Phenotype Definition

Phenotype denotes the expression of genes through a particular trait. While the genotype is the genetic constitution or makeup that brings about a particular phenotype when expressed. The central dogma proposes that DNA is transcribed into RNA, which can then be transported to the site of protein synthesis where they are translated.

What is Phenotype?

Phenotypes are the expression of a particular genotype that is produced via proteins coded by these genes. This process can get complicated. Even in unicellular bacteria there occur much genomic content, the genes in the code for thousands of proteins.

The genes may interact with each other or the environment to produce a particular phenotype. There is an interplay at the protein stage also, that regulates expression. Proteins are involved in the formation of various enzymes that catalyze a variety of reactions like cellular respiration required to synthesize energy.

Generally one gene codes for a particular protein. The amount and efficiency of proteins produced depend on the genotype that denotes the alleles present. Some of these genotypes may code for functional proteins, while other forms may code for less efficient proteins or in some cases may not code for protein at all. If the trait follows Mendelian inheritance and is regulated by 1 protein.

Then, the genotype for that particular protein will determine if the normal protein is expressed or dysfunctional. For example, people having cystic fibrosis (CF), show a mutation in the gene that codes for a protein involved in the transportation of chlorine and salt across membranes.

If the genotype involves a mutant type that is non-functional then the protein formed is dysfunctional. Since the protein involved is not expressed properly it will result in many health problems.

Phenotype Examples

i. Melanin

Melanin is a pigment that gives dark color to tissues. The different concentrations of melanin in eyes, hair, and skin gives different phenotypes. There are a few genes that are involved in the synthesis of melanin while others are involved in its regulation and distribution. Condition in which no melanin production occurs is known as albinism.

People who have this condition irrespective of their race shown no pigment, their skin and hair are white while their eyes are a shade of pink. Albinism occurs due to dysfunctional genes and is present in the gene pool of a population at low frequency. The mutation rate may be low, but the trait gets passed through reproduction.

Melanin pigment protects against harmful UV radiation. Albinism increases the vulnerability to various eye and skin cancers and is observed in only a small percentage of the population. The pathways of melanin production are similar in humans and animals. So animals also do experience albinism as melanin is a common pigment in mammals.

There may be other pigments present in the case of other groups and their mutation will similarly affect the phenotype pattern. If the mutations are advantageous then they get selected in the case of animals adapted to cold regions that show partial albinism.

ii. Mendel’s Peas

Gregor Mendel laid the foundations for some of the basic concepts in genetics. He selected peas to study the phenotypes and the phenotypic ratio of the offspring. He conducted monohybrid crosses that revealed that a factor has certain contrasting or alternating forms known as alleles. When he crosses yellow peas with green peas, the progeny would be either yellow or would contain show both phenotypes in the offspring.

The gene codes for the yellow pigment in the pea plant. In the absence of this pigment, the pea appears green in color due to the presence of chloroplasts. For each gene, organisms receive one allele each from their parent. One copy of a dominant allele is sufficient to produce the phenotype.

The green allele is recessive and would require the genotype to be recessive homozygous for expression o this trait. A Punnett square helps in depicting the cross of 2 individuals for a given trait.