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Do you ever wonder why you look the way you do or why you have certain preferences? Well, it's all determined by your genotype! Even though you can't see it with your naked eye or a microscope, it's an important part of who you are. Scientists use special technology to study it, like microarrays and DNA-PCR, or super-computers and mass-sequencing technology. Your genotype is made up of DNA sequences that decide things like your height, eye color, personality, and even your favorite foods. So, even though you can't see it, your genotype plays a big role in making you unique!

Definition of Genotype

Genotype refers to the specific alleles of a particular gene, which help determine how an organism looks and behaves. Every living thing has, and the combination of alleles in those genes create a unique genotype. The phenotype, on the other hand, is the visible characteristics of an organism - like how it looks. So, to sum it up, genotype is all about the specific alleles of genes, while phenotype is about how those genes actually manifest themselves in an organism's appearance and behavior.

Terms for Describing Genotype

What are some terms we must understand when describing genotype?

Homozygosity is the state of a homozygous organism for a given trait. In other words, both of its alleles for that gene are the same. Let's use cystic fibrosis to examine this. There are two possible alleles on the gene that control whether someone gets cystic fibrosis or not. F is the normal variant, and f is the mutated cystic fibrosis variant. F is the dominant allele, which means there must only be one copy of it for an individual not to have cystic fibrosis. If f is the recessive allele,  there must be two copies of it for the individual to have the disease. There are two possible homozygous genotypes at this gene: either someone is homozygous dominant, has the genotype (FF), and doesn't have cystic fibrosis, or someone is homozygous recessive, has the genotype ff, and has cystic fibrosis.

Heterozygosity is the state of a heterozygous organism for a given trait; its alleles for that gene are different. Let's continue with our previous example. For someone to be heterozygous at the gene controlling cystic fibrosis, their genotype would have to be Ff. Because this gene acts on the principles of Mendelian inheritance (one allele exhibits complete dominance over the other), this person would NOT have cystic fibrosis. They would be a carrier; their genotype shows the presence of a mutant allele, but their phenotype is the same as someone who is homozygous dominant and doesn't have any mutant alleles at all.

Carrier: a term in genetics used to describe a person who has just one copy of a mutant, recessive allele and thus doesn't have the mutant phenotype.

Although we've mentioned this word previously, we will also use this opportunity to define what an allele is. We will define three terms that - as different as they sound - have similar meanings and usages. All three words are important when describing genotype:

1. Allele

2. Mutation

3. Polymorphism

Allele Definition:

An allele is a variant of a gene. In the cystic fibrosis gene mentioned above, the two alleles are F and f. Alleles can be dominant or recessive. They are organized into pairs on chromosomes, which are the total physical representation of our DNA and genetic material. Some genes have more than two alleles, but there are always at least two present because, by definition, they require variation.

Want an example of a gene with more than two alleles (called polyallelic)? Keep reading; there's one below. Human blood groups ABO!

Mutation Definition:

For an allele to be called a mutation, it usually has three factors -

It appeared spontaneously in an organism. Such as a cancer cell developing a mutation or if something goes wrong during reproduction and a newly formed organism develops a mutation. It is deleterious. Deleterious means that it is harmful to the organism. It is rare. Typically it has to be an allele present in less than 1% of the population!

Polymorphism Definition:

Polymorphism refers to any allele that is not a mutation: thus, it occurs more frequently than mutations, is not typically deleterious, and does not necessarily appear spontaneously (or de-novo) in an organism for the first time.

Types of Genotypes

With genes that have only two possible alleles, which follow the principles outlined by Mendelian genetics, there are three types of genotypes:

1. Homozygous dominant

2. Homozygous recessive

3. Heterozygous

Dominant Genotypes:

When it comes to describing genotype, it's important to understand the concept of dominant genotypes. There are two types of dominant genotypes that follow the pattern of Mendelian Inheritance. The first is the homozygous dominant genotype (AA), which has two copies of the dominant allele. The other is the heterozygous genotype, which implies dominance without needing to add the word 'dominant.' This is because when an organism is heterozygous at a gene, one of the alleles will always be dominant and will control the phenotype according to Mendelian genetics.

Dominant genotypes always have dominant alleles, but they may also have recessive alleles. They occur more commonly in a population because of Mendel's Law of Dominance, which states that the dominant allele will always control the phenotype of a heterozygote. This means that dominant phenotypes will naturally be more prevalent in any population, as they encompass both homozygous dominant and heterozygous genotypes. Understanding dominant genotypes is crucial when studying genetics and heredity.

Recessive Genotype

When following the patterns of Mendelian Inheritance, there is only one type of recessive genotype. It is the homozygous recessive genotype (for example, aa). It is typically denoted with two lower case letters, but it can also be capitalized. When it is capitalized, it will be followed by some mark like an apostrophe or asterisk (F'), or the recessive allele will be explicitly obvious to you.

What are the Tools We Have for Determining Genotype?

When determining the genotype, we can use Punnett squares. These are used primarily in Mendelian patterns of inheritance. Punnett squares are tools in biology that help us analyze the prospective genotypes of offspring of two organisms (often plants) when we cross them. When we know the genotype of the two parents, we can see the ratios of the genotypes of their future children. For example, if two homozygous dominants are crossed, we can see that all of their offspring will be heterozygotes (Fig. 1).

Figure 1: Homozygous cross leading to 100% heterozygote offspring. Source: Palomar College

Sometimes, a Punnett square is not enough, especially when examining genotypes for human disorders (like cystic fibrosis). It can tell us the genotype of parents, but not grandparents and other ancestors. When we want a bigger picture demonstration of a genotype, we use something called a pedigree.

A pedigree is a chart that can help us determine genotypes and patterns of inheritance based on phenotypes of family members (Fig. 2).

An example of a pedigree for a family
An example of a pedigree for a family

Examples of Genotype

Genotypes are best understood in relation to the phenotype they contribute to. The table below will show a possible genotype and phenotype pair (Table 1).

Table 1: Some examples of genotypes and the phenotypes they cause.

Remember that not all characteristics follow the principles of Mendelian inheritance. Human blood types, for example, have three possible alleles for each gene; A, B, and O. A and B exhibit codominance, meaning they both are expressed simultaneously; while O is recessive to both of them. These three alleles combine to produce four possible different blood types - A. B, O, and AB. (Fig. 3).

Possible human blood types, due to codominance, and multiple alleles
Possible human blood types, due to codominance, and multiple alleles

Genotype - Key Takeaways Genotype is the genetic sequence that makes up an organism or the specific alleles an organism has for a gene. Phenotype refers to the organism's physical/apparent characteristics. Genotype acts in combination with external and environmental factors to help determine phenotype. There are three genotypes in Mendelian genetics; homozygous dominant, homozygous recessive, and heterozygous. Punnett squares and pedigrees are tools we can use in genetics to help us determine genotypes of existing or future progeny.


how do I know my genotype?

You can do a genetic test such as PCR or a microarray. Or, if you know your parent's genotype, you can figure out the possible genotype you might have by doing a Punnett square. 

what is the difference between genotype and phenotype?

Genotype is what an organism's alleles are, regardless of what it looks like. Phenotype is the way an organism looks, regardless of what its alleles are.

What is a genotype?

A genotype is the specific alleles an organism has for a given trait. 

What are 3 examples of genotype?

Three examples or types of genotype include 1) homozygous dominant 2) homozygous recessive3) heterozygous

Is AA a genotype or phenotype?

AA is a genotype. It shows what the alleles for a particular gene are, in this case, a homozygous pair of A alleles.

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