Physical appearances of individuals are determined by their genotypic ratio and chromosomal inheritance. This phenomenon was put forth by Mendel, and is popularly known as Mendel's Laws of Inheritance. In case of polygenic traits, the characteristic features of individual result from interaction of different genes. In layman's language, we can exemplify with an individual having high blood pressure.
High blood pressure does not result due to activity of a single gene. This particular phenotype is the net result of the interaction of cholesterol, obesity, transporter, and addictive genes. The combined effect of these multiple genes or polygenes is the reason behind high blood pressure.
Genetic Phenomenon of Polygenic Traits
Polygenic traits are not only observed in human beings, but are also present in animals, birds, insects, flies, etc. In terms of human genetics, these traits are defined as inherited characteristics when two or more genes are involved in determining the phenotype of an individual. Thus, the cumulative effects of genes are responsible for determining many traits, like weight, height, shape, color, and metabolic rate.
- These traits do not follow Mendel's patterns of inheritance.
- They are recognized by their expressions that result from gradation of continuous variation.
- Additive effects of two or more separate pair of genes control the continuous variation.
- The traits are quantified by measuring the variation rather than counting.
- The phenotypic expressions vary in wider range as a result of contributing pairs of genes.
These traits are also known as quantitative or multifactorial traits. To understand these synonyms, you need to understand the mechanism of genetic inheritance. They are controlled by two or more genes at different loci on different chromosomes or it is the trait that is controlled by non-allelic genes. They are known as quantitative traits, as their phenotypic expression is dependent on multiple alleles located on different chromosomes.
The quantitative aspect (e.g., how tall or how short) or phenotype of the individual is represented as a trait value. When frequency of individuals with polygenic traits is plotted against the trait value, the graph is a bell-shaped curve. These traits are a result of additive effects of contribution of each gene in loci, and therefore they do not follow typical dominant and recessive patterns.
The second aspect of polygenic genes are that the traits are determined by environmental variations. It means that two or more individuals can be genetically same, but can differ in their physical appearance. Polygenic traits can be determined by knowing the number of quantitative trait loci that play an important role in determining the genetic architecture of an individual.
Examples in Humans
- Eye color
- Skin color
- Cleft palate
- Congenital heart disease
- Congenital dislocation of hip
- Pyloric stenosis
- Neural tube defects
- Diabetes mellitus
- Manic depression
- Ischaemic heart disease
- Spina bifida
Glossary of Terms
- Genes: Unit of heredity in a living organism.
- Chromosomes: Organized structure of DNA and protein that embeds genes.
- Alleles: Different forms of genes in a single genetic locus.
- Genetic Locus: Specific location of gene or DNA sequence in chromosome.
- Phenotype: Observable characteristics, morphology, and physiological properties of an organism
- Genotype: Information in genes that determines the phenotypic traits.
- Quantitative Trait Locus (QTL): Region of DNA found in different chromosomes, associated with a particular phenotypic trait.