In a cross involving a heterozygous red flower plant (Rr) and a white flowered plant (rr). What is the probability that the offspring will be Rr?

Step 1: Analyze the question We are crossing a heterozygous red plant (Rr) with a white plant (rr). We need the probability that an offspring is Rr (heterozygous).

Step 2: The concept This is a test cross (a cross of a heterozygote with a homozygous recessive). Each parent gives one allele to the offspring through its gametes, and a Punnett square shows all the possible combinations.

Step 3: Solution

Gametes:

• Rr parent produces R and r
• rr parent produces r and r

Punnett square:

| | r | r | |---|---|---| | R | Rr | Rr | | r | rr | rr |

Result: 2 Rr and 2 rr out of 4 offspring.

• Genotypic ratio: 1 Rr : 1 rr (50% Rr, 50% rr)
• Probability of Rr = 2/4 = 1/2

(Phenotypically, half the offspring will be red and half white.)

Step 4: Answer The probability is 1/2, which is option A.

Note that options B (1) and C (2) cannot be right, since a probability is never greater than 1. Option D (1/4) would be the probability for one specific box of the Punnett square, but there are two Rr boxes, so you must add them.

Step 5: Exam tip For a test cross, Rr × rr always gives a 1:1 ratio. Cross Rr × Rr gives 1:2:1 genotypically (RR : Rr : rr), where Rr is also 1/2. Always check that your probability is between 0 and 1, which quickly eliminates options like C.