Mendel to Molecules
Common field peas, Pisum sativum, were the focal plant species studied by Gregor Mendel (1822-1884), the father of modern genetics. He documented the presence and absence of different physical traits in over 29,000 pea plants spanning many generations. The traits he focused on were flower color, seed shape, pod shape, pod color, flower position, stem length, embryo color.
Prior to Mendel, many thought that most traits of offspring must be a blended version of parental traits. Mendel showed that this is not generally the case and that offspring traits are determined by the combination of heritable units passed on by parents. We now call these heritable units for a particular characteristic, alleles.
In Pisum sativum, the characteristics that Mendel examined are each controlled by just one set of alleles. These alleles can combine in a homozygous (two copies of the same allele) or heterozygous (one copy of two different alleles) fashion depending on parental alleles. For example: The Yellow and Green pea trait is controlled by the alleles called Y and y. The Round and Wrinkled pea trait is controlled by the alleles called R and r. Alleles are, in a sense, different versions of a single characteristic.
Mendel suggested that inheritance of traits followed specific rules or laws:
The Law of Segregation
Each trait is linked to a pair of alleles that are passed down, one from each parent. A characteristic that is exhibited by the offspring will depend on which version of the characteristic is dominant and whether or not the offspring received one of these dominant alleles from one of its parents.
The Law of Independent Assortment.
The inheritance of one characteristic is independent from the inheritance of another characteristic. For example, whether or not a pea plant has yellow or green seeds is independent of whether or not the pea plant has white or purple flowers. In other words, the presence or absence of a particular version of one characteristic (e.g. seed color) is not related to the presence or absence of a particular version of a different characteristic (e.g. flower color).
DNA, Central Dogma and the R Allele
DNA is made up of long strings of nucleotides. There are four nucleotides in DNA: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). These nucleotides are held together by a phosphate-sugar “backbone” and are complemented by a parallel strand of nucleotides making a DNA molecule “double-stranded”. Parallel strands are held together by hydrogen bonds between bases; Adenine binds to Thymine, Cytosine binds to Guanine. Discrete sections of DNA carry sequences of nucleotides that, when processed, are responsible for proteins or functional RNA chains. These sections are called genes. When making a protein, the DNA is first transcribed into a single-stranded piece of messenger RNA (mRNA) using an enzyme called RNA polymerase. The newly made mRNA is then translated through a ribosome into a long chain of amino acids. This chain of amino acids folds up to make a functional protein. Proteins build the organism, including all measurable and observable characteristics.
Amazingly, Mendel lived decades before the advent of microbiology and the discovery of genes, DNA, or chromosomes. He discovered the laws of inheritance without any basic understanding of the molecular basis of inheritance. We now know the characteristics that Mendel described correspond to specific genes within the pea plant genome. The “R” allele is a specific allele of the SBE1 gene that codes for a functional starch branching enzyme. This enzyme converts amylose starch to amylopectin starch within a pea seed. The “r” allele is a different variant of the SBE1 gene that codes for a non-functioning starch branching enzyme. Without a functional starch branching enzyme, amylopectin starch cannot be formed.