ABSTRACT
Although it has been recognized for thousands of years that traits are passed from
organisms to their offspring, the science of genetics began with the experiments of
Gregor Mendel. By crossing various strains of peas, Mendel was able to deduce
several principles of genetics. The most basic is unit inheritance, which states that
inheritance is determined by discrete quantities known as genes and genes don’t mix.
For example, when Mendel crossed round peas (i.e. peas with a smooth surface)
and wrinkled peas, all of the offspring were round, the offspring were not “kind of
wrinkled.” In this example, we think there is some gene which controls if the pea
is wrinkled or not, and the gene takes 2 values; round and wrinkled. We call the
different values taken by the gene alleles. Peas (and humans) have 2 copies of all
genes, one from each parent. The set of 2 alleles of a gene for an individual is known
as the genotype of the individual. In contrast, an observable feature of an organism
is referred to as the phenotype of the individual. For example, if a pea plant has the
allele for round (denoted R) and wrinkled (denoted r) at the pea shape gene, the
genotype is Rr (or rR, the order of the symbols is irrelevant). If a pea plant has this
genotype, it will appear round, hence we say it has the round phenotype.Another way
to think about the relationship between phenotype and genotype is that genotypes
help determine the phenotype (in conjunction with environmental effects). The term
genotype is also used to refer to the alleles an individual has at more than one gene. If
we also know which pairs of alleles were inherited from which parent, then we know
the individual’s haplotype. For example, suppose there are 2 genes, each denoted by
a distinct letter, and each with 2 alleles, which we will denote Aa and Bb. If someone
has the genotype A, A and B, b then we know one parent must have had the pair of
alleles A, B and the other parent must have had the alleles A, b, hence in this case
we know haplotype of this individual. In contrast, if the genotype had been A, a and
B, b then the parents could have 2 possible configurations for their alleles: A, B and
a, b or A, b and a, B. In the latter case we cannot deduce the haplotype from the
genotype.
