Question

What is dihybrid test cross ratio?
A. 9: 3: 3: 1
B. 3: 1
C. 1: 1
D. 1: 1: 1: 1

Answer 1

Hint: The crossing of dihybrid F1 generation with a homozygous recessive condition is known as dihybrid test cross. Here, one parent is in double heterozygous condition and the other is in double homozygous condition.

Complete answer:
Dihybrid test crossing is an experimental cross in which we took one parent from F1 generation with heterozygous condition of two traits and another parent is in double homozygous condition which will produce different types of gametes which will further fertilise to form new individuals.
Now, in this experiment we consider that all Mendel’s laws of inheritance are true or valid. Thus, gametes which will be produced are as follows:
Let’s consider that we have a parent having heterozygous conditions “Yellow and round (YyRr) “and another parent whose homozygous condition is “Green and wrinkled (yyrr)”. Then the gametes produced will be for heterozygous conditions “Yellow and round (YyRr) “is YR, Yr, yR, and yr and for homozygous condition is “Green and wrinkled (yyrr)” is yr, yr, yr and yr.
Now, when they cross with each other they will produce individuals that are yellow round (YyRr), yellow wrinkled (Yyrr), green round (yyRr) and green wrinkled (yyrr) and count for each type of individual is:
> Yellow round (YyRr) = 1
> Yellow wrinkled (Yyrr) = 1
> Green round (yyRr) = 1
> Green wrinkled (yyrr) = 1

	Double heterozygous F1 Dihybrid	Double homozygous recessive parent
Type of Genotype	Yellow Round (YyRr)	Green wrinkled (yyrr)
Type of gametes	YR Yr yR yr	yr yr yr yr

Test cross progeny

YyRr yyRr (1: 1: 1 :1) 25%: 25%: 25%: 25%

Yyrr Yyrr

Thus, cross ratio, that is, Yellow round (YyRr): Yellow wrinkled (Yyrr): Green round (yyRr): Green wrinkled (yyrr) = 1: 1: 1: 1.

Thus, option ‘D’ is correct.

Note: Please note that dihybrid cross is between two different genes that have different characters and follows Mendel’s laws thus, it will have both parents in double heterozygous condition and hence, will have cross ratio 9: 3: 3: 1 but in dihybrid test cross one parent is double heterozygous condition and other parent is in double homozygous condition. So, the cross ratio is 1: 1: 1: 1.