Quantitative genetics (Inheritance of Multiple Genes)

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Quantitative genetics (Inheritance of Multiple Genes) Quantitative Genetics Inheritance of Multiple Genes Certain Characteristics of Quantitative Inheritance

Quantitative genetics (Inheritance of Multiple Genes)

Quantitative genetics (Inheritance of Multiple Genes)
The phenotypic traits of the different organisms may be of two kinds, viz., qualitative and quantitative. The qualitative traits are the classical Mendelian traits of kinds such as form (e.g., round or wrinkle seeds of pea); structure (e.g., horned or hornless condition in cattles); pigments (e.g., black or white coat of guinea pigs); and antigens and antibodies (e.g., blood group types of man) and so on. We have already discussed in previous chapters that each qualitative trait may be under genetic control of two or many alleles of a single gene with little or no environmental modifications to obscure the gene effects. The organisms possessing qualitative traits have distinct (separate) phenotypic classes and are said to exhibit discontinuous variations.

The quantitative traits, however, are economically important measurable phenotypic traits of degree such as height, weight, skin pigmentation, susceptibility to pathological diseases or intelligence in man; amount of flowers, fruits, seeds, milk, meat or egg produced by plants or animals, etc. The quantitative traits are also called metric traits. They do not show clear cut differences between individuals and forms a spectrum of phenotypes which blend imperceptively from one type to another to cause continuous variations.In contrast to qualitative traits, the quantitative traits may be modified variously by the environmental conditions and are usually governed by many factors or genes (perhaps 10 or I00 or more), each contributing such a small amount of phenotype that their individual effects cannot be detected by Mendelian methods but by only statistical methods.

Such genes which are non-allelic and effect the phenotype of a single quantitative trait, are called polygenes or cumulative genes. The inheritance of poly genes or quantitative traits is called quantitative inheritance, multiple factor inheritance, multiple gene inheritance or polygenic inheritance. The genetical studies of qualitative and quantitative traits are called qualitative genetics and quantitative genetics, respectively. The major differences between the two are following :

Qualitative genetics	Quantitative genetics
It deals with the inheritance of traits of kind, viz., form, structure, colour, etc.	It deals with the inheritance of traits of degree, viz., heights of length, weight, number, etc.
Discrete phenotypic classes occur which display discontinuous variations.	A spectrum of phenotypic classes occur which contain continuous variations.
Each qualitative trait is governed by two or many alleles of a single gene.	Each quantitative trait is governed by many non-allelic genes or polygenes.
The phenotypic expression of a gene is not influenced by environment.	Environmental conditions effect the phenotypic expression of polygenes variously.
It concerns with individual matings and their progeny.	It concerns with a population of organisms consisting of all possible kinds of matings.
In it analysis is made by counts and ratios.	In it analysis is made by statistical methods.

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Examples of Quantitative Inheritance Calculating Number of Multiple Genes Transgressive Variation Modifying Factors Heritability

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