Genotype vs. Phenotype

Genotype determines what phenotype is seen in individuals
Rabbits with different colored fur. Getty/Hans Surfer

Ever since Austrian monk Gregor Mendel did artificial selection breeding experiments with his pea plants, understanding how traits are passed down from one generation to the next has been an important field of biology. Genetics is often used as a way to explain evolution, even if Charles Darwin did not know how it worked when he first came up with the original Theory of Evolution. Over time, as society developed more technology, the marriage of evolution and genetics became apparent.

Now, the field of Genetics is a very important part of the Modern Synthesis of the Theory of Evolution.

In order to understand how genetics plays a role in evolution, it is important to know the correct definitions of basic genetics terminology. Two such terms that will be used repeatedly are genotype and . While both terms have to do with traits shown by individuals, there are differences in their meanings.

The word genotype comes from the greek words “genos” which means “birth” and “typos” which means “mark”. While the entire word “genotype” does not exactly mean “birth mark” as we think of the phrase, it does have to do with the genetics an individual is born with. A genotype is the actual genetic composition or makeup of an organism.

Most genes are made up of two or more different alleles, or forms of a trait. Two of those alleles come together to make the gene. That gene then expresses whatever trait is dominant in the pair.

It could also show a blending of those traits or show both traits equally, depending on which characteristic it is coding for. The combination of the two alleles is an organism’s genotype.

Genotype is often symbolized using two letters. A dominant allele would be symbolized by a capital letter, while the recessive allele is represented with the same letter, but only in the lower case form.

For example, when Gregor Mendel did his experiments with pea plants, he saw the flowers would either be purple (the dominant trait) or white (the recessive trait). A purple flowered pea plant may have the genotype PP or Pp. A white flowered pea plant would have the genotype pp.

The trait that is shown due to the coding in the genotype is called the phenotype. The phenotype is the actual physical features shown by the organism. In pea plants, like in the example above, if the dominant allele for purple flowers is present in the genotype, then the phenotype would be purple. Even if the genotype had one purple color allele and one recessive white color allele, the phenotype would still be a purple flower. The dominant purple allele would mask the recessive white allele in this case.

The genotype of the individual determines the phenotype. However, it is not always possible to know the genotype by looking only at the phenotype. Using the purple flowered pea plant example above, there is no way to know by looking at a single plant whether the genotype is made up of two dominant purple alleles or one dominant purple allele and one recessive white allele. In those cases, both phenotypes would show a purple flower.

To figure out the true genotype, the family history can be examined or it can be bred in a test cross with a white flowered plant, and the offspring can show whether or not it had a hidden recessive allele. If the test cross produces any recessive offspring, the genotype of the parental flower would have to be heterozygous, or have one dominant and one recessive allele.