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Gregor Mendel and his pea plants are the bane of many a
middle school science student learning genetics for the
first time.
But Mendel himself may have been one of the luckiest
people ever to put pen to paper in the name of science.
Not to say he didn’t work incredibly hard for many years to
make the discoveries he did.
It’s just that his chosen organism, the common pea, was about as good
as it gets for research on genetic traits.
And as a result, Mendel and his lucky peas helped build
the field of genetics as we know it.
But if he’d chosen other pea traits to focus on, or another
model organism altogether, he would have had his work
cut out for him, because it turns out those laws have more
exceptions than not.
Mendel’s work was a huge step in explaining how certain
traits are passed from one generation onto the next.
Things like blood group or eye color.
Though when Mendel started this work way back in the
1850s, he wasn’t actually looking at human traits, but
traits of the common pea, Pisum sativum.
He selectively bred pea plants and showed how a
dominant characteristic can be passed down and remain
visible across generations.
On the other hand, recessive traits may seemingly skip
generations, but pop up in the future.
The recessive trait is still being passed along over
generations, but they’re typically overshadowed by
dominant traits, so they don’t show up as observable
traits, or phenotypes.
His work centered around seven observable traits in pea
plants, like plant height, seed shape, pod shape, and pea
color.
And most of these traits had two forms, like smooth
or wrinkled shaped seeds.
That simplicity is important – introductory textbooks even
today might present traits as having one dominant and one
recessive form.
But remember that eye color and blood groups come in more than two flavors.
These traits and their binary level of simplicity allowed
Mendel to work out the Law of Segregation: that each
parent organism passes down a single discrete unit for a
trait, which we now call a gene, to its offspring.
His pea breeding also led him to come up with the Law of
Independent Assortment: That traits are passed down
independently and don’t influence one another’s inheritance.
So whether or not a seed was smooth or wrinkled didn’t influence if it was also
yellow or green.
Together, these made up Mendel’s Principles of
Inheritance.
Of course, Mendel’s work on these pea traits happened
long before we even knew what genes actually were, so
kind of a big deal.
His findings basically let us understand that genes retain
their identity across generations, and don’t fuse or blend
together over time!
As in, a pea with smooth seeds can have offspring with
wrinkled seeds because the wrinkled gene is still in there
to be passed along for a future generation to express.
But he actually got super, super lucky with his peas,
because those rules have a lot of exceptions.
It wasn’t until the early 1900s that his ideas got some
traction, and people started conducting experiments to
flesh them out further.
The geneticists William Bateson, Edith Rebecca Saunders,
and Reginald C. Punnett found that not all of their pea
plant crosses followed Mendelian rules.
Some traits appeared more frequently than Mendel’s ratios
would predict.
Which would only make sense if they were tagging along with other traits – if they
weren’t sorting independently.
As a result, the group concluded that some traits were
likely coupled together, although they weren’t totally clear
on how.
And other researchers filled in the gaps, including Thomas
Hunt Morgan.
He used fruit flies to study genes that broke Mendel’s rules
of independent assortment.
In his experiments, he bred fruit flies with white eyes,
instead of the normal red.
But strangely, he didn’t breed any white-eyed females,
only males.
This made him wonder if the eye color trait was only being
passed down by males.
His contemporaries had recently worked out that sex in
flies was determined by the inheritance of a specific
chromosome.
Female flies get two X chromosomes while males get one.
So he explored the idea that not only was the white eye
color recessive, it was linked to the X chromosome.
That’s why it wasn’t showing up in the first generations of
females he was breeding, since they were all still getting a
dominant red-eyed gene from their maternal side.
That masked any white-eyed genes being passed along.
It wasn’t until Morgan crossed white-eyed males with a
previous generation of their own female offspring that
white-eyed females finally popped up.
That’s thanks to their set of both paternal and maternal white
eye color-linked X chromosomes.
Morgan’s work provided evidence for the chromosome
theory – the idea that genes travel in groups on
chromosomes.
This experiment shows that traits unrelated to sex can still be linked to sex chromosomes.
But it also neatly explains why traits can travel together,
in general.
In Mendel’s case, he lucked out because his choice traits
were mostly found on separate chromosomes, so genetic
linkage wasn’t factoring into his data.
That allowed him to come up with a fairly simple pattern of inheritance,
and let everyone else fill in the complicated bits.
Basically, you could say that Mendel walked so that later
scientists could run.
He got lucky in finding a model organism that showed off
the rules of inheritance in the simplest way possible.
Well, some people have raised the question of whether it
was just luck, or if he did a little massaging of the
evidence to make his point clearer.
Regardless, in laying out the rules, he gave later scientists
the framework they needed to understand how the
exceptions were working.
Which is really what any scientist hopes to leave for the
next generation.
And two hundred years out, we’re still talking about it.
People call Mendel the father of genetics.
His pea plants laid the foundation for the genomics investigations
we have today.
And Brilliant’s course on computational biology has an
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