Genotypic Frequency: Definition & Explanation

Instructor: Sarah Phenix
In this lesson, we will explore what genotype frequency is. We'll examine many important facets of genetic inheritance such as what the term genotype refers to, what a phenotype is, what role alleles play in inheritance and what the variations of genotypes are that result in enabling genotype frequency calculations.

Genotype Frequency

Have you wondered how geneticists figure out how often one particular trait will be expressed over another? Well, if you have, then you've been curious about something called genotype frequency, which is the likelihood that a particular genotype will appear when parental genotypes are crossbred. Now, genetics can be a bit confusing because there are so many terms; therefore, before we jump into genotype frequency, let's take a moment to revisit some of the common terms, such as genotype, phenotype and alleles (oh my!).

Genotype v. Phenotype

Ok, so what is a genotype? Well, the term, genotype, refers to the genetic expression of a particular trait. Not to be confused with phenotype, which refers to the physical manifestation of the trait. So, let's put this into context.

Say you were breeding flowers that came in two colors (or phenotypes) purple and white. Now, what that means is that there are two different allele possibilities (or variations) that code for flower petal color - one allele codes for white (we'll notate that as 'b') while the other codes for purple ('B'). So, allele is the term used to refer to the different genetic possibilities ('b' or 'B') that code for a trait.

Genotype versus Phenotype
Genotype versus Phenotype

Now, when flowers pollinate, pollen from the male anther joins the female stigma; therefore, flowers, just like people, get both maternal and paternal chromosomes. So why is this important? Well, it all comes back to alleles.

Alleles and Phenotype

The resulting offspring will have two chromosomes (one maternal and one paternal) that each carry an allele for flower petal color. If you lined up the complimentary maternal and paternal chromosomes, you would find that the alleles coding for flower petal color are located in the same places on the two chromosomes, but that the alleles don't necessarily code for the same color of flower petal.

In other words, you could think of alleles like different varieties of cake recipes. One recipe may be for a chocolate cake and the other for a vanilla cake but, at the end of the day, they are both variations of recipes for cake. So, if there are two alleles that each code for a different color, than which color wins out? Well, now you're talking about dominant and recessive alleles.

Dominant v. Recessive

Ok, so here's where it starts to get really interesting. Now, alleles come in two varieties, dominant and recessive. You can think of this relationship as very similar to human personality types - if you have two friends and one has a dominant personality, while the other is quiet, or recessive, than the dominant personality will always get their way. So, in the case of alleles, whenever a dominant allele is present, the phenotype that it codes for will always shine through.

Dominant versus Recessive Alleles
Dominant versus Recessive Alleles

Therefore, if you have two dominant alleles, known as homozygous dominant, each coding for purple flowers (BB) than the phenotype would be purple. Conversely, if you have two recessive alleles, known as homozygous recessive, each coding for white flowers (bb) than the phenotype would be white. But, if you have one dominant and one recessive allele, known as heterozygous (Bb), than the phenotype would express the dominant trait, which, in this case, is purple. Now, with that being said, you can see that by looking at the genotype you can always determine the phenotype; however, due to the heterozygote, you cannot accurately determine the genotype from merely looking at the phenotype.

Punnett Squares & Genotype Distribution

Ok, so how do scientists figure out what possible genotypes can arise from two crossbred parents? Well, they use something called a Punnett square. A Punnett square is a tool for determining the genotype, and therefore phenotype, possibilities from two parental genotypes.

Punnett Square of Genotype Frequency
Punnett Square of Genotype Frequency

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