Quantitative genetics The phenotypic traits of the different

Quantitative genetics The phenotypic traits of the different organisms may be of two kinds, viz. , qualitative and quantitative. The quali tative traits are the classical Mendelian traits of kinds such as form (e. g. , round or wrinkled 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. 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, shape, skin pigmentation, metabolic activity, reproductive rate, behaviour, eye-facet or bristle number in Drosophila, susceptibility to pathological diseases or intelligence in man; amount of flowers, fruits, seeds, milk, meat or egg produced by plants or animals, etc. Economically important traits such as body weight gains, mature plant heights, egg or milk production records

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 100 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 nonallelic and affect the phenotype of a single quantitative trait, are called polygenes or cumulative genes. The inheritance of polygenes or quantitative traits is called quantitative. 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 1 -. Characters of kind. 1. Characters of degree 2 -Discontinuous variation; phenotypic class. distinct 2. Continuous variations; phenotypic form a spectrum. 3. Single gene effects. 4 -Concerned with individual matings and their progeny. 3. Polygenic control; effects of single genes too slight to be detected 4. Concerned with population of organisms consisting of all possible kinds of matings. 5. Statistical analyses give estimates 5. Analyzed by making counts and ratios of population parameters such as the mean and standard deviation.

CHARACTERISTICS OF MULTIPLE GENES Multiple genes for quantitative traits have following characteristics: 1 - Each contributing allele in the series of multiple genes produces an equal effect 2 - Effects of each contributing allele are cumulative or additive 3 - There is no dominance 4 -There is no epistasis (masking of the phenotypes(among genes at different loci. 5 - There is no linkage involved. 6 -The environmental conditions have considerable effect on the phenotypic expression of polygenes for the quantitative traits.

EXAMPLES OF QUANTITATIVE INHERITANCE 1. Kernel Colour in Wheat A whole grain or seed of a cereal plant such as corn, wheat, barley, etc. , is called kernel. Kernel colour in wheat is a quantitative trait and its inheritance was studied by Swedish geneticist When he crossed a certain red strain to a white strain, he observed that the F 1 was all light red and that approximately 1/16 of the F 2 was as extreme as the parents, i. e. , 1/16 was white and 1/16 was red. He interpreted these results in terms of two genes, each with a pair of alleles exhibiting cumulative effect P : Red kernel × White kernel R 1 R 1 R 2 R 2 r 1 r 2 F 1 : Light red R 1 r 1 R 2 r 2

Each of the contributing alleles R 1 or R 2 adds some red to the phenotype of kernel colour, so that the genotypes of whites contain neither of these alleles and a red genotype contains only R 1 and R 2 alleles. histograms in Here five phenotypic classes are obtained in F 2; each ‘dose’ of a contributing allele for pigment production increases depth of colour.

F 2 : Summary of checker board derived results, i. e. , F 2 genotypic and phenotypic ratios Genotype Genotypic ratio R 1 R 2 1 Number of contributing alleles 4 Phenotype ratio red 1 Medium red 4 Light red 6 1516 R 1 R 2 r 2 2 R 1 r 1 R 2 2 3 R 1 r 1 R 2 r 2 4 R 1 r 2 1 2 r 1 R 2 R 1 r 2 r 2 1 2 Very light red r 1 R 2 r 2 2 1 r 1 r 2 1 0 White 1 { 116} colorless Color red 4

2 - Skin Colour in Man Another classical example of polygenic inheritance was given by Davenport (1913) in Jamaica. He found that two pairs of genes, A-a and B-b cause the difference in skin pigmentation between negro and caucasian people. These genes were found to affect the character in additive fashion. Thus, a true negro has four d o m i n a n t genes, AABB and a white has four recessive genes aabb. The F 1 offspring of mating of aabb with AABB, are all Aa. Bb and have an intermediate skin colour termed mulatto.

Negro White P 1 : AABB × aabb gametes : AB ↓ ab F 1 : Aa. Bb Mulattoes Intermediate skin colour Mulattoes Intercross : Aa Bb × Aa. Bb mating of two such mulattoes produces a wide variety of skin colour in the offspring, ranging from skins as dark as the original negro parent to as white as the original white parent.

Male AB Female AB AABB Ab a. B ab AABb Aa. BB Aa. Bb Like negro Darker than mulattoes like mulattoes Ab AABb AAbb Aa. Bb Aabb a. B Darker than mulattoes Aa. BB Like mulattoes Aa. Bb Like Lighter than mulattoes aa. BB aa. Bb Darker than mulattoes Like mulattoes Aa. Bb Aabb aa. Bb aabb Like mulattoes Lighter than white mulattoes ab

F 2=negro color (116) , Colour between mulattoes and negro(416) , color of mulattoes (616) , Colour between mulattoes and white (416), white skin color(116). SIGNIFICANCE OF QUANTITATIVE GENETICS Quantitative genetics has great agricultural importance and has helped in the increase of yield of various economically important crops. Q Which of the following human phenotypes would appear to be based on polygene inheritance : intelligence, absence of incisors, height, phenylketonuria, ability to taste phenylthiocarbamide, skin colour, eye colour ? Answer/ All traits showing practically continuous variation are likely to be due to polygenes. Here, the traits of intelligence, height, skin colour, and eye colour probably involve polygenes.