Welcome to week 3 of Science Friday. To read weeks 1 and 2, you can checkout the #sciencefriday tag on the website (note that you will need to change the filter to “all”). This week, I will cover the inception of modern day genetics as a result of the groundbreaking work of Gregor Mendel. Last week, I talked about how using precise, controllable, and reproducible experiments to test hypotheses was not the norm centuries ago. While the field of physics had certainly progressed much further by the nineteenth century in this regard, with geniuses such as Faraday and Maxwell, biology was still a bit behind. This week’s information comes from two sources: one is a print biology textbook I had lying around, and the other is from [url=http://www.biography.com/people/gregor-mendel-39282?page=1]this web page[/url]. Before Mendel, the generally accepted notion of inheritance from parent to offspring was the “blending” theory. For example, it was thought that if a plant with red flowers was crossed (“mated”) with a plant with blue flowers, the offspring would have purple flowers. Furthermore, once these two traits were “mixed,” they were inseparable and no new traits would appear in successive generations. Along came Mendel in the mid-1800s. Amazingly enough, despite the understanding of biology he accelerated, he was not a biologist. He had extensive training in mathematics, physics, and chemistry. In order to determine the nature of inheritance, Mendel crossed various pea-plants to find patterns. Why pea-plants? Well, for one, they were available. But moreover, they reproduced frequently and with many offspring, which made seeking patterns much more practical. Over many thousands of crosses, Mendel came up with his two laws of inheritance: [b]The Law of Segregation:[/b] Each parent contributes one copy—or [i]allele[/i]— of a gene to its offspring, for a total of two alleles in the fertilized egg [b]The Law of Independent Assortment:[/b] The inheritance of a particular [i]character[/i] (e.g., flower color) is independent of the inheritance of a separate character (e.g., seed texture). In other words, exhibiting red flowers does not necessarily mean the plant must exhibit smooth seed texture. Both of these conclusions were made prior to the discovery of genes, chromosomes, or the process of meiosis. The discovery of all these important biological phenomena in subsequent decades only confirmed Mendel’s descriptions, with only one exception, which I’ll discuss one paragraph after next. One of the reasons Mendel was able to make such accurate conclusions based on empirical data was his adroit mathematical skills, uncommon among biologists of the time (goes to show you how important math is, irrespective of scientific discipline). In the early twentieth century, another famous biologist, Thomas Morgan, conducted crosses with fruit flies. He noticed that the pattern of inheritance for certain characters was not indicative of Mendel’s laws proposed only a few decades ago. However, the new inheritance patterns were still predictable, showing their own unique pattern. This unique pattern of inheritance violated Mendel’s second law, but only in a very special circumstance: if the gene loci (points) are located on the same chromosome, inheritance is not independent. Understanding the process of meiosis, we can consider this further. During meiosis, homologous chromosomes (i.e., mother-father pairs) exchange alleles for the same genes. This is one of the reasons why sexual reproduction yields so much diversity. If two gene loci are located near one another, then they are more likely to remain together, whereas if they were located further from one another, they may be separated during crossover. Any [i]new[/i] genotypic expression the resultant offspring would exhibit, as a result, would be more common in the latter situation. Regardless of some inaccurate generalizations, Mendel’s work was instrumental to the study of human inheritance. He laid a generally accepted myth to rest and provided the framework for extraordinary feats such as the mapping of the genetic disorders. I hope you guys enjoyed this week’s Science Friday. As always, please provide feedback. If you have any questions regarding my sources or content, do not hesitate to ask. I will most likely not be posting a Science Friday next week, but intend on returning the week after.