It is an established fact that genes are transmitted from parents to the offspring and from generation to generation. Transmission genetics is the study of the patterns of inheritance from generation to generation . This study is often called as Mendelian inheritance for eukaryotic organisms. It is because the principles behind the study were first deduced and proposed by an Augustine monk, Johann Gregor Mendel. The inheritance of characteristics in garden peas (Pisum sativum) is studied by Mendel. He acquired the experience of growing pea plants during childhood as his parents were farmer.
The reasons he chose the garden peas are:
- The peas can be easily obtained from his monastery garden.
- The peas can be grown easily.
- The peas have a short generation time.
- The characteristics for peas can be easily observed and compared.
- The crossing for the peas can be controlled: can either self-pollinated or cross-pollinated.
Mendel studied seven characteristics of the pea plant, that is, seed shape, seed colour, flower colour, pod shape, pod colour, flower and pod position and stem length.
Mendel monohybrid cross and the law of segregation
Monohybrid cross was the earliest experiment carried out by Mendel. The particular characteristic that he used in this experiment is the stem length, where he cross two pure-breeding plants with alternative expressions, that is a tall plant and a dwarf plant. Mendel crossed a tall plant (the 'male' parent) and a dwarf plant (the 'female' plant) by transferring the pollen to the stigma of the plant. He noticed that all progenies for the first generation, F1 were tall plants. Two plants from this generation were crossed, which is known as selfing. The progenies in the second generation, F2 were a mixture of tall plant and dwarf plant. The approximate ratio for tall plants to dwarf plants is 3:1. The law of segregation can be defined as: For a diploid organism, the characteristics are determined by a pair of alleles (which Mendel called as factors). The pair of hereditary determinants, that is genes, segregate in a way that the gamete will have an equal chance to contain either allele of the pair in the formation of gametes. 
Mendel dihybrid cross and the law of independent assortment
The characteristics Mendel studied in dihybrid cross were seed shape and seed colour. He crossed the pure-breeding round seeds, yellow seed colour with pure-breeding wrinkled seeds, green seed colour. All the offspring in F1 generation were round seeds, yellow seed colour. Then, he self-crossed among the F1 generation and he found that the phenotypes and ratios produced in the F2 generation were 9 round seeds with yellow seed colour, 3 round seed with green seed colour, 3 wrinkled seeds with yellow seed colour and 1 wrinkled seeds with green seed colour. The law of independent assortment can be defined as: For a particular gene, each of a pair of allele can segragate independently with the segragation of the other pair of allele in the formation of reproductive cells. 
- ↑ 1.0 1.1 1.2 Daniel L. Hartl, 2012. Essential Genetics: A Genomics Perspective. Sixth Edition. Burlington: Jones Bartlett Learning.
- ↑ Daniel L. Hartl, 2011. Genetics: Analysis of Genes and Genomes. Eighth Edition. Burlington: Jones Bartlett Learning.