About 2000 years ago, Aristotle claimed:
We’re each a mixture of our parents’ traits, with the father supplying life force and the mother supplying the building blocks.
Over the next 2000 years, our understanding of heredity didn’t change much. We had a general understanding that if two parents had blonde hair, their offspring would probably have blonde hair. We took this concept and applied it to the breeding of livestock, to the glorification of royal bloodlines, and to the subjugation of entire ethnic groups.
It wasn’t until the mid 20th century that we discovered the role chromosomes, DNA, and genes played in heredity. This post will explore how our understanding of heredity and genetics has developed over time. We will start with the father of genetics, Gregor Mendel.
In the mid 19th century, an Austrian monk named Gregor Mendel demonstrated that inheritance followed particular patterns. Over the course of a decade, Mendel investigated the patterns of inheritance in mice, in honeybees, and most famously in garden peas.
Mendel designed an experiment where he would breed pea plants until they would always produce an offspring identical to the parent (purebred). Once Mendel had purebred pea plants of different characteristics, i.e. short vs tall, he would breed them together and see how traits were inherited in subsequent generations. Surprisingly, Mendel found specific patterns of inheritance for all the features he studied. The key findings were:
- One form of a feature, such as tall, always concealed the other form, such as short, in the first generation after the cross. Mendel called the visible form the dominant trait and the hidden form the recessive trait.
- In the second generation, after plants were allowed to self-fertilize (pollinate themselves), the hidden form of the trait reappeared in a minority of the plants. Specifically, there were always about 3 plants that showed the dominant trait (e.g., tall) for every 1 plant that showed the recessive trait (e.g., short), making a 3:1 ratio.
- Mendel also found that the features were inherited independently: one feature, such as plant height, did not influence inheritance of other features, such as flower color or seed shape.
Based on these findings, Mendel proposed a model of inheritance in which:
- Characteristics such as flower color, plant height, and seed shape were controlled by pairs of heritable factors that came in different versions.
- One version of a factor (the dominant form) could mask the presence of another version (the recessive form).
- The two paired factors separated during gamete production, such that each gamete (sperm or egg) randomly received just one factor.
- The factors controlling different characteristics were inherited independently of one another.
These findings contradicted the popular theory at the time of blended inheritance. Blended inheritance said that if you crossed a tall pea with a short pea you would get a medium pea. Mendel showed that you would, in fact, get a tall pea or a short pea. In humans, this all gets blurred since we have so many different genes that contribute fractionally to our height. Luckily for Mendel, there was only one gene that determined whether a pea plant was tall or short. Let’s try an example.
Punnett square example
In humans, the trait of wet or dry ear wax works in much the same way that tall and short worked in Mendel’s pea plant experiments. Whether we have dry or wet ear wax is controlled by one gene having one of two possible arrangements. In this case, wet ear wax is dominant and dry ear wax is recessive.
We get one ear wax determining gene or allele from our mom and one from our dad. In this example, our mom gave us a dominant wet gene and our dad gave us a recessive dry gene. We might be tempted to say we will automatically have wet ear wax since it is dominant, but there is something else going on. We have to go up one generation and find out what genes our grandparents gave to our parents. If our mom inherited two wet genes we would call that homozygous, if she inherited one dry and one wet we would call that heterozygous.
For this example, let’s say our mom was heterozygous and our dad was homozygous. We can use a punnett square to determine our phenotype (physical characteristics):
We would have a 50% chance of having either wet or dry ear wax. As powerful as Mendel’s findings were, he still had no idea what the mechanism for all of this was. It would take another 40 years until the chromosome theory of inheritance was developed.
In the early 20th century, Walter Sutton and Theodor Boveri simultaneously discovered that chromosomes were the mechanism for Mendelian inheritance. Boveri and Sutton observed meiosis and reproduction in animals and saw that chromosomes act in the same way that Mendel’s laws described. Since chromosomes separated in the same way that the Ww x WW example above shows, one chromosome might have the wet trait and one might have the dry trait.
Thomas Morgan would later use the famous fruit fly experiment to show that traits can be sex specific. Morgan observed that a white-eyed trait only occurred in male fruit flys. He correctly determined that the trait must be carried on the X chromosome since that was the only way to explain the way inheritance was playing out. Ultimately, this confirmed that chromosomes were the mechanism of inheritance.
The chromosome theory was still missing a key factor though. No one understood how information was coded on a chromosome. The discovery of DNA and genes comes next.
In the mid 20th century, Watson and Crick discovered that DNA was the molecular basis of inheritance. Watson and Crick showed that DNA had a double helix structure of base pairs linking acidic strands. This structure would lend itself well to something that stores and expresses information. Since each base in this structure only matches with one other base, it would be easy to divide it in half and easily repopulate the other half.
A DNA strand on a chromosome can code for multiple things. A section of a DNA strand that codes for a specific protein is called a gene. The different variations that the gene can take on are called alleles. For example, a gene might contain information for producing eye color pigment. RNA would make a copy of that gene, than transport and translate it into the protein we need.
The human genome has 6 billion base pairs spread over 46 chromosomes. This is the equivalent of about 1.5 gigabytes of computer storage.
For most of human history, people realized that some form of inheritance occurred between parents and offspring. It would only be in the most recent 200 years that we began to understand the underlying mechanisms of that inheritance. Today, we understand that information is encoded in DNA and is passed on through chromosomes.