Mendelian inheritance

Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children, and underly much of genetics. They were initially derived from the work of Gregor Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.

History
The laws of inheritance were derived by Gregor Mendel, a 19th century Austrian monk, who was conducting plant hybridity experiments. Between 1856 and 1863, he cultivated and tested some 28,000 pea plants. His experiments brought forth two generalizations which later became known as Mendel's Laws of Heredity or Mendelian inheritance. These are described in his paper "Experiments on Plant Hybridization" that was read to the Natural History Society of Brno on February 8 and March 8, 1865, and was published in 1866.

Mendel's results were largely neglected. Though they were not completely unknown to biologists of the time, they were not seen as being important. Even Mendel himself did not see their ultimate applicability, and thought they only applied to certain categories of species. In 1900, however, the work was "re-discovered" by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. The exact nature of the "re-discovery" has been somewhat debated: De Vries published first on the subject, and Correns pointed out Mendel's priority after having read De Vries's paper and realizing that he himself did not have priority, and De Vries may not have acknowledged truthfully how much of his knowledge of the laws came from his own work, or came only after reading Mendel's paper. Later scholars have accused Von Tschermak of not truly understanding the results at all.

Regardless, the "re-discovery" made Mendelism an important but controversial theory. Its most vigorous promoter in Europe was William Bateson, who coined the term "genetics", "gene", and "allele" to describe many of its tenets. The model of heredity was highly contested by other biologists because it implied that heredity was discontinuous, in opposition to the apparently continuous variation observable. Many biologists also dismissed the theory because they were not sure it would apply to all species, and there seemed to be very few true Mendelian characters in nature. However later work by biologists and statisticians such as R.A. Fisher showed that if multiple Mendelian factors were involved for individual traits, they could produce the diverse amount of results observed in nature. Thomas Hunt Morgan and his assistants would later integrate the theoretical model of Mendel with the chromosome theory of inheritance, in which the chromosomes of cells were thought to hold the actual hereditary particles, and create what is now known as classical genetics, which was extremely successful and cemented Mendel's place in history.

Mendel's law of segregation


Mendel's law of segregation, also known as Mendel's first law, essentially has four parts.
 * 1) Alternative versions of genes account for variations in inherited characters. This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. Each human has a gene that controls height, but there are variations among these genes in accordance with the specific height the gene "codes" for.
 * 2) For each character, an organism inherits two genes, one from each parent. This means that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same (true-breeding organisms, e.g. ww and rr in Fig. 3), or different (hybrids, e.g. wr in Fig. 3).
 * 3) If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. In other words, the dominant allele is expressed in the phenotype of the organism. However this does not always hold true: Today, we know several examples that disprove this "law", e.g. Mirabilis jalapa, the "Japanese wonder flower" (Fig. 3). This is called incomplete dominance. There is also codominance on a molecular level, e.g. people with sickle cell anemia, when normal and sickle-shaped red blood cells mix and prevent malaria.
 * 4) The two genes for each character segregate during gamete production.  This is the last part of Mendel's generalization. The two alleles of the organism are separated into different gametes, ensuring variation.

During his experiments, Mendel encountered some traits that did not follow the laws he had encountered. These traits did not appear independently, but always together with at least one other trait. Mendel could not explain what happened and chose not to mention it in his work. Today, we know that these traits are linked on the same chromosome.

Mendel's law of independent assortment

 * Also known as Mendel's Second Law

The most important principle of Mendel's law of independent assortment is that the emergence of one trait will not affect the emergence of another. While his experiments with mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with mixing two traits showed 9:3:3:1 ratios (Fig. 2). Mendel concluded that each organism carries two sets of information about its phenotype. If the two sets differ on the same phenotype, one of them dominates the other. That way, information can be passed on through the generations, even if the phenotype is not expressed (F1 generations, figures 1 and 2).'''

Mendel's findings allowed other scientists to simplify the emergence of traits to mathematical probability. A large portion of Mendel's findings can be traced to his choice to start his experiments only with true breeding plants. He also only measured absolute characteristics such as color, shape, and position of the offspring. His data was expressed numerically and subjected to statistical analysis. This method of data reporting and the large sampling size he used gave credibility to his data. He also had the foresight to look through several successive generations of his pea plants and record their variations. Without his careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.