Mendelian inheritance

Mendelian inheritance is a set of rules about genetic inheritance.
The basic rules of genetics were first discovered by a monk named Gregor Mendel in the 1850s, and published in 1866. For thousands of years, people had noticed how traits are inherited from parents to their children. However, Mendel's work was different because he did experiments on plants, and designed those experiments very carefully.
In his experiments, Mendel studied how traits were passed on in pea plants. He started his crosses with plants that bred true, and counted characters that were either/or in nature (either tall or short). He bred large numbers of plants, and expressed his results numerically. He used test crosses to reveal the presence and proportion of recessive characters.
Mendelian genetics
Limitations
Mendel's laws apply widely, but not to all living things. They apply to any organism which is diploid (has two paired sets of chromosomes) and which engages in sexual reproduction. They would not apply to bacteria, for example, or to asexual reproduction. They do apply to the great majority of plants and animals.
Mendel's laws
Mendel explained the results of his experiment using two scientific laws:
1. Factors, later called genes, normally occur in pairs in ordinary body cells, yet separate during the formation of sex cells. This happens in meiosis, the production of gametes. Of each pair of chromosomes, a gamete only gets one. The factors (genes) determine the organism's traits, and are inherited from its parents. As the pair of chromosomes separate, each gamete only receives one of each factor. This Mendel called the Law of segregation.
Mendel also noted that versions of a gene could be either dominant or recessive. We call those different versions alleles.
2. Alleles of different genes separate independently of one another when gametes are formed. This he called the Law of independent assortment. So Mendel thought that different traits are inherited independently of one another.
The second law is only true if the genes are not on the same chromosome. If they are, then they are linked to each other. This was the next great discovery after Mendel: that genes were carried on chromosomes. The closer they were on the chromosomes, the less likely was crossing over between them.
Mendel's laws explained the results he got with his pea plants. Later, geneticists discovered that his laws were also true for other living things, even humans. Mendel's findings from his work on the garden pea plants helped to establish the field of genetics. His contributions were not limited to the basic rules that he discovered. Mendel's care towards controlling experiment conditions along with his attention to his numerical results set a standard for future experiments.
Consequences
When the chromosome pairs are separated in a gamete, they are randomly segregated. A gamete might have any proportion from 100% maternally derived to 100% paternally derived chromosomes.
In crossing-over, sections are exchanged between pairs of chromosomes during meiosis. This increases the number of genetically different individuals in a population, which is important in evolution.
As a consequence of 1 & 2, except for identical twins, no two siblings have identical genetics.
Diagammatic examples
The ratios in the diagrams below are statistical predictions. In a large number of crosses, the numbers of offspring with these features will approximate to the ratios given.
References
Classical genetics