Fundamentals: DNA. Modern Synthesis

While the understanding of DNA, genetics, and heredity matured over hundreds of years, an essential milestone occurred in the early 20th century as Darwinian evolution and Mendelian genetics transitioned from theory to fact. The combining of Darwinian evolution (natural selection) and Mendelian genetics (inherited DNA) was termed the Modern Synthesis.

Today, Gregor Mendel is known as the founder of genetics, and genetics, of course, is the foundation of the biotechnology industry! But, in the mid-1800s, when Mendel began his scientific inquiries into how traits are passed on from one organism to the next, virtually nothing scientific was known about it. While traditional breeders of livestock and crops had been slowly modifying organisms over at least thousands or tens of thousands of years, scientifically proving that biology could be changed through domestication had yet to be achieved.

It wasn’t until Mendel started his pea plants experiments that crucial information on genetics emerged (Figure 1-10). Mendel found that when he cross-pollinated pea plants with different colored seeds, some of those seed colors were preferentially passed down to the next generation of plants. But, looking further down the line of pea plant generations, he found that some seed colors eventually re-emerged!

Figure 1-10. Pea plant. Photo by Crepessuzette from Pixabay

His experimental procedure, and therefore the first genetic experiments, went something like this:
1. Yellow seed pea plants were carefully bred with other seeded yellow pea plants to ensure he had “pure-breed” yellow pea plants.
2. The same was done with green seed pea plants.

3. Yellow and green seed pure-bred plants were then carefully cross-pollinated. This is quite easy to do. Today, you can use a Q-tip and gently rub a blossom of one plant to gather pollen, then rub it in another plant blossom, and lastly, rub it back in the first plant.
4. The plants grew, matured, and produced seeds
5. The color of the seeds were identified and kept for similar future experiments.

The pure-bred plants would always produce the expected colored seeds: yellow produced yellow seeds and green produced green seeds. However, when he did the first cross-pollination of yellow and green plants, all of the first generation offspring seeds were yellow! It looked as though some genetic information for yellow seeds was preferred over green (Figure 1-11)!

Figure 1-11. Mendel’s pea plants experiment provided the basis for modern genetics

But, things ended up getting a bit more complicated. What do you think happened when these first-generation yellow seeds were grown into plants and cross-bred? You might expect this preferential yellow genetic information to cause only yellow seeds. But the result included the reemergence of green seeds! And the most interesting part was they appear in a ratio of one green seed for every three yellow seeds.

This was the first time that rigorous scientifically derived information about how traits were passed down from a parent to its offspring was available. It was also the first instance that showed some “genetic rules” for which information is passed to the offspring. To explain this, Mendel coined the terms “recessive” and “dominant” genes. The green seeds were the ones with the recessive trait and the yellow seeds the dominant trait.

Mendel’s studies broadened beyond seed color into different plant traits, including height, blossom colors, pea pod shapes, colors, and more. In these studies, he found other “genetic rules” existed! Today, our hair color, eye color, skin color, and a lot more are well-known examples of how genetics are passed down from parents to offspring.

Modern synthesis combined Darwinian evolution theory and the scientifically rigorous Mendelian genetics to create a unified framework to understand how biological organisms can change through time. Mendelian genetics, now just genetics, turned out to be the mechanism or the “how” and “why” different genetic material is passed down from parent to offspring.

Darwinian evolution allowed scientists to see that genetics applied to all organisms, not just pea plants. Modern synthesis also included some other key mechanisms in which DNA changes occur in organisms and populations:

Mutations: when changes in the DNA sequence happen due to copy errors. While mutations can be increased due to environmental factors, this is often considered independent of selective environmental pressures (natural selection). In most cases, mutations are harmful to an organism; however, in a minority of cases, a mutation can lead to a benefit that can help an organism survive in their environment. In other words, mutations do occur randomly but can become a part of natural selection.

Random genetic drift: is where allele frequency changes in populations. An allele is a DNA sequence (ie.g. gene) that is very similar to another allele but has a slightly different sequence. Alleles are created due to mutations. As organisms reproduce, these different versions of DNA sequences will “move” through a population. Similar to the pea experiments above where the “yellow seed” and “green seed” alleles “moved” through the pea population as the plants were bred.

Gene flow: is where allele frequency changes due to immigration or emigration to/from a population. For example, if there were only true-bred green seed pea plants in a region and a person or bird happened to bring a yellow pea seed to that region. When that pea plant grows, it’s yellow seed allele will become dominant in the region in the coming years as cross-breeding occurs.

A key basis for natural selection, random genetic drift, and gene flow are mutations. Mutations are random and result in different DNA sequences (alleles). While most mutations negatively impact an organism, some can provide a benefit. The beneficial mutations can help an organism survive in a particular environment (natural selection) or can help the organism survive independent of the environment. The alleles can then “move” around and change a population through gene flow and genetic drift. Bit by bit, generation after generation, mutations reshape us, and the world around us as species change or new species emerge.