Polygenic Traits and Quantitative Inheritance: Mendel Prospective

Let us make an in-depth study of the polygenic traits and quantitative inheritance.

Mendel believed that a factor or gene is responsible for the expression of one character. This factor or gene has two alternate forms or alleles.

The flower colour in his peas were either red or white, the seeds either yellow or green, either smooth or wrinkled etc.

These qualitative characters permitted Mendel to make accurate counts of individuals showing one or the other of these alternative traits. For instance, if tall and dwarf plants were crossed, in F₁ only tall plants and in F₂, only tall and dwarf plants were obtained. No plants of intermediate types were ever obtained. Such type of inheritance based on Mendelian ratio is known as qualitative inheritance.

On the other hand, when two or more genes at different loci are responsible for the expression of one character, such genes are known as polygenes or multiple genes and the character or trait they produce is called a polygenic trait.

The inheritance pattern involving multiple genes is known as polygenic or quantitative inheritance. The quantitative traits in man include its height, weight, intelligence and colour and those of plants include their size, shape, number of seeds and fruits etc.

The quantitative traits are also called metric traits. In quantitative inheritance, the offspring’s do not show clear cut differences among them, rather show intermediate character of two parents. The gradations in characters are determined by a number of genes and all the genes have additive effect or are cumulative. It means that each gene has a certain amount of effect, and more is the number of dominant genes, the more is the degree of expression of the character.

Nilsson-Ehle (1908) made a very interesting study on the colour of grain in wheat. He crossed red kerneled variety with white kerneled strain and obtained F₁ plants whose grains were uniformly red but intermediate between red and white parental generations. When the members of F₁ were self-crossed five different phenotypic classes appeared in F₂ in the ratio of 1: 4: 6: 4: 1.

(i) Red (extreme) – 1/16

(ii) Dark red – 4/16

(iii) Medium red – 6/16

(iv) Light red – 4/16

(v) White – 1/16

Later it was found that the kernel colour in wheat is determined by two pairs of genes Aa and Bb. Genes A and B are dominant genes which determine the red colour whereas a and b are recessive alleles. Results of this polygenic inheritance is depicted in Fig. 5.8.

Additional Information:

Pleiotropy:

From our study of Mendel’s laws, we have learnt that one factor or one gene is responsible for the expression of one character. However, there are also such cases where one gene influences more than one phenotypic character. Such a gene, besides producing its character (i.e., major effect), also affects other characters (i.e., secondary effect) of the body. Thus, the ability of a gene to have many effects is known as pleiotropy.

The common example of pleiotropy in man is a hereditary disease called sickle-cell anaemia or cooley’s anemia found among certain African tribes. A recessive gene causes this disease. In homozygous condition the gene causes production of an abnormal hemoglobin. As a result the shape of the red blood cell containing it becomes sickle shaped and distorted.

However, in heterozygous condition, the individuals possess both normal and abnormal hemoglobin and have mild anemia. In Drosophila gene responsible for the size of wing also affects the eye colour, shape of spermatheca, and position of dorsal bristles.

Analyze the following concept & answer the questions:

The Punnett square shown below represents the pattern of inheritance in di-hybrid cross when yellow (Y)is dominant over white (y) and round (R) is dominant over wrinkled (r) seeds.

Question 1:

A plant of type ‘M’ will produce seeds with the genotype identical to seeds produced by the plants of-

(a) Type A

(b) Type J

(c) Type P

(d) Type L

Ans: Type L

Question 2:

What will be the % of “G”?

Ans: 25%

Question 3:

What is the genotype of “F” in above checker board?

Ans: YyRr

Question 4:

Find out the % of “yyrr” if the cross is made between G x F?

Ans: 6.25%

Question 5:

If the cross is made between H x D then the genotypic ratio is

Ans: 1:1:1:1

Question 6:

If the cross is made between “G x F” then it produces:

(i) How many types of phenotypes?

(ii) How many types of genotypes?

(iii) What will be the phenotypic ratio?

(iv) What will be phenotypic ratio?

Ans:

(i) Four types of Phenotypes

(ii) Nine types of genotypes

(iii) 9:3:3:1

(iv) 1:1:1:1:4:2:2:2:2

Question 7:

If the total number of progeny obtained through di-hybrid cross of Mendel is 1280 in F2 generation. How many are recombinants?

Ans: 480