel, an Austrian monk, conducted experiments on pea plants that led to the discovery of the laws of inheritance. His work, which he published in 1866, laid the foundation for modern genetics. Mendel's laws of segregation and independent assortment described how genes are passed from one generation to the next and how traits are inherited. In the early 20th century, Thomas Hunt Morgan and his colleagues at Columbia University conducted experiments on fruit flies that showed that genes are located on chromosomes. This discovery led to the development of the chromosome theory of inheritance, which states that genes
It's not quite accurate to say that everyone inherits two copies of every gene. In most mammals, the X and Y sex chromosomes determine whether an individual is male or female. Females have two X chromosomes, and therefore two copies of every gene. Males, however, have one X and one Y chromosome. For genes that appear only on the X chromosome or only on the Y (some do appear on both), males inherit just one copy. Taken from http://www.eschooltoday.com/science/genetics/images/sex-chromosomes.jpg Other living things have sex chromosomes too. Birds and reptiles have Z and W sex chromosomes. Unlike with X and Y, males have two Z chromosomes, and females have one Z and one W. Some insects and a few mammals have only an X chromosome. Females have two copies and males have one. However, sex is not always determined by chromosomes. With alligators, crocodiles, and most turtles, it's egg incubation temperature. And some fish can change gender in response to cues from the en