1. Polygenic Traits – Quantitative Inheritance

Phenotypic traits are of two kinds: qualitative and quantitative.

Qualitative traits. These are the classical Mendelian traits which have two contrasting conditions controlled by a single pair of genes. These traits may pertain to form (smooth and wrinkled pea seeds), size (tall and dwarf pea plants), colour (black and white guinea pigs), structure (horned and hornless cattle), and so on.

A qualitative trait is fully expressed by a single dominant gene. The presence of two dominant genes does not modify the phenotype. For example, the genotype Bb and BB produce equally black coat colour in guinea pigs. The qualitative traits, being controlled by single genes (called monogenes) are also termed monogenic traits. The inheritance of qualitative traits is known as qualitative or monogenic inheritance. This inheritance produces two distinct phenotypes, e.g., black or white coat colour, thereby showing discontinuous variation.

Quantitative traits. These are the measurable phenotypic traits which do not have two distinct contrasting conditions. Instead, they show a wide spectrum of phenotypes which imperceptably blend from one condition to another as continuous variation. They are usually controlled by more than one pair of genes, and may be modified by environmental factors. For example, light or dark human skin can become lighter or darker depending upon exposure to the sun. The quantitative traits are very common, and include certain human characters such as height, weight, skin colour, hair colour, size of some organs, face form, intelligence, susceptibility to diseases, etc. and many commercially important characters of animals and plants, e.g., milk yield in cattle, meat yield in pigs, egg production in poultry, amount of flowers, size of fruits, seed number and size in beans, grain colour in wheat, cod length in corn, ets. The quantitative traits are also known as metric traits because they can be measured and expressed in terms of units of length, weight and number.

A single phenotypic trait governed by more then one pair of genes is called a polygenic trait. Because of many possible genotypes, polygenic traits show a wide range of phenotypes.

Polygenes (Multiple or cumulative Genes). The quantitative traits, as mentioned above, are governed by several pairs of genes. These genes form a special gene complex known as a polygenic system contributes to the phenotype but to a small degree. Presence of more then one dominant gene makes the phenotype more prominant, It other words, all the dominant genes add up their effects to produce a full phenotype. Two or more pairs of nonallelic genes, which produce a cumulative (additive) effect on the same phenotypic quantitative character are called cumulative genes, or multiple genes, or polygenes. A polygene may also be defined as a gene which causes only a partial expression of a quantitative trait. The dominant polygenes which collectively produce a trait are called contributing alleles and the recessive one are termed non-contributing alleles. The polygenes may occupy two or more different loci on the same homologous chromosome pair or on different non-homologous chromosomes.

Quantitative Inheritance. The inheritance of quantitative traits, also called polygenie traits, is known as quantitative or multifactor or polygenic inheritance.

Polygenes (Multiple or Cumulative Genes). The quantitative traits, as mentioned above, are governed by several pairs of genes. These genes form а special gene complex known as а polygenic system. Each gene of а polygenic system con- tributes tо the phenotype but to а small degree. Presence of more than one dominant gene makes the phenotype more prominent. It other words, all the dominant genes add up their effects tо produce а full phenotype. Two or more pairs of non-allelic genes, which produce а cumulative effect on the same phenotypic quantitative character, are called cumulative genes, or multiple genes, or polygenes. А polygene may also be defined as а gene which causes only a partial expression of a quantitative trait. The dominant polygenes which collectively produce а trait are called contributing alleles and the recessive one are termed non-contributing alleles. The polygenes mау occupy two or more different loci on the same homologous chromosome pair or on different nonhomologous chromosomes.

Quantitative Inheritance. The inheritance of quantitative traits, also called polygenic traits, is known as quantitative or multifactor or polygenic inheritance, in this kind of inheritance, F₁ individuals are very similar to one another and are usually intermediate between the two parents. А cross between two F₁ individuals yields а widely variable F₂ generation having а few individuals like one grand-parent, а few like the other grandparent, and the rest ranging between the two. А good example of quantitative inheritance is seen m the human skin colour.

Human Skin Colour. Human skin color is believed to be controlled by at least three pairs of genes: Аа, Вb and Сс, located in different chromosomes. and inherited independently. The genes for dark color, А, В and С, are incompletely dominant, and the darkness of the skin colour is proportionate to the sum of the dominant genes present. А pure negro (very dark person) has 6 dominant genes (АА ВВ СС) for skin colour. А pure white (very light) person has б recessive genes (aa bb cc) for lack of skin colour.

A cross between а pure negro and а pure white produces F₁ hybrid offspring with the genotype Aa Вb Cc and the skin colour or phenotype mulatto, which is intermediate between the two parents. The F₁ hybrids form 8 kinds of gametes in each sex, giving 64 combinations F₂ generation, having 7 genotypes and as many phenotypes. The skin colour of F2 offspring varies according to the number of genes for pigmentation they inherit. Their skin colour ranges from pure black (АА ВВ СС) in 1/64 individuals through very dark-brown in 6/64 individuals, dark-brown in 15/64 individuals, mulatto or intermediate (Aa Bb Cc) in 20/64 individuals, light-brown in 15/64 individuals, and very light- brown in 6/64 individuals to pure white (aa bb сс) in 1/64 individuals.

Each dominant gene directs the synthesis of the same amount of the pigment melanin which gives colour to the skin Hence, the shade of the skin colour is directly proportional to the number of dominant genes present in the individual.

The above cross shows that it is possible for the parents heterozygous for skin colour to produce children with darker colors and also with lighter colors than themselves.

In wheat also, three different gene pairs control the colour of kernels from white to deep red.

The analysis of the pattern of inheritance of the polygenic traits is difficult because it is hard to delink the effects of the various interacting alleles which influence а simple phenotype. Environment may further complicate the matter. For example, human skin colour changes with age, use of cosmetics and environmental variation such as exposure to sunlight. Human height and intelligence are affected by nutrition and social background.