7.3 MENDELIAN INHERITANCE

The field of genetics began in 1900 with the rediscovery of a paper published in 1866 by Gregor Johann Mendel, an Augustinian monk. Mendel was the first person who successfully explained mode of inheritance of traits from one generation to the next through his experiments on pea plants. The mode of inheritance according to the principle proposed by Mendel is known as Mendelian inheritance. Mendel's work laid the foundation for the science of inheritance, which is why he has been titled as the "Father of Genetics".

First, he developed true- breeding plants, which consistently produced offspring with the same traits after self- fertilization. This was achieved by repeated self- fertilization over successive generations. Mendel worked with seven pairs of contrasting traits, creating true- breeding plants for each.

Mendel then carried out hybridization, where he cross- fertilized plants with different traits. For example, he crossed a round- seed plant with a wrinkled- seed plant to study a single trait (monohybrid cross) and a round, yellow- seed plant with a wrinkled, green- seed plant to study two traits (dihybrid cross).

A monohybrid cross involves parents differing in one trait, while a dihybrid cross involves parents differing in two traits. Mendel ensured specific matings of different pea plants by removing the stamen of one plant and transferring pollen using a paintbrush.

By analyzing the inheritance of one, two, or three traits at a time, Mendel formulated what are now known as Mendel's laws of inheritance.

7.3.1 Inheritance of Single Trait by Monohybrid Cross

Mendel's study of inheritance for a single trait, known as a monohybrid cross, involved crossing two plants that differed in one trait, such as seed shape.

1. Mende1's Procedure and observations He crossed a true- breeding round- seed plant with a true- breeding wrinkled- seed plant. In the first filial generation $(F_{1})$ all offspring displayed the round seed trait (he designated this phenotype as dominant while other as recessive). However, when these $F_{1}$ plants self- fertilized, their offspring $(F_{2})$ exhibited both round and wrinkled seeds in a 3:1 ratio. Further investigation revealed that $25\%$ $F_{2}$ generation were true breed round, $50\%$ were non- true breed round and $25\%$ were true breed wrinkled. So the genotypic ratio in $F_{2}$ generation was 1:2:1. Similar results were obtained when he studied other pairs of contrasting traits. Conclusion/Interpretation of the results Mende1 concluded that: (i) The traits are controlled by factors or elements (now called genes) passed from parents to offspring through gametes. (ii) Each plant carries two factors (alleles) for each trait- one from each parent. (iii) Dominant alleles, like "R" for round seeds, are expressed, while recessive alleles, like "r" for wrinkled seeds, are not expressed in $F_{1}$ but reappear in $F_{2}$ generation. (iv) During gamete formation both alleles of a gene pair segregate from each other and pass into different gamete. (v) The gene pair is restored when gametes are fertilized to make the next generation.

Law of segregation