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Genotype

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Biology

An organism's genotype is not visible to the naked eye. It's not even visible under a microscope. To determine it in a laboratory either takes endless sets of microarrays and DNA-PCR or the power of super-computers and mass-sequencing technology. Yet genotype, in combination with environmental effects, determines so much of what you look like and how you behave - from eye color to height to personality to food preferences. Ultimately, your genotype is an orderly sequence of DNA that encodes the proteins that make you, you.

Definition of Genotype

Genotype is defined as the genetic makeup of an organism. In terms of a particular trait, genotype describes the nature of the alleles of that trait. Every living thing has genes, and the particular alleles of those genes help determine how that organism looks and behaves - its phenotype.

Genotype: the genetic makeup of an organism and the specific alleles of a particular gene.

Phenotype: the apparent characteristics of an organism; the way an organism looks.

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 -

  1. 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.
  2. It is deleterious.
    • Deleterious means that it is harmful to the organism.
  3. 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:

There are two types of dominant genotypes when following the patterns of Mendelian Inheritance. One is the homozygous dominant genotype (AA), which has two copies of the dominant allele. The other is the heterozygous genotype. We do not call this 'heterozygous dominant' because the dominance is implied. The implication is that when an organism is heterozygous at a gene, there are two different alleles, and according to Mendelian genetics, one of the alleles shines through in the phenotype and is dominant. So saying 'heterozygous dominant' would be redundant.

Dominant genotypes always have dominant alleles, they may have recessive alleles, and they occur more commonly in a population. This phenomenon occurs because of Mendel's Law of Dominance, which states that the dominant allele will always control the phenotype of a heterozygote. Thus, dominant phenotypes will naturally be the most prolific in any population because this phenotype encompasses both homozygous dominant and heterozygous genotypes.

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).

Genotype Homozygous cross leading to 100% heterozygote offspring Palomar.edu.StudySmarterFigure 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).

Genotype An example of a pedigree for a family. Wikimedia. StudySmarterFigure 2: An example of a pedigree for a family. Source: Wiki Commons

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.

Genotype
PPno horn present in European cows
Ppno horn present in European cows
pphorn present in European cows
GGgreen pea plant
Gggreen pea plant
ggyellow pea plant
AOA blood type in humans
AAA blood type in humans
ABAB blood type in humans
BOB blood type in humans
BBB blood type in humans
OOO blood type in humans

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).

Genotype Possible human blood types, due to codominance, and multiple alleles. 123RF Stock Photos. StudySmarterFigure 3: Possible human blood types, due to codominance, and multiple alleles. Source: 123rf.com

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.

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. 

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.

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

Three examples or types of genotype include 1) homozygous dominant 

2) homozygous recessive

3) heterozygous

AA is a genotype. 


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

Final Genotype Quiz

Question

What is the definition of genotype?

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Answer

Genotype is the set of specific alleles an organism has for a given gene

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Question

Which of the following is not a genotype

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Answer

Blue eyes

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Question

How many different types of genotypes are there in Mendelian Genetics?

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Answer

Three

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Question

Which of the following is not a genotype seen in Mendelian Genetics?

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Answer

heterotropic dominant

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Question

What is the difference between genotype and phenotype

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Answer

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

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Question

_____ + Environment = Phenotype

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Answer

Genotype

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Question

All genotypes for all traits have two alleles

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Answer

False

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Question

All genotypes for all alleles follow the principles of Mendelian Inheritance

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Answer

False

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Question

Assuming complete dominance, what possible genotypes can give a dominant phenotype?

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Answer

Homozygous dominant and Heterozygous

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Question

When not in a laboratory setting, what are two tools we can use to examine genotypes of family members?

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Answer

Punnett Squares and Pedigrees

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Question

Punnett squares can tell us two things about genotypes of future offspring. What are they?

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Answer

The possible genotypes and their ratios.

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Question

What are all the possible genotypes that we can see in offspring of this cross? Assume Mendelian Inheritance.


AA x Aa


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Answer

We can see AA and Aa progeny only.

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Question

What are the ratios of the possible genotypes that we can see in the offspring of this cross? Assume Mendelian Inheritance. 

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Answer

We will see a 1:1 ratio of AA to Aa. 

50% AA and 50% Aa.

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60%

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