Question

A dihybrid plant on self-pollination produced 400 phenotypes with 9 types of genotype. How many seeds will have genotype TtRr?
A) 200
B) 100
C) 50
D) 150

Answer 1

Cross of dihybrid means cross between two different genes that differ in two observed traits. Di indicates there are two different traits (e.g. R and T) and hybrid means that each trait has two different alleles (e.g. R and r or T And t).

Complete answer:
The self-pollination of a dihybrid plant yields.
If F1 generation is allowed to self-pollinate.

TtRr × TtRr
Possible gametes are TR, Tr, Tr, tr.

| TR| Tr| tR| tr
---|---|---|---|---
TR| TTRR| TTRr| TtRR| TtRr
Tr| TTRr| TTrr| TtRr| Ttrr
tR| TtRR| TtRr| ttRR| ttRr
tr| TtRr| Ttrr| ttRr| ttrr

The genotypic ratio is 1TTRR: 2TTRr: 1TTrr: 2TtRR: 4TtRr: 2Ttrr: 1ttRR: 2ttRr: 1ttrr.
Hence, out of the total 400 phenotypes, the number of seeds having genotype TtRr is $\dfrac{4}{16} \times 400 = 100$.

Additional information: Phenotype is defined as the observable characteristics of an organism which is determined by genetic make-up. While genotype refers to the genetic characteristics of an individual. The dihybrid cross helps us to identify the pattern of two different traits at same time.

Hence the correct answer is option, B.

Note: The Mendel stated that each inherited trait is defined by a gene pair and there was a relationship between completely dominant-recessive traits. The dihybrid ratio was 9:3:3:1.
The inheritance of one trait is not dependent on the inheritance of the other trait.