Call Us: US - +1 845 478 5244 | UK - +44 20 7193 7850 | AUS - +61 2 8005 4826

The Punnet square protocol

Mendel’s particulate inheritance hypothesis

Mendel’s elegantly simple experiment clearly disproved both the blending inheritance and the inheritance of acquired characteristics hypotheses. He proposed an alternate hypothesis, theparticulate inheritance hypothesis. He predicted that the inherited phenotypes don’t blend from generation to generation. Rather, he suggested that the offspring inherit discrete ‘particles.’ If one of these particles is dominant, the dominant phenotype will be expressed. If both particles are recessive, the organism will express the recessive phenotype.

Alleles, genes and the genotype

Today, we call Mendel’s ‘particles’ alleles. In sexual reproduction, an allele from one parent combines with an allele from a second parent during fertilization. The combination of the two alleles is known as a gene. We now know that a gene is a segment of DNA that codes for a specific mRNA, which in turn codes for a specific protein. There are three possible combinations of genes. Two dominant alleles represent a genotype termed homozygous dominant, and are notated with two capital letters (e.g. AA). Two recessive alleles in a gene are said to be homozygous recessive, notated with two lowercase letters (e.g. aa). A genotype with both a dominant and recessive allele are heterozygous, and are expressed with a capital letter followed by a lower case letter (e.g. Aa).