Gregor Mendel lived in an Austrian monastery and tended
the monastery garden. In 1865, through his observations of the garden
pea plants that grew there, Mendel developed three basic principles
that—although ignored at the time by his scientific colleagues—would later
become the foundation for the new science of genetics.
Every pea plant contains both male and female reproductive
parts and will normally reproduce through self-pollination. Mendel
noticed that the self-pollinating pea plants in his garden were
true breeding: they all produced offspring with characteristics
identical to their own. Mendel looked at seven different characteristics,
or traits, that showed up in all of the plants. Each
of these traits had two contrasting natures, only one of which would show
up in a given true-breeding plant. For example, plant height could
be either short or tall: short, true-breeding plants would only
produce short offspring, and tall plants would only produce tall
offspring. At some point, Mendel wondered what would happen
if he manually mated these true-breeding plants with each other—would
a tall plant mated with a short plant produce a tall, medium, or
short offspring? Focusing on only one trait at a time, Mendel cross-pollinated
plants with each of the seven contrasting traits and examined their
offspring. He called the original true-breeding parents the
P (for parental) generation and called their first set of offspring
the F1 (for “first filial,”
from the Latin word filius, meaning son). The F1 offspring
that result from two parents with different characteristics are
also called hybrids.
Law of Dominance
When Mendel crossed a purebred tall plant with a purebred
short plant, all of the offspring in the first generation (the F1 generation)
were tall. The same thing happened with the other pairs of contrasting
traits he studied: hybrid offspring in the first generation always showed
just one of the two forms.
Mendel used the word dominant to describe
the form that dominated the phenotype, or physical appearance, in
the F1 generation. The
other form he called recessive, because the characteristic
receded into the background in the F1 generation.
Mendel was the first to realize that hereditary information for
two different forms of a trait can coexist in a single individual,
with one form masking the expression of the other form. This principle, referred
to as the law of dominance, provided the basis for Mendel’s subsequent
Law of Segregation
Mendel discovered that mating a tall pea and a short pea
would produce an F1 generation
of only tall pea plants. But, he wondered, were these offspring
tall pea plants really identical to their tall parents, or might
they still contain some element of their short parents? To answer
this question, Mendel let all seven types of hybrid F1 generation
plant self-pollinate, producing what he called the F2 (second
Lo and behold, in each F2 generation
some of the recessive forms of the traits—which had visibly disappeared
in the F1 generation—reappeared!
Approximately one fourth of the F2 plants
exhibited the recessive characteristic, and three fourths continued
to exhibit the dominant form of the trait, like their F1 parents.
This 3:1 ratio of dominant to recessive remained consistent
in all of the F2 offspring.
Mendel came up with a simple but revolutionary explanation
for the results he saw in the F2 generation.
He concluded that within an individual, hereditary information came
in paired units, with one unit derived from each parent. Each simple
physical trait, such as stem height, was determined by the combined
action of a single pair of units. Each unit could come in either
a dominant form, which he denoted with a capital letter “A,” or
a recessive form, which he denoted with a lowercase “a.” Two units
with two possible forms gave four possible combinations: AA, Aa,
aA, and aa; since Aa and aA were equivalent, there were really only
three functional combinations. Because “A” is dominant over “a,” both
AA and Aa produced plants with the same physical characteristics.
Only “aa” produced a plant that showed the recessive characteristic.
Mendel realized that the results he saw in the F2 generation
could only be explained if, during the formation of reproductive
cells, paired units are separated at random so that each gamete
contains only one of the two units. This postulate is now known
as the law of segregation.