Question

In ${ F }_{ 2 }$ generation quantitative inheritance of 1:4:6:4:1 is obtained instead of
(a)9:3:3:1
(b)8:6:4:1
(c)7:4:1:4
(d)6:6:4:7

Answer 1

Quantitative inheritance is another word used to describe polygenic inheritance. Inheritance of skin color of human beings is an example of polygenic inheritance. Polygenic inheritance is defined as the inheritance in which there are multiple genes responsible for the expression of one phenotype.

Complete answer:
In polygenic inheritance, as multiple genes are controlling a single phenotype, there is a range of phenotypes present instead of the normal two phenotypes expressed by dominant and recessive alleles. For example, in the case of the plant height of the pea plant, there are only two phenotypes possible either tall or dwarf but in the case of the height of trees which is a type of polygenic inheritance we find an entire range of heights for trees. Similarly, in the ${ F }_{ 2 }$ generation of quantitative inheritance a range of phenotypes are possible as shown in the ratio 1:4:6:4:1 instead of the normal ratio of 9:3:3:1.

Additional Information:
Let us look at the polygenic inheritance of skin color in detail.
-The phenotype of skin color is controlled by three genes whose dominant allele is denoted by capital letters A, B, and C, and recessive alleles are represented by small letters a, b, and c. The dominant alleles cause more pigmentation which means more melanin production in the body whereas the recessive alleles induce low melanin production.
-The genotype which has all dominant alleles AABBCC, the phenotype will be very dark skin due to the highest amount of melanin. Whereas the genotype with all recessive alleles aabbcc will have the lightest skin color due to the lowest amount of melanin.
-Each dominant allele increases the pigmentation by one unit, thus there are a number of intermediate phenotypes with a range of skin shades.
-A person with half dominant alleles and half recessive alleles will have a medium amount of melanin and intermediate skin color. Such a genotype is known as mulatto.
So, the correct option is ‘9:3:3:1’.

Note: -The ratio of 9:3:3:1 is the phenotypic ratio seen in the ${ F }_{ 2 }$ generation of the dihybrid cross.
-A dihybrid cross is known as a cross in which the inheritance pattern of two characters is studied together for example seed color and seed shape were studied together by Mendel.
-The observations of the dihybrid cross resulted in the third law of Mendel also known as the Law of Independent Assortment.