Polygenic Traits

Physical appearance of individuals is determined by genotypic ratio and chromosomal inheritance. This phenomenon was put forth by Mendel, 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 single gene. This particular phenotype is the net result of interaction of cholesterol genes, obesity genes, transporter genes and addictive genes. The combined effect of these multiple genes or polygenes, is the reason behind high blood pressure. That was an elementary approach.

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, polygenic traits is defined as inherited characteristics when two or more genes are involved in determining the phenotype of individuals. Thus, the cumulative effects of genes are responsible for determining many traits like, weight, height, shape, color and metabolic rate of individuals.

• The 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 continuous variation.
• The traits are quantified by measuring the variation, rather than counting.
• The phenotypic expression vary in wider range as a result of contributing pairs of genes.

These traits are also known as quantitative traits 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 trait value. When frequency of individuals with polygenic traits is plotted against trait value, the graph is bell shaped. Polygenic traits are a result of additive effects of contribution of each genes in loci and therefore they do not follow typical dominance and recessive patterns. The second aspect of polygenic genes are, the traits are determined by environmental variations. It means that an individual can be genetically same, but can differ in their physical appearance, while rest are congenial disorders. Polygenic traits can be determined by knowing the number of quantitative trait loci, that plays an important role in determining the genetic architecture of an individual.

Examples in Humans

• Weight
• Height
• Eye color
• Intelligence
• Behavior
• Skin color

Polygenic traits examples with disorders in genetic components are:

• Cleft palate
• Cancer
• Diabetes
• Autism
• Congenital heart disease
• Congenital dislocation of hip
• Talipes
• Pyloric stenosis
• Neural tube defects
• Schizophrenia
• Diabetes Mellitus
• Glaucoma
• Hypertension
• Manic depression
• Ischaemic heart disease
• Maniac depression
• Eczema
• Dermatitis
• Spina bifida
• Anencephaly

Polygenic traits are an important genetic phenomenon, governing different variations and characteristic in individuals. Polygenic traits in humans are interesting to study and observe.

NOTE
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, 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.