Genotype vs Phenotype

Hi, and welcome to this review of genotypes and phenotypes! What exactly is a genotype? How are genotypes and phenotypes related? How can we apply these terms to real-life situations? In this video, we’re going to answer all of those questions. We’ll compare and contrast genotypes and phenotypes to help us understand what purpose they serve in the world of genetics.

Before we can fully understand genotypes and phenotypes, we first have to understand alleles.

Genes occur at specific locations on each chromosome and are made up of a specific chemical sequence of adenine, guanine, cytosine, and thymine bases. However, the sequence of a gene on one copy of a chromosome may vary a little from the sequence on the other copy of the chromosome. If that’s the case, this sequence variant for the same gene is our allele. An allele is a variant of a gene on a chromosome that helps determine the heredity of an organism. Since we have two copies of each gene (one from mom and one from dad), that means that all somatic cells have two alleles for each trait. Together, these two alleles determine the trait you inherit.

Genotype and Phenotype Examples

Let’s take height for example. Although there are a few different factors that contribute to height, we’ll keep it simple and say someone can either be tall or short. Let’s say the gene that codes for height on one chromosome has a sequence that reads like this: ACGTC. Let’s say this allele codes for “tall.” The gene that codes for height on the other chromosome has a sequence that reads: AGGTC. This allele codes for “short.” Again, because the gene is slightly varied (one sequence has a cytosine as the second base, while the other has a guanine) this is considered an allele. We can assign arbitrary upper and lowercase letters to our alleles to keep them organized. Let’s keep it simple and go with a capital T for “tall” and a lowercase t for “short” like this:

So we can have the same gene coding for the same trait on both chromosomes, but the sequence variant, or allele, is what produces different phenotypes. Therefore, a phenotype is the physical trait that is determined by the present alleles. Phenotypes help us observe and analyze genetic mutations, especially when we are thinking about an experiment to track inheritance of a trait. If we want to know if the dumpy wing trait will appear in the offspring of a male and female fruit fly, all we have to do is look for this trait in their progeny.

Sometimes just seeing the phenotype isn’t enough. Maybe we want to know the exact allele pairing that caused that trait. This is where the genotype comes in. A genotype is the allele pairing inherited for a particular gene that produces a specific phenotype. In reality, genotype refers to all the genes in an organism and phenotype refers to all observable traits of an organism, but in practice they’re usually used to refer to a single gene in question.

Now let’s bring everything together and look at how genotypes and phenotypes actually affect inheritance. Remember that being a diploid organism means you don’t have just one copy of an allele to contend with, you have two! Different combinations of those two alleles are what lead to the observable phenotype, or trait. An individual with two copies of the same allele is said to be homozygous for that trait and an individual with one copy of one allele and another copy of another allele is said to be heterozygous for that trait. So going back to our height example, an individual would be homozygous for height if he or she had the TT genotype or the tt genotype. An individual with a Tt genotype is therefore heterozygous. “tT” is understood to be the same as “Tt” so that is why there is only one option here.

This is important because whether an individual is homozygous or heterozygous for a trait can mean the difference between passing on an inherited mutation or not. Some alleles have been identified to be mutant alleles, meaning they contain a mutation that could cause a change or disease. If an allele is not a mutant allele, it is called a wildtype allele. This type of allele is considered to be “normal.” So how do we know if a mutant allele will cause a change or not? It depends on the way it pairs with the second allele for that gene. Another way to say this is that it depends on if the alleles are dominant or recessive.

Mutant alleles can be dominant, meaning they will override another allele, or recessive, meaning they will not override another allele. Dominant alleles are usually represented with a capital letter, and recessive alleles are represented by a lowercase letter. Let’s look at this in more detail using a dimple mutation. Let’s say the dominant allele means you have dimples, so we’ll assign it the capital letter “D.” And let’s say the recessive allele means you don’t have dimples. We’ll assign it the lowercase “d.” If we draw a simple Punnett square, we can determine a genotypic and phenotypic ratio for dimples. For this example, we have mom and dad who are both heterozygous for dimples with the genotype “Dd”.

If we fill in the Punnett square by dragging each allele down and across for each box, this is our result. Let’s start with the genotypic ratio. We have one offspring that is homozygous dominant “DD,” two offspring that are heterozygous “Dd,” and one that is homozygous recessive “dd” so the ratio is 1:2:1. Since the dominant allele is “D,” offspring with the genotypes DD and Dd will all have dimples because the “D” allele overrides the recessive allele. We also have one homozygous recessive offspring where there is no dominant allele, so this combination will yield no dimples. With this we can say the phenotypic ratio is 3:1 dimples to no dimples. We can also say that 75% of the offspring will have dimples while 25% will not. This example shows how the genotype and phenotype, while related, can give us different, but equally important, information about inheritance.

That’s really all there is to it! Let’s go over a quick review question to test your knowledge.

Which of these correctly describes a main difference between genotypes and phenotypes?

1. Genotypes are coding genes while phenotypes are non-coding genes
2. Genotypes are recessive alleles while phenotypes are dominant alleles
3. Genotypes are genetic information while phenotypes are environmental information
4. A genotype is an organism’s set of genes while a phenotype is its physical traits

I hope this review was helpful! Thanks for watching, and happy studying!