Mendels' experiments

Through the selective growing of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain traits show up in offspring plants without any blending of parent characteristics. For instance, the pea flowers are either purple or white: intermediate colors do not appear in the offspring of cross-pollinated pea plants. Mendel observed seven traits that are easily recognised and apparently only occur in one of two forms:

• flower color is purple or white
• flower position is axil or terminal
• stem length is long or short
• seed shape is round or wrinkled
• seed color is yellow or green
• pod shape is inflated or constricted
• pod color is yellow or green

This observation that there are traits that do not show up in offspring plants with intermediate forms was critically important. The leading theory in biology at the time was that inherited traits blend from generation to generation. Mendel picked common garden pea plants for the focus of his research. Garden peas can be grown easily in large numbers and their reproduction can be manipulated. Pea plants have both male and female reproductive organs. As a result, they can either self-pollinate or cross-pollinate with another plant. In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.

In cross-pollinating plants that either produce purple or white flowers exclusively, Mendel found that the first offspring generation (F1) always has purple flowers. Realise that in this experiment both the starting parent plants were homozygous for flower color. However, the following generation (F2) consistently has a 3:1 ratio of purple to white. This 3:1 ratio occurs in later generations as well. This is the key to understanding the basic mechanisms of inheritance.

The experimental results

He came to three important conclusions from these three experimental results:

• The inheritance of each trait is determined by "units" or "factors" (now called genes) that are passed on to descendents unchanged
• An individual inherits one such unit from each parent for each trait
• A trait may not show up in an individual but can still be passed on to the next generation.

With all of the seven pea plant traits that Mendel examined, one form appeared dominant over the other. In other words, it masked the presence of the other allele. For example, when the genotype for flower color is purple / white (heterozygous), the phenotype is purple. However, the dominant purple allele does not alter the recessive white one in any way. Both alleles can be passed on to the next generation unchanged.

The conclusions of his experiments

Mendel's observations from these experiments can be summarised in two principles:

• The principle of segregation
• The principle of independent assortment

According to the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of alleles occurs during the process of sex cell formation (i.e. meiosis ).