Gregor Mendel and the Laws of Inheritance

Gregor Mendel was an Austrian monk and scientist who lived from 1822 to 1884.  Much of his research involved producing hybrid pea plants and studying the traits found in the offspring.  While the significance of this research was not realised at the time, its results became the basis of modern genetics.

The basis of his research was cross breeding pea plants with particular traits that only occur in two distinct forms, and observing the traits displayed by the offspring.  The following traits were used in his experiments:

*Stem length: tall or short

*Flower colour: purple or white

*Flower location: axial or terminal

*Seed colour: yellow or green

*Seed shape: smooth or wrinkled

*Pod colour: yellow or green

*Pod shape: inflated or constricted

Mendel crossed purebred pea plants with different forms of a particular trait and discovered that the offspring had one of the two displayed by the parents rather than a new form that was a cross between the two.  For example, crossing a plant producing yellow seeds with one producing green seeds led to offspring with yellow seeds, and not a third colour that was part way between yellow and green.  This went against the common theory at the time which suggested that the offspring would be a blend of the two parental characteristics.

He further experimented by crossing two of these offspring and found that the new offspring produced had a three to one ratio of the forms from the original parents.  In the case of the colour example used above, three quarters of this generation of offspring were yellow and one quarter green.  This showed that traits that do not show up in an individual can be passed on to the next generation.

In each of the seven different traits Mendel experimented with, he discovered that one of the two forms of the trait was “dominant” and could mask the presence of the other “recessive” trait, however the latter could still be passed on to future generations.  He also found that inheriting one trait did not make the pea plant more likely to inherit another.  For example, when crossing tall plants with yellow seeds and short plants with green seeds he found the expected ratios in the two generations of offspring, and was able to produce plants with combinations not found in the parents i.e tall plants with green seeds and short plants with yellow seeds.

Mendel’s work led to two important principles, or laws, which as the basis of the genetics we use today:

*The principle of segregation

Each trait is made up of two units or “alleles”.  During reproduction, only one of these is passed on to each offspring, and which allele is passed on is down to chance.  In the case of the pea plant experiments, the original parents were purebred and their alleles were the same.  This is called homozygous  The offspring however had one allele from each parent and can be described as heterozygous.  When they were bred, it was down to chance whether they passed on a dominant or recessive allele, which led to both of the original forms of the trait being present in their offspring.

*The principle of independent assortment

Pairs of alleles for different traits are passed on to the offspring independently of one another.  This allows the offspring to have combinations of traits not found in either parent- for example the plant may inherit flower colour from the female parents and seed colour from the male parent.

Mendel’s work provided the basis for modern genetics, however we have since discovered there are some exceptions to his rules.  They apply with traits which have only two forms, a dominant and a recessive, and are located either on different chromosomes or far enough apart on the same chromosome to behave as though they are on different ones.  In other cases, the mechanisms for inheritance are more complex and do not follow Mendel’s laws.