Quantitative Genetics- An Overview

Introduction

Quantitative genetics focuses on heredity in a group of individuals for traits determined by many genes simultaneously. It focuses on the relationship between parents and offspring. The phenotype of the offspring may be determined by the phenotype of the parent but not the genotype. Many traits in humans and other organisms are genetically influenced but do not show single gene patterns of inheritance. They are influenced by the combined action of many genes and are characterized by continuous variation. These are called polygenic traits. Continuously variable characteristics that are both polygenic and influenced by environmental factors are called multifactorial traits, for example, height, intelligence, hair color, etc.

What Are the Types of Quantitative Traits?

There are mainly three types of quantitative traits:

1. Continuous Traits - Most phenotypic variation in the population is not manifested in a few easily distinguishable categories. Instead, the traits vary continuously from one phenotypic extreme to another, for example, human height, milk production in cattle, the growth rate in poultry, yield in corn, and blood pressure in human beings. These traits are called continuous traits because there is a continuous gradation from one phenotype to the next.

2. Meristemic Traits - In this type of quantitative trait the phenotype is determined by counting. For example, the number of skin ridges forming fingerprints, the number of kernels on an ear of corn, the number of eggs laid by a hen, the number of bristles on the abdomen of a fly, and the number of puppies in a litter.

3. Threshold Traits - these are those quantitative traits that have only two or a few phenotypic classes but their inheritance is determined by the effects of multiple genes together with the environment. For example, threshold including twinning in cattle as well as parthenogenesis in turkeys. In many threshold traits disorders, the phenotypic classes are ‘affected’ versus ‘not affected’, for example, diabetes, schizophrenia, and congenital abnormalities like spina bifida.

What Are the Genetic Factors that Affect Quantitative Traits?

The following are the genetic factors that can affect quantitative traits-

• Additive Gene Action - Each type of allele has a specific metric value that is added to the phenotype. It is the most important type of gene action that controls quantitative traits.

• Dominant Gene Action - Homozygous dominant and heterozygous genotypes contribute the same to the phenotype.

• Epistatic Gene Action - It is the interaction of two genes controlling a trait.

What Is the Difference Between Qualitative and Quantitative Genetics?

Qualitative Genetics

• It is also called monogenic.

• It deals with the inheritance traits of a kind, for example, form, structure, color, etc.

• Discrete phenotypic classes occur that display discontinuous variations.

• The phenotypic expression of the gene is not influenced by the environment.

• It is concerned with individual mating and their progeny.

• The analysis is made by counts and ratios

Quantitative Genetics

• It is also called polygenic.

• It deals with the inheritance traits of a degree, for example, height, weight, etc.

• The spectrum of phenotypic classes occurs which contains continuous variation.

• Environmental conditions affect the phenotypic expression of polygenes.

• The analysis is made by statistical method.

What Are the Characteristics of Quantitative Inheritance?

The inheritance of quantitative traits is knowns as a quantitative inheritance. It is also known as multiple-factor inheritance or blending inheritance. The characteristics of quantitative inheritance are as follows:

• The segregation phenomenon occurs at an indefinitely large number of gene loci.

• If a substitution of an allele occurs in a gene locus then such allelic substitutions have trivial effects.

• The genes for multiple traits have different biochemical functions but similar phenotypic effects, therefore, the phenotypic effects of gene substitutions are interchangeable.

• Phenotypic expressions are subject to modification by changes in the environment.

• Natural populations show a considerable genetic variation in every quantitative trait.

• Polygenes also have pleiotropic effects, which means, one gene can modify or suppress more than one trait.

What Are the Principles of Quantitative Traits Inheritance?

The principles of quantitative traits inheritance are as follows:

• Quantitative traits are influenced by the combined effects of numerous genes hence these are called polygenic or multifactorial traits.

• The genes usually follow Mendelian laws of inheritance however multifactorial traits can have numerous other possible phenotypic categories.

• Environment influences the phenotypic differences between adjacent genotypes.

What Are the Variance Components of a Quantitative Trait?

The metric value (phenotypic value) for a specific individual is the result of genetic factors, environmental factors, and the environmental factors that interact with the genetic factors. The sum of these factors in a population of individuals segregating for a quantitative trait contributes to the variance of that population. The total variance can be partitioned like VP=VG+VE+VGE, where -

• VP is the total phenotypic variation of the segregated population.

• VG is the genetic variation that contributes to the total phenotypic variation.

• VE is the environmental contribution to the total phenotypic variation.

• VGE is the variation associated with the genetic and environmental factor interactions.

What Is the Role of Environment in Quantitative Genetics?

Quantitative characters are considerably affected by the environment. The main result of this effect is that the relationship between genotype and phenotype is partially or completely hidden, that is, the phenotype does not reveal the genotype. For example, phenotype= genotype + environment (P=G+E). If the phenotype is 0 then the phenotype will be equal to the genotype. However, the effect of the environment is seldom zero. So phenotype is the joint action of genotype and environment. The effectiveness of selection depends on the proportion of phenotype due to the genotype. Therefore, it is important to know the extent to which the environment influences different quantitative characters. To estimate the effect of the environment on a character, a large number of strain/genotypes are grown in a replicated trial, and the data is subjected to analysis of variance as per the experimental design used. The genotype and environmental interaction signify that the relative performance of various genotypes is affected by the environment. For example, the performance of genotype ‘A’ may be superior to genotype ‘B’ in one environment but in another environment inferior to that of ‘B’. If genotype and environment interaction is absent, genotype ‘A’ will be superior to genotype ‘B’ in all the environments.

Why Is Quantitative Genetics Important?

The importance of quantitative genetics is as follows -

• It helps to identify different disease variations.

• It helps to understand complex disorders that are caused by multiple genetic and environmental factors.

• It helps to differentiate between the genetic and environmental causes of a particular condition.

• It provides prevention methods and genetically tailored treatment options.

Conclusion

Quantitative genetic variation has traditionally been studied with the use of statistical analyses to partition the components of phenotypic variation, usually without the knowledge of the genes involved. However, recent developments in genomic technology determine that it is increasingly possible to identify the specific genes that underlie the variation in quantitative traits.