Mendelian Genetics: Principles of Heredity
Throughout history, investigators have sought to understand the inheritance of hereditary characteristics. This understanding began with the work of Gregor Mendel.
Life of Mendel
Gregor Mendel was born on July 20, 1822, in the Czech Republic. His father was a veteran of the Napoleonic Wars, and his mother was the daughter of a gardener. After a childhood marked by poverty, he entered the monastery in 1843, where he took the name Gregor and was ordained a priest in 1847. He lived in the Abbey of St. Thomas and was sent to Vienna, where he received his doctorate in mathematics and science in 1851. In 1854, he became an assistant professor. In 1856, he began experiments with peas. He presented his experimental results on plant hybrids in 1866, but they were largely ignored. In 1868, he was appointed abbot of the monastery. His research was rediscovered in 1901, when a monk reached similar conclusions, leading to the recognition of Mendel’s laws. He died on January 6, 1884, without knowing the importance of his discoveries to the world of science.
Mendel’s Investigations
Mendel cross-bred two varieties of peas: one green and one yellow. He covered the stigma of the flowers and introduced pollen from the yellow flower onto the stigma of the green flower, and vice versa. He obtained yellow seeds. When these seeds were allowed to self-fertilize, he observed that 75% were yellow and 25% were green. Later, he studied how two characters are transmitted together, looking at the color and shape of the seeds (smooth and rough). He crossed two pure varieties: one yellow smooth and one green rough. The first generation was all yellow smooth. When he crossed these, he found 9 yellow smooth, 3 yellow rough, 3 green smooth, and 1 green rough.
Mendel’s Laws
- Law of Uniformity: When crossing two pure strains that differ in one character, the first generation offspring will uniformly express the dominant character.
- Law of Segregation: In the second generation, recessive traits that were not expressed in the first generation reappear in the ratio of 3 dominant to 1 recessive.
- Law of Independent Assortment: For two or more characters, each character is transmitted to the next generation independently of any other character, leading to all potential combinations of these characters.
Inheritance of Blood Groups
There are four blood groups:
- A: A agglutinogen and beta agglutinins in plasma
- B: B agglutinogen and alpha agglutinins in plasma
- AB: A and B agglutinogens, no agglutinins in plasma (universal recipient)
- O: No agglutinogen, alpha and beta agglutinins in plasma (universal donor)
Rh Factor
The Rh factor is an agglutinogen found in rhesus macaque blood and also present in some humans. Individuals either have this factor (Rh-positive) or do not (Rh-negative).
Genotypes:
- Group A: IAIA or IAi (if recessive)
- Group B: IBIB or IBi (if recessive)
- Group AB: IAIB
- Group O: ii
Intermediate Inheritance
In Mendel’s studies, there was absolute dominance of one allele over the other, so the heterozygous phenotype expressed the dominant character. However, in some cases, intermediate inheritance occurs, where the heterozygote expresses an intermediate phenotype between the two parents.