Phenotype- Definition, Expression, Types, Examples, Significance

The whole set of characteristics that an organism exhibits, as determined by the expression of its genes, as well as by environmental influences and random variation, is known as its phenotype.

The physical, biochemical, physiological, and behavioral characteristics of an organism make up its phenotype.
This could be an observable characteristic such as a disease, blood type, protein variant, or other.
Due to environmental changes, as well as the physiological and morphological changes brought on by aging, the phenotype of an individual may vary continuously throughout their lifetime.
The method and accuracy of observation have a significant impact on phenotype.
The following relationship has frequently been used to illustrate how components interact:

Genotype + Environment + Random variation → Phenotype

Etymology

The word “phenotype” is adapted from the German word phänotypus, based on the Greek word phaino, meaning “shining” or “appearance.”
Wilhelm Ludvig Johannsen developed the genotype-phenotype concept in 1909 to distinguish between hereditary and environmental variations.
He argues that whereas a person’s phenotype represents the observable structural and functional characteristics that are formed by the interaction between genotype and environment, the genotype of an individual represents the totality of hereditary factors.
This distinction resulted from Johannsen’s research on heritable variation in plants, which also had an impact on his pure-line theory of heredity.
With the increased knowledge of epigenetic, environmental, and molecular alterations, the word phenotype has evolved and has gained multiple definitions.

Table of Contents

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Expression of Phenotype

Three major factors contribute to the expression of the phenotype. They are:

Genetic factors
Environmental influences
Random genetic variations

1. Genetic factors

A trait is said to be homozygous if the two alleles are made up of the same genes, e.g. AA or aa.
A trait is considered heterozygous if the two alleles consist of different genes, e.g. Aa.
The expression of the trait (A) depends on the presence of the dominant allele, like AA or Aa, whereas the expression of the recessive trait (a) depends on the lack of the dominant allele, like aa.

This is a case of complete dominance, and it follows Mendelian inheritance.

The expression of a trait will not follow the Mendelian pattern in cases of codominance, incomplete dominance, and polygenic inheritance.

Since both alleles in a pair are dominant (e.g. AB) in codominance, the alleles of a gene pair in a heterozygote will be fully expressed.

The resulting phenotype in incomplete dominance will be an intermediate of the two alleles’ effects. This is due to the incomplete expression of the dominant allele.

As a result, the phenotype of the heterozygous child will be intermediate to that of the parents.

2. Environmental Influences

An organism’s appearance may change due to exposure to environmental factors such as nutrition, temperature, oxygen levels, humidity, light cycles, and mutagens.
Even identical organisms can exhibit diverse phenotypes when subjected to well-controlled lab conditions.
Environmental factors can sometimes have a greater impact on phenotypes than genetic factors. So a phenotype and genotype may or may not be directly related.
Light-colored skin frequently exposed to the sun will darken due to increased melanin production.
The temperature impacts the color of a Siamese cat’s coat. Siamese cats with lighter coats tend to reside in warmer climates than those with darker coats.

3. Genetic Variation

The fundamental source of genetic variation is mutation.
A single-base alteration, also known as a single-nucleotide polymorphism, is a change in a single DNA base and is the most frequent type of variant.
The mutation may be a missense, nonsense, or frameshift.
The random genetic variation could alter the physical trait or at least the fitness of an organism.
Single-gene pathogenic variants frequently have several phenotypic effects on several organ systems. A variant in the VHL gene2 might have effects on the reproductive system, pancreas, brain, kidney, and retina.
Changes in genes are vital as they enable evolution and natural selection.

What is Extreme Phenotype?

When the parents’ alleles combine, a hybrid is produced with a phenotype that is more or higher than the phenotypes of its parents. This is known as an extreme phenotype.
The formation of extreme phenotypes is known as transgressive segregation.
Depending on how the transgressive phenotype impacts the overall fitness of the offspring, it may be useful or damaging.
Extreme phenotype can be observed in the offspring from a cross between Helianthus annus and Helianthus petiolaris.
The two sunflower species can produce hybrids that possess transgressive traits.
In contrast to their parents, hybrids can flourish in environments where their parents fail.
They can thrive in salt marshes and sand dunes.

