Genotype Definition

The genotype of an organism is the chemical composition of its DNA, which gives rise to the phenotype, or observable traits of an organism. A genotype consists of all the nucleic acids present in a DNA molecule that code for a particular trait. The outward appearance, or phenotype, is the result of interactions of proteins being created by the DNA. Modern DNA analyzing techniques have made it easier to identify which segments of DNA are responsible for various phenotypes.

A genotype has different alleles, or forms. The different alleles are produced by mutations to the DNA, and may give rise to beneficial or detrimental changes. In bacteria, the DNA exists in a ring and only one allele for each genotype is present. Sometime, an allele will mutate in a beneficial way, the organism will reproduce more and the genotype will increase in the population. In sexually reproducing organisms, there are two alleles present in each organism, which can have complex interactions with each other, and other genes. Mutations can occur in these alleles, new combinations can arise during meiosis, and infinite amount of variety can be created. These combinations of genotype give rise to the enormous variety of life on Earth.

Examples of Genotype

Example #1: Eye Color

Although a particular genotype consists of many nucleic acids, scientists typically represent genotypes with single letters, or two letters in the case of sexually reproducing organisms that receive one allele from each parent. For example, the trait for eye color could be represented with the letter “E”. Varieties, or alleles, of that trait that are dominant will be designated by capital letters. Therefore, “E” will represent brown eyes. Traits that are recessive are written in lower case. The allele for blue eyes is recessive to the allele for brown eyes, so we can call it “e”.

A set of parents has brown eyes. Having brown eyes only tells us their phenotype, not their genotype. The parents could be “Ee”, “EE”, or there could be one of both. A single “E” allele in the genotype will result in the brown-eyed phenotype, even if the parent harbors a recessive “e” allele as well. The parents conceive a child. The child has blue eye. This tells us that the child is homozygous recessive, or “ee”, because only two recessive alleles can produce blue eyes. This also tells us a lot about the parents. The baby, having two “e” alleles, got one from each parent. Therefore, each parent has one “e” allele to give, while having the brown eyed phenotype. This shows us that the parents have the heterozygous “Ee” genotype. If either parent where homozygous dominant, “EE”, the baby would have received at least one dominant “E” allele, giving it brown eyes.

Example #2: Cystic Fibrosis

For a long time, it was not known why some children would develop a thick mucus in their airways, causing them to be short of breath and wheezy. The children had a variety of other symptoms, such as the inability to process food efficiently, gas, and weight loss. Until recent advances in medical and genetic sciences, many children died at a very early age. After years of research, cystic fibrosis was found to be caused by a defect in a gene that produces salt channels across cell membranes. These salt, or ion-channels, are used to maintain pH levels in various cells, remove waste, and remove nutrients from the intestines.

The genotype of people with cystic fibrosis is homozygous recessive. In other words, they carry two copies of the non-functioning allele for the gene that creates specific ion-channels. Some people, known as “carriers” can have a functioning, normal phenotype, while having a heterozygous genotype. This means that a carrier can pass a non-functioning allele on to their child. Unknowingly, two carriers can both pass on the non-functioning allele, and the child with receive a non-functioning homozygous recessive genotype. However, if one or both carrier parents passes on their good allele, the child will not have the symptoms of cystic fibrosis. If the child receives one functioning and one non-functioning allele, they will be a carrier as well. The child will not be carrier if they receive two functional alleles.

However, in parents who are both carriers, the genotypic ratio of offspring will be set at 1 normal: 2 carriers: 1 cystic fibrosis genotype. This genotypic ratio can be counted in a Punnett square. Place the heterozygous parents on the sides of the square, separating their individual alleles (Aa and Aa below). Then, simply fill in the boxes with the two alleles that would be received by each potential child. It can quickly be seen by counting that the genotypic ratio is 1AA:2Aa:1aa. In this case the phenotypic ratio would be 3 normal: 1 cystic fibrosis.

Incomplete dominance

Related Biology Terms

  • Dominant – Having the ability to mask the effects of a recessive allele.
  • Recessive – An allele that only shows phenotypic effects in the presence of another recessive allele.
  • Heterozygous – A genotype containing two types of alleles.
  • Homozygous – A genotype containing one type of allele.


1. Two hedgehogs with the genotypes “AA” and “Aa” reproduce a large litter of offspring. What is the estimated genotypic ratio of the offspring?
A. 1AA : 2Aa : 1aa
B. 2AA : 2Aa
C. 3AA : 1Aa

Answer to Question #1
2. Congratulations, you are madly in love and want to have children. First things first, you swing by your local geneticist and find out that you and your wife are both carriers for cystic fibrosis. What are the chances that you will have a child with cystic fibrosis?
A. 100%
B. 50%
C. 25%
Answer to Question #2

3. Albinism is another trait caused by a homozygous recessive genotype. Albinism is the inability to produce melanin, a pigment that colors our hair, skin and eyes. Many animals use melanin, and many animals can experience albinism. However, to produce a phenotype of albinism, two recessive non-functioning alleles are needed. In wild populations albinism is seen in relatively low numbers, a consequence of natural selection against it, and its recessive nature. Why then do we have entire populations of lab rats that are entirely albino, meaning the entire population contains recessive alleles?
A. Scientist release non-white rats into the wild.
B. These rats were genetically engineered to be white.
C. Only white rats were used, therefore only white alleles got added to the gene pool.

Answer to Question #3

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