Question

Which of the following statements is/are correct about F2 generation obtained by Mendelian dihybrid cross?
A. $1/{16}^{th}$ of the progenies are pureline for recessive traits.
B. $1/8^{th}$ of the progenies are pure lines for recessive as well as dominant traits for both the characters.
C. $1/4^{th}$ of the progenies are heterozygous for both the characters.
D. $9/{16}^{th}$ of the progenies show recessive phenotypes for both the characters.

• A. A, C, D
• B. A, B, C, D
• C. B, C, D
• D. A, B, C

Answer 1

The correct option is D A, B, C

Let’s understand it using Mendel’s dihybrid cross in which two pea plants pureline for producing round, yellow seeds (RRYY) and wrinkled, green seeds (rryy) are crossed.
F1 offspring RrYy after undergoing selfing produces F2 generation.
On F1 selfing, we have RrYy x RrYy.
Gametes produced from F1 hybrids include RY, Ry, rY and ry and are in the ratio of 1: 1: 1: 1.
$\begin{array}{ccccc}\text{Gametes}& \text{RY}& \text{Ry}& \text{rY}& \text{ry}\\ \text{RY}& \text{RRYY}& \text{RRYy}& \text{RrYY}& \text{RrYy}\\ & \text{(Round, yellow)}& \text{(Round, yellow)}& \text{(Round, yellow)}& \text{(Round, yellow)}\\ \text{Ry}& \text{RRYy}& \text{RRyy}& \text{RrYy}& \text{Rryy}\\ & \text{(Round, yellow)}& \text{(Round, green)}& \text{(Round, yellow)}& \text{(Round, green)}\\ \text{rY}& \text{RrYY}& \text{RrYy}& \text{rrYY}& \text{rrYy}\\ & \text{(Round, yellow)}& \text{(Round, yellow)}& \text{(Wrinkled, yellow)}& \text{(Wrinkled, Yellow)}\\ \text{ry}& \text{RrYy}& \text{RrYy}& \text{rrYY}& \text{rrYy}\\ & \text{(Round, yellow)}& \text{(Round, green)}& \text{(Wrinkled, yellow)}& \text{(Wrinkled, green)}\end{array}$
Mendel’s phenotypic ratio in a dihybrid cross is 9 : 3 : 3 : 1.
The F2 phenotypic ratio is:-
yellow round : yellow wrinkled : green round : green wrinkled = 9 : 3 : 3 : 1, as shown above.
9/16 show dominant phenotypes
Out of 16 plants 4 plants are heterozygous. Therefore, $1/4^{th}$ of all the plants produced are heterozygous.
Pureline implies that the individual is true breeding or carries homozygous alleles for a particular gene.
Out of 16 plants, only one plant is pureline recessive, rryy(wrinkled, green)
Out of 16 plants, two plants are completely purelines for dominant and recessive traits for both the characters, RRYY and rryy. Hence, the fraction of such plants is given by $2/{16}^{th}$ = ⅛.