Gregor Johann Mendel (1822–1884): Founder of Modern Genetics
As a
young man, Mendel spent his time doing genetics research and teaching math,
physics, and Greek to high school children in Brno (now in the Czech Republic).
Mendel studied the inheritance of vari-ous traits of the common garden pea,
Pisum sativum, because he was able to raise two generations a year. He studied
many differ-ent physical traits of the pea, such as flower color, flower
position, seed color and shape, and pod color and shape. Mendel grew differ-ent
plants next to each other, looking for traits that mixed together. Luckily, the
traits he studied were each due to a single gene that was either dominant or
recessive, although he did not know this at the time. Consequently, he never
saw them “mix.” For example, when he grew yellow peas next to green peas, the
offspring looked exactly like their parents. This showed that traits do not
blend in the offspring, which was a common theory at the time.
Next
Mendel moved pollen from one plant to another with different traits. He counted
the number of offspring that inherited each trait and found that they were
inherited in specific ratios. For example, when he cross-pollinated the yellow
and green pea plants, their offspring, the F1 generation, was all yellow. Thus
the yellow trait must dominate or mask the green trait. He then let the F 1
plants produce offspring, and grew all of the seeds. These, the F 2 generation,
segregated into 3/4 yellow and 1/4 green. When green seeds reappeared after
skipping a generation, Mendel concluded that a “factor” for the trait—what we
call a gene nowadays—must have been present in the parent, even though the
trait was not actually displayed.
Mendel demonstrated many principles that form the
basis of modern genetics. First, units or factors (now called genes) for each
trait are passed on to successive generations. Each parent has two copies of
each gene but contributes only one copy of the gene to each offspring. This is
called the principle of segregation. Second, the principle of independent
assortment states that different off- spring from the same parents can get
separate sets of genes. The same phenotype (the observable physical traits) can
be represented by different genotypes (combinations of genes). In other words,
although a gene is present, the corresponding trait may not be seen in each
generation. When Mendel began these experiments, he used purebred pea
plants—that is, each trait always appeared the same in each generation. So when
he first crossed a yellow pea with a green pea, each parent had two identical
copies or alleles of each gene. The green pea had two green alleles, and the
yellow pea had two yellow alleles. Consequently, each F 1 offspring received one yellow allele and one
green allele. Despite this, the F1 plants all had yellow peas. Thus yellow is
dominant to green. Finally when the F1 generation was self-pollinated, the F2
plants included some that inherited two recessive green alleles and had a green
phenotype (Fig. A) .
Mendel published these results, but no one
recognized the significance of his research until after his death. Later in
life he became the abbot of a monastery and did not pursue his genetics
research.
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