So now that we've distinguished between the dominant and the recessive alleles in our last lesson video, in this video, we're going to distinguish between the genotype and the phenotype. The genotype, as its name implies with the root gene in it, is the genetic combination of alleles in an individual organism. In diploid organisms that have 2 copies of every gene or 2 alleles, the genotype is always written as a pair of letters. There are homozygous genotypes, and there are also heterozygous genotypes. The root homo means the same, and so homozygous genotypes are organisms that have 2 identical alleles for the same exact gene. The genotype could have 2 identical dominant alleles or 2 capital Y's, or the genotype could have 2 identical recessive alleles or 2 lowercase y's. The 2 capital Y's that are identical here would be homozygous dominant, and the 2 lowercase Y's that are identical would be homozygous recessive, and we distinguish between those down below in our image. The heterozygous genotype, the root 'hetero', means different, and so it's no surprise that heterozygous genotypes are individuals that have 2 different alleles for the same gene. Having a dominant allele and a recessive allele, which are 2 different alleles, would create a heterozygous genotype. If we take a look at our image down below, notice that the left column here has the genotype, the combination of alleles in an individual. Notice that the first two, numbers 12 here correspond with number 12 down below. They are referring to homozygous genotypes, meaning that the individual has 2 identical alleles. The top one here is referring to a homozygous dominant genotype because it has 2 capital Y's, 2 dominant alleles. Notice that the individual down here with the homozygous has a homozygous recessive genotype because it has 2 lowercase Y's, 2 recessive alleles. This third genotype has 1 dominant allele and 1 recessive allele, 1 capital Y and 1 lowercase y, which means that it has 2 different alleles for the same gene and it is a heterozygous genotype. Again, the genotype is always written as a pair of letters. It's the genetic combination of alleles.

What is the phenotype? The phenotype, as its name implies with the pH, is the physically expressed trait that results from the genotype. The phenotype is the physical expression of the genotype. It is physically expressed in this scenario as either the yellow color of the pea or the green color of the pea. You can see down below in our chart, we have the phenotype on the right column. Notice that the homozygous dominant genotype leads to a yellow color phenotype. The homozygous recessive genotype leads to a green color phenotype. Notice that the heterozygous genotype leads to a yellow color phenotype. The reason that the heterozygous genotype leads to a yellow color and not a green color is that the dominant allele will dominate and exert its effect over the recessive allele when they are both together. The recessive allele is going to be masked when it's in the presence of the dominant allele. The dominant allele is going to dominate and create a pea with a yellow color. Just by visually looking at the color of these peas, we would not be able to distinguish between the homozygous dominant and the heterozygous pea because they both have yellow-colored peas. However, there are ways that we can test to see what the genotype is of plants that have these specific phenotypes, and we'll get to talk more about that later in our course.

But for now, this here concludes our lesson on the difference between the genotype and the phenotype. How the genotype is a genetic combination of alleles or a pair of letters, whereas the phenotype is going to be a physically expressed trait, such as the color of a specific pea. We'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.