Question

A pure tall and a pure dwarf plant were crossed to produce offsprings. Offsprings were self crossed, then find out the ratio between true breeding tall to true breeding dwarf plant?

• A. 1 : 1
• B. 3 : 1
• C. 2 : 1
• D. 1 : 2 : 1

Answer 1

The correct option is A 1 : 1

A true breeding plant is always homozygous, i.e., it carries the two copies of same allele of a particular gene.

Let's suppose that tall allele is represented by T and that for dwarf is represented by t. Hence, the genotype of pure tall plant should be "TT" and that of pure dwarf plant should be "tt". The pure tall plant (TT) will produce only one type of gametes all having one copy of "T" allele. Similarly, the dwarf plant (tt) will produce only one type of gametes all having one copy of "t" allele. Hence, the cross between pure tall and a pure dwarf plant will produce all tall plants (Tt).

This heterozygous tall plant produces two types of gametes, half carrying "T" and other half with "t".

Selfing of the heterozygous tall plants (Tt) produce tall and dwarf plants in 3 : 1 ratio. However, the genotypic ratio comes to be 1 TT : 2Tt : 1 tt. Hence, ratio between true breeding tall to true breeding dwarf plants is 1 (TT) : 1 (tt). The correct option is A.