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Mendel's Experiments
Throughout the 1850s and 1860s, Mendel selectively cross-bred varieties of the common pea plant and discovered that certain characteristics (including flower color, stem length and seed shape) occurred in only two possible forms. For example, flowers were always either purple or white, rather than mixing. When Mendel crossed a white-flowered plant with a purple-flowered plant, the offspring were a mix of pure purple-flowered plants and pure white-flowered plants in various ratios.
Mendel's findings contradicted the major theories of the day about how traits were passed from parent to child. The dominant theory of the day proposed that traits of parents were mixed in the offspring, while Darwin's theory of "pangenesis" theorized that inherited characteristics could be passed from parent to child. Both theories were disproved by Mendel's experiments with selective breeding. Mendel expressed his findings in four laws that later became known as Mendel's Principles of Heredity. These principles, rediscovered in the early 20th century, form the basis of modern genetics.
Principle of Unit Characters
The Principle of Unit Characters states that characteristics are passed from parents to offspring as individual traits. These traits are passed on unchanged as a pair of genes. In Mendel's experiments, this explained why breeding a purple-flowered pea and a white-flowered pea never produced an offspring with purpley-white flowers. It also explains why children are never a perfect blend of their parents: instead, they inherit some pure characteristics from the father and other pure characteristics from the mother. This principle also explains why acquired characteristics are not passed on: traits are passed on unchanged, so the child of a pair of bodybuilders won't be born with bulging biceps.
Principle of Dominance
The Principle of Dominance posits that some traits are dominant and can mask or prevent the expression of other, recessive traits. The gene for the recessive trait is present but masked by the dominant trait and can be passed to the next generation. Dominant and recessive genes explain a great deal about the way characteristics are passed from parents to offspring. It explains why, for example, a blue-eyed father and brown-eyed mother will have all brown-eyed children. The children receive the genes for both eye colors, but only the dominant gene (for brown eyes) is expressed. It also explains why two brown-haired people might have a red-haired son: the genes for red hair were present in both parents, but masked by the dominant gene for brown hair. Only when both alleles are for red hair is that trait expressed. This is also why inbreeding is dangerous: it increases the chances of offspring receiving two recessive genes for a genetic disease, like cystic fibrosis or Tay-Sachs disease.
Principle of Segregation
The Principle of Segregation says that each individual receives one gene for each trait from each parent. An individual has two genes for each trait, one received from the mother and the other from the father. This accounts for how traits are passed from parent to child. Each parent passes a full set of genes to the child; the offspring has two genes for each characteristic. In humans, the male sperm and female egg each has a single full set of genes, half of a DNA strand. They combine at fertilization to create the full DNA double helix that contains all the genetic information of the individual.
Principle of Independent Assortment
The Principle of Independent Assortment states that genes separate randomly and are passed to offspring independent of each other. The combination of genes that are passed from parent to offspring are not necessarily present in either parent. For example, in Mendel's experiments with pea plants, the genes for wrinkled seeds, yellow pods and white flowers are all inherited independently and distributed at random among the offspring. This accounts for why parents never know what combination of traits their children will have, and no child ever looks exactly like one parent: each offspring is a random mix of traits from each parent. For example, if the mother is blonde and tall, a blonde child has no better chance of being tall than a dark-haired child.