is through the process of mutation that there exists more than one version of a particular
Once we have different version of the same gene (alleles) we can begin to follow these
alleles through generations to detect the patterns of inheritance.
Gregor Mendel is considered to be the Father of Genetics and he formulated the first two
laws of inheritance. These two laws are often referred to as Mendel’s laws.
In Mendel’s first law arose from his theory Mendel said something along the lines of “I
think sexually reproducing organisms are diploid (have two versions of each gene), but
they only give one version to their offspring. His experiment strongly supported this
theory and it became known (many years later) as Mendel’s First Law: Segregation of
In Mendel’s time scientists did not yet have microscopes powerful enough to allow us to
see chromosomes, and back then it was not even know that DNA is the hereditary
material. Mendel came up with his Laws by doing genetic experiments with garden peas.
Let’s look at his experiments. We’ll need some more terms.
Genotype is a term that refers to the specific alleles of a particular gene that are present
in an individual.
Phenotype is a term we use to refer to an observable trait or character of an organism.
The phenotype is determined by the genotype (but is also influenced by the environment).
Why? It is because:
• Genes are specific regions of the DNA that encode the information to make
proteins. Usually one gene encodes one protein.
• Proteins are essential to the individual cells of the organism and are essential to
• The proteins determine the appearance and functions of the organism.
• Genes encode the proteins and the proteins determine the specific characteristics
of the organism.
Before he began his experiments, Mendel created sets of true breeding lines. This was
If two true breeding individuals, with the same phenotype are mated, their offspring
will have the same phenotype (for the trait being studied) as the parents; i.e. they breed
true to their form.
Mendel first did a monohybrid cross.
A Monohybrid cross is one in which the inheritance for one genetic locus is being
studied, and the starting parents have different genotypes for the one locus being studied.