Wize University Biology Textbook > Mendelian Genetics

Genotype and Phenotype

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Alleles
Different versions of the same gene are called alleles. For example, everyone has genes responsible for determining eye color. Some people have a version, or allele, that encodes for brown eyes, while others have the alleles that encode for blue.


An individual's complete set of alleles makes up their genotype, while the products of what's encoded by those alleles make up their phenotype.
Several alleles may interact with one another in different ways to give a specific phenotype.
Different alleles arise through mutations, which are changes in the gene's nucleotide sequence.
Mutations may arise through:
Errors in replication;
Environmental factors;
Chemicals, etc.
Photo by Thomas Samuel for ACC-BioinnovationLab / CC BY

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Genotype and Phenotype
Simply put, the genotype is what is in your genetic code (alleles of a gene). The phenotype is the observable physical trait that you have as a result of those alleles.

Genotype: allele combinations that you have
The key to differentiate genotype from phenotype is the idea that not all of your genes when expressed are actually visible as physical traits. Let's take the classic example of eye color.

Wize Concept
A is the allele for brown eyes, while a is the allele for blue eyes. Because brown is the darker color, if you have one A and one a allele (your genotype is Aa), you are going to have brown eyes (this your phenotype). Even though your genes encode for both, when the color brown is expressed in your eyes it completely overshadows the blue, such that it is not visible.

Two of the same alleles: homozygous
Example: allele A and A
Two different allele: heterozygous
Example: allele A and a



Phenotype: physical trait that results due to the allele combination that you have
If you have two AA, you have brown eyes.
If you have an A and a (Aa), you have brown eyes.
If you have aa, you have blue eyes.
Therefore, two genotypes (AA and Aa) can result in one phenotype (brown eyes).


Wize Concept
Because homozygous AA and heterozygous Aa look identical, it is said that A is the dominant allele. The other allele, not seen in the phenotype, is called recessive.

Extra Practice

Inheritance of traits practice problem 3

In cats, big ears are dominant over short ears, and the gene is autosomal. Another gene controls eye colour, and is X-linked. "E" gives brown eyes, "e" gives blue eyes, and Ee gives green eyes.  
Brown eyed male with the homozygous genotype for big ears is mated with a female with short ears and green eyes. What kittens could be produced?
Note: X-linked traits are on the X chromosome. Females have two X chromosomes while males have one X chromosome and one Y chromosome. 

The bacterium Mycobacterium tuberculosis is capable of degrading
cholesterol due to the presence of a cluster of genes encoding cholesterol-degrading enzymes. In the lab, you removed the gene that encodes the cholesterol transport and the bacterium is no longer able to bring cholesterol in to the cell. The inability of the bacterium to grow on cholesterol is a change to the ________________, while the removal of the gene is a change to the __________________.

Which of the following described alleles?

Genome

A diploid cell that is 2n = 4 receives a2, b2, and c2 alleles from one parent, and a1, b2, and c2 alleles from the other parent. Alleles a and b are on the same chromosome, while allele c is on another one. 
Draw the resulting cell after DNA replication.

Genome

A cell has a genotype a1/a2, c3/c3. Draw the cell before and after DNA replication.

Inheritance of traits practice problem 3

In cats, big ears are dominant over short ears, and the gene is autosomal. Another gene controls eye colour, and is X-linked. "E" gives brown eyes, "e" gives blue eyes, and Ee gives green eyes.  
Brown eyed male with the homozygous genotype for big ears is mated with a female with short ears and green eyes. What kittens could be produced?
Note: X-linked traits are on the X chromosome. Females have two X chromosomes while males have one X chromosome and one Y chromosome. 

Cell cycle practice problem 2

A cell is 2n=82n=82n=8and has the genotype a2/a2, b1/b2, c1/c2, d2/d2. Draw the cell: 
a) in "G1 phase"
b) At "metaphase"

An individual's complete set of alleles makes up their:

Assume the following dihybrid crosses where for a particular gene, A represents the dominant allele and a, the recessive allele, and for another trait, B represents the dominant allele and b, the recessive allele. Genes show complete dominance.

Which of the following described alleles?

Which of the following describes alleles?	

Which of the following statements is false?

Practice: Alleles

Which of the following describes alleles?	

Practice: Bacterium

The bacterium Staphylococcus aureus is able to produce a compound called staphyloferrin B. In the lab, you removed the gene that encodes the enzymes responsible for producing this compound. The inability of the bacterium to produce staphyloferrin is a change to the ________________, while the removal of the gene is a change to the __________________. 

Is this phrase true or false?
"An allele can be defined as a variation in the nucleotide sequence of the genes that always result in an observable phenotype"

Which of the following described alleles?