Genetic variation is the basis of evolution. But because Mendel was able to see the physical representation of genetic variation in his pea plants, it also allowed him to understand genetic inheritance for the very first time.
His first law of dominance says that when an individual inherits two different forms of the same gene, one variant is often dominant over the other. For example, when Mendel crossed plants with round peas with plants with wrinkled peas, he found that the round pea shape was dominant over the wrinkled. Only plants that inherited two versions of the wrinkled type would have wrinkled peas. In other words, the wrinkled pea type is recessive. In human disease genetics, variants that cause monogenic diseases are often recessive, whilst the ‘normal’ variant is dominant.
His second law concerns how variants segregate over time. Remember that we have two copies of each of our chromosomes. When we have children we only pass on one of each of the copies, with our partner passing on just one of their two copies. Mendel’s second law states that the two versions of a given gene will always be split apart and won’t both get inherited together. So if a disease is monogenic and recessive, then the disease variant of the gene will be inherited from both parents.
Mendel’s third law says that genes underlying different traits will be independently inherited. In his experiments, the gene for round peas didn’t always get inherited with the gene for tall plants, so it’s not the case that all tall plants have round peas.
Whilst these laws form the bedrock of modern genetics, they can be violated and when they are, we can learn something new about the inheritance of different traits and diseases.