Exceptions to Mendelian rules
There are many reasons why the ratios of offspring phenotypic classes may depart (or seem to depart) from a normal Mendelian ratio. For instance: • Lethal alleles Many so called dominant mutations are in fact semidominant, the phenotype of the homozygote is more extreme than the phenotype of the heterozygote. For instance the gene T (Danforth's short tail) in mice. The normal allele of this gene is expressed in the embryo. T/+ mice develop a short tail but T/T homozygotes die as early embryos. • Incomplete or semi- dominance Incomplete dominance may lead to a distortion of the apparent ratios or to the creation of unexpected classes of offspring. A human example is Familial Hypercholesterolemia (FH). Here there are three phenotypes: +/+ = normal, +/- = death as young adult, -/- = death in childhood. The gene responsible codes for the liver receptor for cholesterol. The number of receptors is directly related to the number of active genes. If the number of receptors is lowered the level of cholesterol in the blood is elevated and the risk of coronary artery disease is raised. • Codominance If two or more alleles can each be distinguished in the phenotype in the presence of the other they are said to be codominant. An example is seen in the ABO blood group where the A and B alleles are codominant.
A number of hypotheses were suggested to explain heredity, but Gregor Mendel, was the only one who got it more or less right. His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brno (now in the Czech Republic). While Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to humans and other animals because the mechanisms of heredity are essentially the same for all complex life forms. But Mendelian inheritance not common in organelle gene. Through the selective growing of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring plants without any blending of parent characteristics. This concept is reveled during the reappearance of the recessive phenotype in the F2 generation where allele remains particulate during transmission and are neither displaced nor blended in the hybrid to generate the phenotype. Flower color in snapdragons is an example of this pattern. Cross a true- breeding red strain with a truebreeding white strain and the F1 are all pink (heterozygotes). Self-fertilize the F1 and you get an F2 ratio of 1 red: 2 pink: 1 white. This would not happen if true blending had occurred (blending cannot explain traits such as red or white skipping a generation and pink flowers crossed with pink flowers should produce only pink flowers). Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated. Pea plants have both male and female reproductive organs. As a result, they can either selfpollinate themselves or cross-pollinate with another plant.