Question

Due to the cross between TTRr and ttrr the resultant progenies showed how many percent plants would be tall, red-flowered?
(a)50%
(b)75%
(c)25%
(d)100%

Answer 1

Hint: When two parents differing from each other in two or more pairs of contrasting characters are crossed, then the inheritance of one pair of characters is independent of the other pair of characters.
Mendel proposed “the law of independent assortment” which tells us that in a dihybrid cross, the segregation of one pair of traits is independent of the other.

Complete step-by-step answer:
Let the gene for the tall habit of a plant be represented by ‘T' and dwarf habit is represented by ‘t’. Let the gene for red-flowered be represented by ‘R' and that of white-flowered be represented by gene ‘r’. Then, the genotypes of the parents would be Heterozygous tall, red-flowered- TTRr and Homozygous dwarf, white-flowered – ttrr.
Phenotypes of parents – Tall red × Dwarf white
Genotypes of parents – TTRr × ttrr
Gametes – TR, Tr, tr, tr
F1 generation -

male/female	TR	Tr
tr	TtRrtall red	Ttrrtall white
tr	TtRrtall red	Ttrrtall white

Phenotypes of the F1 generation - tall red-2, tall white-2
Genotypes of the F1 generation - TtRr and Ttrr
So the percentage of tall red flowers in the cross between TTRr and ttrr is ‘50%’.
So the correct answer is 50%.
From the above results, it is obvious that the inheritance of character of tallness i.e. represented by ‘T’ is in no way linked with the red colour i.e. represented by ’R’ of the flower. Similarly, the character of dwarfness i.e. represented by ‘t’ is not linked with the white colour i.e. represented by ’r’ of the flower. This is due to the fact that in the above cross, the two pairs of characters segregate independently. In other words, there is an independent assortment of characters during inheritance.
Note: Law of independent assortment is not universally applicable because many genes are located on one chromosome i.e. they are lined. Thus, they pass through gametes in the form of a linkage group during crossing over. However, recombinations are caused due to the crossing over of genes that takes place during meiosis phase of cell division. Therefore, The law of independent assortment is applicable only for traits(one form of visible feature) which are present on different chromosomes. Thus the law of independent assortment is not universally applicable for all crosses.