In the mid-19th century, Mendel conducted cross-breeding experiments using
pea plants in a monastery garden in the Austrian city of Brünn (today Brno).
Through statistical analysis of his observation findings on the separation of traits, he
elucidated a mode of inheritance rooted in the concept of the gene. The pea plants
that Mendel used for his experiments (Pisum sativum) self-fertilized through a
process whereby the pistil received flower from a stamen within the same flower. It
was therefore possible to obtain genetically fixed organisms over several generations of breeding with almost no natural crossing, thereby maintaining a pure
lineage. Peas were also easy to cultivate and had a short growth period, bearing fruit
easily and proving simple to hybridize artificially.
Moreover, the seven genetic traits that Mendel focused on in his research were
controlled by simple genes on different chromosomes. This resulted in clear genetic
separation in later breeding generations, a fact that proved most suitable for the
discovery of genetic laws. In the process, he refuted the hitherto accepted notion of
blending inheritance and proved that the traits were governed by certain factors
(genes). He used the term “dominant” to refer to traits found in the first generation
of cross-breeding and “recessive” for traits that did not appear. This phenomenon
became known as the “law of dominance,” whereby dominant traits alone appear in
the first hybridized generation.
Mendel used pea plants to conduct his research on modes of inheritance.
Cross-breeding of a pair bearing conflicting traits, such as a plant bearing round
seeds on one bearing wrinkled seeds, would have a first generation (F 1 ) consisting
solely of round seeds. Those seeds were then sown and raised on their own, and the
7324 second-generation (F 2 ) hybridized seeds obtained through self-fertilization
(the pistil’s receipt of pollen from the same flower’s stamen) were examined.
A total of 5474 seeds were found to be round and 1850 wrinkled, giving a ratio of
2.96 to one. Cross-breeding experiments between plants with yellow and green
cotyledons resulted in an F 1 consisting entirely of yellow cotyledons, while the ratio
among the 8023 F 2 plants was 6022 yellow to 2001 green, or 3.01 to one (see
Fig. 2.1). The cross-breeding experiments for the seven traits thus showed that the
F 2 generation always showed parental traits at a ratio of roughly 3:1 (Bateson and
Mendel 2013; Mendel et al. 1993; Orel 1996).
From these experimental findings, Mendel found that only one parental trait
appeared in the first hybridized generation, while the other trait did not appear at all.
He referred to the trait that did appear as dominant and the trait that did not appear
as recessive. He also hypothesized a “genetic factor” that was transmitted for
parents to offspring to cause the trait’s appearance, which he indicated by means of
a symbol. For instance, he would refer to the dominant of two conflicting traits with
a capital “A” and the recessive genetic factor (hereafter “gene”) with a lower-case
“a.”
As shown in Fig. 2.2, parental somatic cells contain two genes for each trait.
When a male or female gamete such as pollen of an ovum is produced, however,
these separate and are each distributed as a distinct gamete. The fertilized egg
resulting from the combination of male and female gametes—the offpsring’s
somatic cell, in other words—contain a set of two genes, or one each from the
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2 Introduction to Molecular Biology
pea plants in a monastery garden in the Austrian city of Brünn (today Brno).
Through statistical analysis of his observation findings on the separation of traits, he
elucidated a mode of inheritance rooted in the concept of the gene. The pea plants
that Mendel used for his experiments (Pisum sativum) self-fertilized through a
process whereby the pistil received flower from a stamen within the same flower. It
was therefore possible to obtain genetically fixed organisms over several generations of breeding with almost no natural crossing, thereby maintaining a pure
lineage. Peas were also easy to cultivate and had a short growth period, bearing fruit
easily and proving simple to hybridize artificially.
Moreover, the seven genetic traits that Mendel focused on in his research were
controlled by simple genes on different chromosomes. This resulted in clear genetic
separation in later breeding generations, a fact that proved most suitable for the
discovery of genetic laws. In the process, he refuted the hitherto accepted notion of
blending inheritance and proved that the traits were governed by certain factors
(genes). He used the term “dominant” to refer to traits found in the first generation
of cross-breeding and “recessive” for traits that did not appear. This phenomenon
became known as the “law of dominance,” whereby dominant traits alone appear in
the first hybridized generation.
Mendel used pea plants to conduct his research on modes of inheritance.
Cross-breeding of a pair bearing conflicting traits, such as a plant bearing round
seeds on one bearing wrinkled seeds, would have a first generation (F 1 ) consisting
solely of round seeds. Those seeds were then sown and raised on their own, and the
7324 second-generation (F 2 ) hybridized seeds obtained through self-fertilization
(the pistil’s receipt of pollen from the same flower’s stamen) were examined.
A total of 5474 seeds were found to be round and 1850 wrinkled, giving a ratio of
2.96 to one. Cross-breeding experiments between plants with yellow and green
cotyledons resulted in an F 1 consisting entirely of yellow cotyledons, while the ratio
among the 8023 F 2 plants was 6022 yellow to 2001 green, or 3.01 to one (see
Fig. 2.1). The cross-breeding experiments for the seven traits thus showed that the
F 2 generation always showed parental traits at a ratio of roughly 3:1 (Bateson and
Mendel 2013; Mendel et al. 1993; Orel 1996).
From these experimental findings, Mendel found that only one parental trait
appeared in the first hybridized generation, while the other trait did not appear at all.
He referred to the trait that did appear as dominant and the trait that did not appear
as recessive. He also hypothesized a “genetic factor” that was transmitted for
parents to offspring to cause the trait’s appearance, which he indicated by means of
a symbol. For instance, he would refer to the dominant of two conflicting traits with
a capital “A” and the recessive genetic factor (hereafter “gene”) with a lower-case
“a.”
As shown in Fig. 2.2, parental somatic cells contain two genes for each trait.
When a male or female gamete such as pollen of an ovum is produced, however,
these separate and are each distributed as a distinct gamete. The fertilized egg
resulting from the combination of male and female gametes—the offpsring’s
somatic cell, in other words—contain a set of two genes, or one each from the
24
2 Introduction to Molecular Biology