Once upon a time, in the mid-19th century, there was a friar named Gregor Mendel who lived in what is now the Czech Republic. Mendel had various interests such as beekeeping, gardening, and pursuing his religious vocation as a friar. However, he had a deep passion for science. It was this love for scientific exploration that led Mendel to make groundbreaking discoveries in the field of genetics.
Mendel’s journey into the world of genetics began when he noticed an interesting pattern among the flowers in his pea plant garden. Most of the flowers were purple, but occasionally, he would come across a plant that produced white flowers. Being born into a family of farmers, Mendel decided to experiment with breeding the purple-flowered plants together and separately breeding the white-flowered plants. After several generations, he observed that the plants were consistently producing flowers of only one color.
Before his death in 1884, Mendel conducted a series of experiments in which he crossed purple-flowered and white-flowered plants. However, it wasn’t until the early 1900s that a British geneticist named Reginald Punnett, one of Mendel’s successors, realized the need for a tool to track and analyze the different combinations of plant traits that led to specific flower colors. This marked the birth of the Punnett square.
The Punnett square is a diagram that helps determine all the possible outcomes of crossing plants with different traits. Mendel referred to the different forms or versions of a gene that can be passed from parent to offspring as “factors.” Today, we use the term “alleles” to describe these variations. Mendel observed that purple flowers were dominant traits, meaning they would always be expressed, while white flowers were recessive traits, only visible when paired with another recessive allele. In his pea plants, each organism had two alleles for flower color, one inherited from each parent. If the alleles were the same, the organism was homozygous, and if they were different, the organism was heterozygous. The collection of genetic instructions is referred to as the genotype, while the observable traits, such as the color of the pea flower, are known as the phenotype.
To construct a Punnett square, one simply draws a square and divides it into four quadrants. The top of the square represents the possible alleles from the mother, while the left side represents the alleles contributed by the father. Let’s label one version as “M” and the other as “m.” If both parents are heterozygous, carrying both alleles for the gene determining flower color, the Punnett square can display the potential genotypes and phenotypes of the offspring based on the presence of both alleles (M and m).
Reginald Punnett, in 1905, wrote “Mendelism,” considered the first genetics textbook. He later co-founded the Journal of Genetics in 1910, solidifying his place in the history of genetic research.