What is Recombinant Phenotype?

One of the crucial biological processes that contribute to more variation in the phenotypes of organisms is meiosis.
The homologous chromosomes come together, particularly during the metaphase of meiosis I, to swap genes through homologous recombination.
The four daughter cells will have distinct chromosomes as the homologous chromosomes reach the end of meiosis (telophase II).
Some of them develop into gametes with recombinant genes.
If such a gamete is fertilized by a wild type, it will produce offspring with a recombinant phenotype, which is distinct from that of either of its parents.

What is the Phenotypic Ratio?

A Punnett square is a method that can be used to determine all potential allelic pairings in a test cross.
It is an illustration of alleles using grids and letters.
Both the genotypic ratio and the phenotypic ratio can be determined using the Punnett square.
The phenotypes of the offspring, i.e., the frequency of offspring exhibiting certain traits or combinations of traits, can be used to determine the phenotypic ratio.

In a dihybrid cross AaBb x aabb, where

Body color – A for blue and a for black
Wing morphology – B for normal wings and b for vestigial wings

The expected phenotypic ratio is 1:1:1:1, based on the four possible different phenotypes:

AaBb (blue, normal-winged fly)
aaBb (black, normal-winged fly)
Aabb (blue, vestigial-winged fly)
aabb (black, vestigial-winged fly)

Parents	AaBb	aabb
Gametes	AB, Ab, aB, ab	ab

AaBb	aaBb	Aabb	aabb
1	1	1	1

Significance of Phenotype data

The pathogenic problems in plants, animals, and other species can be identified, described, and diagnosed using phenotype data.
Understanding of an organism’s growth, purposes, evolution, and interactions with the environment can be improved through observations of its phenotypes.
The study of uncovered phenotypes can lead to the development of effective microrobots and antimicrobial materials, new methods of medicine delivery, treatments for the adverse effects of aging, and enhancements to crop qualities.
Similarly, disease phenotypes investigations into their genomic and environmental origins frequently involve the modification of model organisms and/or the examination of wild populations and ancestors, particularly in the case of plants.

References

Biology Online. (2022). Phenotype. Accessed from: https://www.biologyonline.com/dictionary/phenotype
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Inspirit. (2022). Environmental Effects on Phenotype. Accessed from: https://www.inspiritvr.com/ap-bio/heredity/environmental-effects-on-phenotype-study-guide
IU School of Medicine. (2022). What is a Phenotype? Accessed from: https://medicine.iu.edu/research-centers/musculoskeletal/clinical-service-cores/informatics-methodology/what-is-a-phenotype
National Human Genome Research Institute (NIH). (2022). Phenotype. Accessed from: https://www.genome.gov/genetics-glossary/Phenotype
National Library of Medicine (NIH). (2017). Understanding Genetic Variance and Phenotype Expression. Available from: https://www.ncbi.nlm.nih.gov/books/NBK425811/
Passarge E. (2001). Phenotype and Genotype. In Color Atlas of Genetics. Second Edition. Thieme Stuttgart. New York, pg. 138
Peirson E. (2012). Wilhelm Johannsen’s Genotype-Phenotype Distinction. Accessed from: https://embryo.asu.edu/pages/wilhelm-johannsens-genotype-phenotype-distinction
Verma P.S. and Agarwal V.K. (2005). Genetics, Human Genetics and Eugenics: Introduction. In Cell Biology, Genetics, Molecular Biology, Evolution and Ecology. Multi-color Edition. S. Chand & Company Ltd. Ram nagar, New Delhi, pg 3-9
Vocabulary.com. (2022). Phenotype. Accessed from: https://www.vocabulary.com/dictionary/phenotype