Ryan K. answered • 03/30/19
Former Teacher Specializing in SAT Prep
In biology, there are three different types of genotypes used for 'regular' Punnett Squares.
There is homozygous recessive, homozygous dominant, and heterozygous.
If 'A' is dominant and 'a' is recessive, then:
homozygous recessive is aa
homozygous dominant is AA
heterozygous is Aa
Remember that 'homo' means the same and that 'hetero' means different. So if someone is homozygous for a trait, that person's genotype will include two of the same letter-case (AA or aa). If someone is heterozygous for a trait, that person's genotype will have one capital and one lower case letter.
When talking about genotypes, it's likely that you'll be creating or will soon start to create Punnett Squares. These are used to find the percent probability of an offspring presenting (having) a certain trait. Before one can make a Punnett Square, he or she must determine the genotypes of each parent. It is NOT POSSIBLE to determine a person's genotype if that person presents the dominant trait, as it is not possible to differentiate between someone who is homozygous dominant and heterozygous for a specific trait simply by looking at that person. However, if a parent presents a recessive phenotype, then you know that the genotype of that parent must be homozygous recessive.
To further answer the question I believe you are trying to ask, I will give you a specific, real-life example about myself. I have brown eyes, which are dominant over blue eyes. Let's say 'B' represents brown eyes and that 'b' represents blue eyes. Just by looking at me, you know that my genotype for eye color cannot be homozygous recessive (bb). My genotype could be either homozygous dominant (BB) or heterozygous (Bb). Remember, one can always potentially ascertain more information by looking at one's parents. As my mom has blue eyes, her eye color genotype must be homozygous recessive (bb). As my mom only has recessive genes to pass down to me, I know that I am heterozygous (Bb) for eye color. Going further, my wife also has blue eyes. Therefore, there is a 50% chance that our children will be homozygous recessive (bb) for eye color and will present with blue eyes. There is also a 50% chance they'll be heterozygous (Bb) for eye color and present with brown eyes.
Remember, only one capital letter is required for the dominant trait to be present. Again, it is not possible to determine an exact genotype of individuals who present a dominant trait without knowing information about the individuals' parents. Dominant parental phenotypes can only supply you with a limited amount of information, but if a parent presents a recessive gene, one can immediately determine that parent's genotype to be homozygous recessive.
I hope that you found this information to be helpful!
