Answer:
d) roan × roan
Explanation:
From the question, the roan coat color (mixed red and white hairs) occurs in the heterozygous (Rr) offspring of red (RR) and white (rr) homozygotes. i.e the F1 progeny of heterozygous (Rr) are foremed as a result of the cross between red (RR) and white (rr) homozygotes.
So which cross in the cycle will produce offspring in the ratio of 1 red:2 roan:1 white...?
Let us take a look at the options one after the other.
If we take a look at the first option, red and roan : the cross between both will yield the following result.
RR × Rr = ( RR, Rr, RR, Rr) ; The result will yield 2 red : 2 roan , which is incorrect!.
IN option B
red × white, the result will yield;
RR × rr = ( Rr,Rr,Rr,Rr) ; All heterozygous Rr offspring (All roan)
IN option C
white × roan = rr × Rr
= (Rr, Rr, rr, rr) 2 roan : 2white. This option is still not the right answer.
In the last option, we have the cross between roan × roan...i.e if our previous f1 progeny (right from the question), if they self-crossed, we will have
Rr × Rr = RR,Rr, Rr, rr
(1 red: 2 roan : 1 white). This is the right answer!
I hope that helps alot!.
