Gregor Mendel, a pioneer in the study of heredity, made
history by carefully selecting traits in pea plants that were clearly different
from each other. He focused on seven pairs of contrasting characteristics—such
as tall vs. short plants—because they were easy to identify and consistently
passed from one generation to the next.
These traits came from what are known as true-breeding
varieties—plants that produced offspring identical to themselves,
generation after generation.
Studying One Trait at a Time
Establishing Pure Tall Plants
Mendel first worked with a variety of pea plants that grew
tall, nearly one meter in height. By allowing these tall plants to
self-pollinate over many generations, he produced a line of pure tall plants.
Each generation consistently grew tall, showing no variation.
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| Mendel’s seven pairs of contrasting traits in garden peas |
Establishing Pure Dwarf (Short) Plants
At the same time, he did the same with short plants. By
repeatedly crossing them with other short plants, Mendel created a line of pure
dwarf plants—all offspring remained short in height.
What Happened When Tall and Short
Plants Were Crossed?
Mendel then took a bold step: he crossed pure tall plants
with pure short plants. The result was unexpected—all the offspring in this
first generation (called the F₁ generation) were tall. There wasn’t a
single short plant.
But the real surprise came in the second generation (F₂).
When he allowed the F₁ tall plants to self-pollinate, he grew 1,064 new
plants. Out of these, 787 were tall and 277 were short—a ratio close
to 3 tall plants for every 1 short plant.
This ratio puzzled Mendel. If both parents appeared tall,
how did the short trait come back? This mystery led him to a deeper
understanding of how traits are inherited.
Mendel’s Key
Insight: Traits Come in Pairs
Through careful observation and calculation, Mendel
concluded that every trait is controlled by two units, one inherited
from each parent. He called these units factors—what we now call genes.
- When
both inherited units are the same, the plant is said to be homozygous.
- When
the two units are different, the plant is heterozygous.
For example:
- A
plant with two tall genes (one from each parent) is homozygous
tall.
- A
plant with one tall gene and one short gene is heterozygous—but
still grows tall because the tall trait is dominant.
In this case:
- The
dominant trait is the one that shows up in the plant (tall).
- The
recessive trait is the one that’s hidden unless both units carry it
(short).
How Traits Are Written in Genetics
Genetic traits are represented by letters:
- A
capital letter (T) shows a dominant gene (tall).
- A
lowercase letter (t) shows a recessive gene (short).
So:
- TT
= Homozygous tall (tall from both parents)
- Tt
= Heterozygous tall (tall from one parent, short from the other)
- tt
= Homozygous short (short from both parents)
Even though Mendel didn’t use modern genetic terms, his work
laid the foundation for them. The word gene was first used by Wilhelm
Johannsen in 1909, and the term genetics was introduced by William
Bateson in 1912. The different forms of a gene—like T and t—are called alleles.
Genotype vs. Phenotype: What You See
vs. What’s Inside
- The
phenotype is the visible trait—such as whether the plant is tall or
short.
- The
genotype is the actual pair of genes the plant carries—like TT, Tt,
or tt.
Even if two plants look the same (both tall), their
genotypes might be different. One could be TT, while the other is Tt.
Generational Labels in Mendel’s Experiments
Mendel used specific symbols to keep track of generations:
- P
(Parental Generation): The original pair of pure tall
and pure short plants.
- F₁
(First Filial Generation): The first set
of offspring—all of which were tall.
- F₂
(Second Filial Generation): The next
generation, which showed the 3:1 ratio of tall to short plants.
The word "filial" comes from Latin and
simply means "offspring."
Final Thoughts
Mendel’s simple yet powerful experiments with pea plants
revealed that inheritance follows clear and predictable patterns. By focusing
on a single trait at a time and observing how it was passed from parents to
offspring, he discovered principles that still guide genetic science today.
His legacy lives on in every genetics classroom, lab, and
discovery—and it all began with a few rows of carefully grown peas.
