Dec 7, 2015

Continuously Varying Trait

A continuously varying trait is a heritable characteristic that exhibits a range of phenotypic variation within a population, as opposed to discrete traits that are either present or absent. Examples of continuously varying traits include height, weight, skin color, and eye color. These traits are influenced by multiple genes and environmental factors, and their expression can be affected by interactions between genes and the environment. The study of continuously varying traits is important in fields such as genetics, evolutionary biology, and ecology, as it provides insights into the mechanisms underlying the variation and adaptation of organisms to their environments.

 

The term ‘variation’ describes the characteristics shown by organisms belonging to the same natural population or species. A study of the phenotypic differences in any large population shows that two forms of variation occur, discontinuous and continuous.

 

Discontinuous Variation

Variation in this case produces individuals showing clear cut differences with no intermediates between them, such as ABO blood groups in humans, pea seeds round or wrinkle, wing length in Drosophila and sex in animals and plants. Characteristics showing discontinuous variation are usually controlled by one or two major genes which may have two or more allelic forms and their phenotypic expression is relatively unaffected by environmental conditions. Since the phenotype variation is restricted to certain clear cut characteristics, this form of variation is also known as qualitative inheritance.

 

Continuous Variation

Many characteristics in a population show a complete gradation from one extreme to the other without any break. For example traits like height, weight, intelligence and skin color in humans and grain color. Continuous variation is also known as quantitative inheritance and its genetic basis are the polygons or polygenic inheritance.

 

Polygenic Inheritance

Polygenic inheritance (Gk. Polys, many, and L. genets, producing) occurs when one trait is governed by several genes occupying different loci on the same homologous pair of chromosomes or on different homologous pairs of chromosomes. Each gene has a contributing and non-contributing allele. The contributing allele is represented by a capital letter and the noncontributing allele is represented by a small letter. Each contributing allele has a quantitative effect on the phenotype and therefore the allelic effects are additive.

 

For example, H. Nilsson Ehle studied the inheritance of seed color in wheat. Genes at three different loci determine seed color. After he crossed plants that produced white and dark red seeds, the F1 plants were allowed to self-pollinate. Each of the F2 produced seeds having any one of the following seven colors.

 

The alleles responsible for the color of the seeds are represented by dots in the diamond shapes. The number of capitals is represented by coloring in the dots. Any capital has the same effect on the color of the seed. Notice that each capital (contributing allele) has a small but equal quantitative effect and that this accounts for the various degrees of color and the observed range in F2 phenotypes.

 

In polygenic inheritance, each contributing allele adds to the phenotype. A bell-shaped curve of phenotypes is also due to environmental effects.


Polygenic Inheritance


Human skin color: It is a quantitative trait which is controlled by three to six gene pairs. The greater the number of pigment specifying genes, the darker the skin. Exposure to sun can affect the skin color and increase the number of phenotypic variations.

 

Human Height: This trait is controlled by many pairs of genes at different loci. There is a wide range in variation of human height. Tallness is recessive to shortness. If there is more dominant alleles in the genotype, shorten will be the individual and if there is more recessive allele in the genotype taller will be the person. Environment and nutrition can effect height resulting in many more phenotypes.




Histogram

The bar chart in the figure plots the proportions of the phenotypes for wheat seed color, human skin color. The vertical bars that are the shortest represent categories with least number of dominant ge.ne. The bar that is the tallest represents the category with greatest number of dominant genes. The reverse is in the case of human height as tallness is recessive to shortness. If a graph line is drawn all around the bar a bell shaped curve is obtained. Such curves are typical of populations that show continuous variation in a trait.

 

Frequency histograms illustrate variations. A frequency histogram is a simple gram. The horizontal or X axis indicates the range of different phenotypes of a trait within a population. The vertical or Y axis indicates the number of individuals or their percentage in the population as seen in the figure.

 

The tongue rolling ability is due to a single dominant gene. It is a discontinuous variation inherited in simple Mendelian fashion. Its frequency diagram forms asymmetric distribution curve, with much greater frequency of phenotype at one end than the other.

 

Human height is a continuously varying trait. Its frequency histogram forms a smooth bell shaped normal distribution curve.