Question

In four-o'clocks, the allele for red flowers is incompletely dominant over the allele for white flowers, so heterozygotes have pink flowers. What ratios of flower colors would you expect emong the offspring of the following crosses: (a) pink x pink, (b) white x pink, (c) red x red, (d) red x pink, (e) white x white, and (f) red x white? If you specifically wanted to produce pink which of these crosses would be most efficient?

Answer 1

Answer: Crossing red with white is the most efficent way to produce pink flowers.

Step-by-step explanation:

Let's call the "red" allele R and the "white" allele r. A red flower would have both copies of R, and its genotype would be RR, a white flower would have both copies or r, being rr, and a pink flower would be Rr (heterozygote).

We can represent crosses between individuals by Punnet squares. From pink flowers you can get gametes R and r, from red flowers you can get only R gametes and from white flowers only r. If we put male gametes in the first row and female gametes on the first column, we get the following cases:

a: Crossing pink with pink, we get a ratio of a red flower to a white flower to two pink flowers.

`\begin{center}\begin{tabular} \ & R & r \\ \ R & RR & Rr \\ \ r & Rr & rr \\ \end{tabular}\end{center}`

b: Crossing white with pink, we get two pink flowers, and two white flowers.

`\begin{center}\begin{tabular}c\ & r & r \\ \ R & Rr & Rr \\ \ r & rr & rr \\ \end{tabular}\end{center}`

c: Crossing red with red, we get only red flowers.

`\begin{center}\begin{tabular}c\ & R & R \\ \ R & RR & RR \\ \ R & RR & RR \\ \end{tabular}\end{center}`

d: Crossing red with pink, we get a ratio of two pink flowers to two red flowers.

`\begin{center}\begin{tabular}c\ & R & R \\ \ R & RR & RR \\ \ r & Rr & Rr \\ \end{tabular}\end{center}`

e: Crossing white with white, we get only white flowers.

`\begin{center}\begin{tabular}c\ & r & r \\ \ r & rr & rr \\ \ r & rr & rr \\ \end{tabular}\end{center}`

f: Crossing red with white, we get only pink flowers.

`\begin{center}\begin{tabular}c\ & R & R \\ \ r & Rr & Rr \\ \ r & Rr & Rr \\ \end{tabular}\end{center}`

Thus crossing red with white is the most efficent way to produce pink flowers.