In this lesson, we will look at the difference between genotype and phenotype. This illustration is a karyotype, which shows all the chromosomes from one individual. Recall that our somatic cells contain 23 pairs of chromosomes and each pair of chromosomes contains one chromosome from mom and one from dad. Recall that our chromosomes contain our genes. It is estimated that humans have around 25,000 genes located on our 23 pairs of chromosomes. Each gene occupies a specific position on a specific chromosome. This place is referred to as the gene's locus. Because our chromosomes are in pairs, a single gene will be found at the same locus on both chromosomes in the pair. Here the colored bands represent the locus or location of a single gene on this particular chromosome. Notice that on the first chromosome the gene is indicated with red, whereas on the other chromosome it is indicated in blue. This is to show you that although this is the same gene, there may be variations of that gene. Different versions of a single gene are called alleles. So for any gene, you may have two of the same alleles for that gene, or you may have two different alleles for that gene. The term "genotype" is typically used to refer to the specific alleles that you have. In our example, we have a red allele and a blue allele. Please not that color is being used here just for simplicity. In reality though, different alleles contain different sequences of DNA. Genotype refers to the specific alleles that an individual has. In contrast, phenotype refers to how those alleles are expressed. We often refer to genetic traits, which basically means the outcome that we get from our genes. Think about making cookies, for example. The instructions for doing so are in the recipe. But if you follow the recipe, you get cookies. Using this analogy, genotype is similar to the recipe. In other words, genotype represents the instructions to be followed. And then phenotype is like the cookies; It is the outcome or what we get from following the instructions. Thus, genotype produces the phenotype. Now let's consider dimples. People who inherit certain alleles will have dimples. We typically indicate alleles by using letters. In our example, let's use "D" for the allele that produces dimples and "d" for the allele that does not. The gene that determines dimples is found at a certain locus on a specific chromosome. But we have two of those chromosomes. This represents that chromosome pair from someone who has two copies of the dimples allele. We would say that this person's genotype is DD. The result of this genotype is that the individual has dimples. Her genotype, DD, produces her phenotype, those lovely dimples when she smiles. But not all situations are this straightforward. Most human traits, or phenotypes, are polygenic, meaning that they result from interaction of multiple genes. But other factors can also affect gene expression. You should think of genotype as providing the potential for a certain trait or phenotype. But in many cases, other factors, collectively referred to as the environment, may alter the outcome or expression of that potential. Here is an example. Consider height. A child with two tall parents may inherit alleles that would provide the potential to grow tall; however, if the child is poorly nourished during its early years, in spite of having a genotype that would predict tallness, the child's growth may be stunted by external factors; in this case - diet. Genotype and environment often interact to determine the phenotype.