Key principles of genetics were developed from Mendel’s studies on peas.
1. Fundamental theory of heredity
Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring.
Mendel found that paired pea traits were either dominant or recessive. When pure-bred parent plants were cross-bred, dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number of second-generation (F2) progeny with dominant or recessive traits and found a 3:1 ratio of dominant to recessive traits. He concluded that traits were not blended but remained distinct in subsequent generations, which was contrary to scientific opinion at the time.
Mendel didn’t know about genes or discover genes, but he did speculate that there were 2 factors for each basic trait and that 1 factor was inherited from each parent.
We now know that Mendel’s inheritance factors are genes, or more specifically alleles – different variants of the same gene. In today’s genetic language, a pure-breeding pea plant line is a homozygote – it has 2 identical copies of the same allele. An F1 cross-bred pea plant is a heterozygote – it has 2 different alleles.
Inheritance of a single trait in peas
Mendel followed the inheritance of 7 pea traits. Dominant traits, like round peas, appeared in the first-generation hybrids (F1), whereas recessive traits, like wrinkled peas, were masked. However, recessive traits reappeared in the second generation (F2). Each individual carries a pair of factors for each trait, and they separate from each other during fertilisation. This is the basis of Mendel’s principle of segregation.