 We know that DNA is the material that codes for traits, but let's look at this in a little bit more detail. You may recall that chromosomes exist in pairs, and in humans we have a total of 23 chromosome pairs. From these total chromosome pairs, half of these chromosomes are inherited from your mother and the other half from your father. These chromosomes contain genes, which are sections of DNA that code for something important, such as a trait, part of a trait, or even a protein. The expression of traits is complex and is controlled by the interaction of multiple genes. One particular gene can have multiple variations, and these different variations are known as alleles. You may inherit two different alleles or the same allele for a particular gene from your parents. These two inherited alleles alongside other gene interactions determine the trait expressed. So let's say you inherit two different alleles from each parent. Which one is expressed? The answer to this question was discovered even before we knew about the existence of DNA. One of the earliest findings on the inheritance and expression of genes was by an Austrian monk by the name Greggelmendl. His work on pea plants showed how traits are passed on from one generation to the next. During his experiments, Mendel noticed that purebred purple flowering pea plants crossed over with purebred white flowering pea plants resulted in a new generation of just purple flowers. He referred to the purple trait as being dominant over the white flowers, as they were the only ones expressed in this new F1 generation. When Mendel allowed these hybrid purple flowering plants from the first generation to self-pollinate, the new generation contained both purple and white flowers. This indicated that the white trait was masked by the dominant purple trait. So he referred to this white trait as recessive. From the classical experiment, Mendel was able to conclude that the variation of traits he observed was due to the interaction of dominant and recessive factors or alleles. At the time, Mendel referred to alleles as factors. During this time, he also wasn't sure about what was causing this dominant and recessive nature, as the discovery of DNA was still many years away. However, due to our current understanding of DNA, we can now conclude that dominant alleles are expressed if there is one allele present. These dominant alleles are represented by a capital letter. So if we look at the flower example, the dominant trait is expressed by a capital P. Recessive alleles are only expressed when there is no dominant allele present. This is represented by a lowercase letter. For example, this can be shown with a lowercase p. When referring to a gene, we often refer to its genotype and phenotype. The genotype refers to the genetic makeup responsible for determining the observable characteristics. The phenotype, however, is the observable or physical characteristics. For example, if we look at the bottom flower, it has the phenotype of the color white, which is expressed by the genotype of the two lowercase p's. I know we've covered a lot of terms so far, but I promise these are the last bits of terminology. The alleles can obtain different genotypic characteristics, depending on if the versions of the gene inherited are different or the same from each other. Due to this, alleles can either be heterozygous or homozygous. Heterozygous is when two different alleles are present for a gene, for example, for the genotype capital P and lowercase p. Homozygous refers to when two of the same alleles are present for a gene. For example, this can be seen for the genotype with two lowercase p's or two uppercase p's. Based off what we just covered, if we look at Mendel's flowering p-plant example again, we can use these principles to add genotypes to each of these flowers. In this video, we covered various concepts within genes and alleles. Here's a summary of the different terms that we came across today.