During reproduction, the inherited factors (now called alleles) that determine traits are separated into reproductive cells by a process called meiosis and randomly reunite during fertilisation.
Mendel proposed that, during reproduction, the inherited factors must separate into reproductive cells. He had observed that allowing hybrid pea plants to self-pollinate resulted in progeny that looked different from their parents. Separation occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants).
3. Principle of independent assortment
Genes located on different chromosomes will be inherited independently of each other.
Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment. For example, he cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).
Inheritance of multiple traits in peas
Mendel cross-bred plants with 2 or more traits and found that each trait was inherited independently of the other and produced its own 3:1 ratio. For example, a plant with round, yellow seeds crossed with a plant with wrinkled green seeds gives a ratio of 9:3:3:1. This is the basis for Mendel’s principle of independent assortment.