Salutations! We were prompted by this post to talk about the
history of DNA and genetics. It may sound a little “high school” but we decided
it’s a great place to start with Mendel. Because you will never ever get away
from him. 3 years of University under our belts and we still get at least 10
minutes about this man every class that deals with DNA/genetics. Just a note
that I realized as I was writing- this is being explained with words Mendel did
not have. Homozygous, Heterozygous, alleles- these are words that come later,
but I am using them for ease.
Gregor Mendel was a 19th century Augustinian
friar. In 1854 he converted 5 acres of monastery
ground into his experimental garden. His target of study- pea plants (Pisum sativum). He worked on these
plants for the next ten years, resulting in his paper, which he entitled Experiments
on Plant Hybridization.
His experiments were such that he isolated traits amongst
pea plants, and examined how these physical (phenotypic) traits were passed on as
each successive generation grew. I will talk about his experiments with tall
and short plants, as this is the one Watson and I are familiar with.
Mendel used 2 varieties of commercially available seeds- one
that promised tall plants (2 metres), and another that promised short plants (~40
centimetres or less). He grew several of each variety, and then cross pollinated
them, so he would now have seeds which he believed would posses traits from
both parent plants (for explanations sake, Generation 0).
Generation one was grown from these new seeds, which
produced plants that were all just as tall as the tall parent plant (2 metres).
Generation 2 (so the children of generation 1) revealed a mix of tall and short
plants. I’ll save you the other 6 traits he looked at, but from generation 2,
Mendel determined that “tall” was the dominant trait, and that “short” was the
recessive allele. Fun fact- dominant and recessive were terms coined by Mendel
directly for his research.
“But Sherls, how could he figure that out, what’s going on”?
For that, I must now MS paint a very useful device that I know and love from
grade 11 biology- the Punnet Square.
A Punnet square is a tool used to examine the traits of
parents, and what the progeny will inherit. These are super handy, even for
modern breeding (I know of them being used for bearded dragon morphs).
In Generation 0, we cross a Homozygous Dominant (a plant
possessing only the “tall” allele) with a Homozygous Recessive (a plant
possessing only the “short” allele). We see from the Punnet Square that 100% of
the offspring will be heterozygous- they will each contain one “tall” allele
and one “short” allele. Tall is dominant because it is the one that is visible.
How do we know that tall is dominant, and that these plants did posses a “short”
allele? We must look at the crossing of Generation 1 plants with other
Generation 1 plants.
In Generation 1, we cross two Heterozygotes. We see from
this Punnet Square that 25% of the plants will be homozygous dominant (appearing tall), 25% will
be homozygous recessive (appearing short), and 50% will be heterozygous (appearing tall, but have one short allele). Mendel, seeing that in
the Generation 2 that there were both tall and short plants, he came up with
what are now known as Mendel’s Laws of Inheritance.
The first law is the Law of Segregation. This law states
that during formation, gametes (egg or sperm cells) will obtain only one allele
from the parent.
The second law is the Law of Independent Assortment. This
law states that genes for different traits are independent from each other. So
you do not have to take the “short” allele if you pick up the wrinkly pea pod
The third law is the Law of Dominance. This law states that
there are dominant alleles and recessive alleles. Any living creature
possessing a dominant allele will display this allele.
(Fun fact: co-dominance and incomplete dominance are also a thing. Let’s say there is an allele for red and an allele for white. With co-dominance both red and white will show in kind of a splotchy kind of way. Incomplete dominance will show up as a mixed pinkish colour instead.)
So, there you have it, that covers all the high points of
Gregor Mendel’s work, which was completed in 1866. However, scientists at the
time were firm believers in Darwinian theory (survival of the fittest), and
generally ignored Mendel’s findings. It wasn’t until 1900 that his works were
rediscovered, by three separate scientists that began working further on what
Mendel had done.
If you remember all this from High School- congrats! This
post is meant to help those new to DNA and Genetics, also because most sources
I have found require you to have some crazy prior knowledge.