This lecture went over the basic principles of Mendelian genetics. Mendel was a monk, working in a monastery in what is now the Czech Republic. He did his research between 1856 and 1863, published his results in 1866 and died in 1884. There was little recognition of his work while he was alive although its importance was rediscovered by several researchers around 1900.
Mendel’s essential breakthrough was that he proposed a new model of "particulate" inheritance for traits which replaced an ill-defined "blending" model which had been accepted up to that point. His success was based on choosing a suitable research organism (the garden pea) and analyzing his results in a systematic, quantitative manner.
Mendel studied seven different traits in peas. He used pure-breeding lines which he developed by allowing the peas to self for multiple generations in his studies. Such lines are important in a variety of types of research today. Pure-breeding (inbred) lines of mice are a notable example.
The basic type of cross that Mendel analyzed was a monohybrid cross, which followed inheritance of a single trait at a time. He crossed two different lines (P or parental generation) to make a hybrid (F1) generation. Such crosses are done by removing the pollen-forming organ (anther) from one plant and bringing in pollen from another plant. In his experiments, when Mendel did the reverse (reciprocal) cross (e.g., for the Female A X Male B cross the reciprocal is Female B X Male A) he got the same result. We will discuss cases where reciprocal results are not the same (e.g., sex linkage, cytoplasmic inheritance) later in the course. Mendel then allowed the F1 plants to self, giving rise to an F2 (second hybrid) generation.
The basic result he found for all seven traits was that in the F1 generation, all the peas were the same and like one of the parents (termed the dominant type). The recessive type (other parent) was not expressed in the F1. In the F2 generation the two types were present in a 3:1 dominant: recessive ratio. This reappearance of the recessive type in the F2 generation was surprising and inconsistent with blending inheritance. Mendel explained it on the basis of a "Law of Segregation" under which genes (inheritance factors) can together in the F1 generation but separated (segregated) during gamete formation to form the F2 generation. Alleles refer to alternate forms of a gene. Homozygous refers to an individual carrying two copies of the same allele. Heterozygous refers to an individual carrying two different alleles for a gene. The genotype refers to the genetic makeup of an organism, the phenotype to the observable characteristics of the organism. Testcross refers to a cross to the homozygous recessive type, used to determine if a dominant type individual is homozygous or heterozygous. A backcross is a cross of an F1 to one of the parents (P).
Dihybrid inheritance studies follow the inheritance of two genes at once. The results from these studies gave rise t Mendel’s second law, the "Law of Independent Assortment". This states that when studying the inheritance of two genes at the same time, the two genes are inherited independently of each other. The result is a 9:3:3:1 ratio in the F2 generation. This ratio can be derived with the Punnett square, the "forked line" method, or (best) with probability calculations. The two basic probability rules are the "Addition Rule" and the "Multiplication Rule". Under the addition rule, the probability of one or the other of two mutually exclusive events is the sum of their probabilities. For example, the chance of being of the dominant phenotype in the F2 of a monohybrid cross is 1/4 (AA) + 1/2 (Aa) = 3/4. Under the multiplication rule, the probability of independent events occurring together is the product of their independent probabilities. For example, in a trihybrid cross, the chance of being homozygous recessive in the F2 at all three genes is 1/4 X 1/4 X 1/4 = 1/64.
In humans, Mendelian genetic studies are challenging because of small family sizes and the lack of controlled crosses. Pedigrees, which will be discussed in the next lecture, were developed to record inheritance data from human families.