A Short History of Genetics
Photo Credit: Shannon Henry
We take for granted the process of passing on our genetic information from parent to offspring. Unsurprisingly, this wasn’t the case about two centuries ago. It was fairly obvious that a child shared some traits of his mother and father: red frizzy hair, icy blue eyes, a crooked nose perhaps. But what was the mechanism of inheritance? It took a German monk to unlock the secret of the gene and carve out a science of his own.
Gregor Mendel took the challenge of discovering exactly why traits are sometimes lost in one generation and reappear in the next. How could two parents with brown eyes produce a child that has blue eyes? This was a mystery that nagged scientists until the middle of 19th century, when Mendel began his experiments. Because of his religious affiliation, he was disallowed from using mice as subjects and instead turned to pea plants to use as models for his breakthrough findings.
Well it seems silly that pioneering a revolutionary branch of science, relevant to all known sexually reproducing species, found its roots in pea plants. But it is often a sign of true genius when you can theorize about phenomena so abstract and miniscule through the use of observable evidence.
Mendel’s Observations
Say you take a green pea plant, which has always produced green plants for generations and generations, and you cross it with a yellow pea plant that has always produced yellow plants. What result would you expect from this experiment? In Mendel’s garden, the offspring of these purebred, or “true breeding” plants, were always completely green. Well, that’s strange. You just combined a green plant and a yellow plant and yet all of the peas turned out to be green; where did the yellow go?
To find out, Mendel decided to take those offspring green plants (the child of the green and yellow) and cross it with itself: green with green. The result was magical: all of a sudden the yellow plants made a resurrection in this generation appearing at an almost exact 25% probability. Repeating this experiment hundreds of times, Mendel recorded the same result: 25% of the weaker version, which he called recessive, and 75% of the stronger version, which he called dominant.
Mendel’s Conclusions
By continuing to cross offspring from different generations and calculating how often different variations appeared, Mendel formed several laws about genetic inheritance: Law of Segregation and the Law of Independent Assortment.
19th Century Understanding
The law of segregation states that each individual has two sets of genetic information: one from mom and one from dad. Moreover, that individual then goes on to pass only one of the two packets of genes for each trait. So when Mendel crossed the pure green and the pure yellow plant, the offspring still had half of the yellow parent’s genes, even though the color was the dominant green. A dominant version of a gene is just one that shows itself if it’s matched up with a recessive version. The only time a recessive version will show is if it’s matched up with another recessive (i.e. yellow and yellow).
Independent assortment, according to Mendel, means that one trait has no bearing on another trait. So if a pea plant inherits a green color, it will have no effect on whether it is tall or short, round or wrinkled, or any other trait he observed.
Modern Understanding
We now know that the two sets of genetic information referred to above is our DNA, packed in individual chromosomes. Humans have 46 total chromosomes and they are separated into homologous pairs: each pair has the same exact genes, except one might have a different version of a gene than the other one. So the first pair of chromosomes might both contain the gene for eye color, except one could code “blue eyes” and the other could code “brown eyes.” The dominant version takes charge and the human will have brown eyes. The chromosomes are what get segregated: only one of each pair goes to your child. No more, no less.
If this still doesn’t make sense, or you’re a visual learner, the video below gives a graphical representation of the two principles. Enjoy!
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