Question

1. the allele for red flower color, r, in a certain plant is incomplete dominant with the allele for white flowers, r. thus, a plant with the genotype rr has pink flowers. tall, d, is dominant to dwarf, d. what would be the expected phenotypic ratio from a cross of rrdd plants with rrdd plants?

a 3:1
b 9:3:3:1
c 50% pink, 50% white, and all tall
d 1:1:1:1, in which 50% are tall, 50% are dwarf, 50% are pink, and 50% are white

Explanation:

Step1: Analyze flower color cross

Parent 1 (flower color): $RR$; Parent 2 (flower color): $R'R'$
Offspring genotypes: All $RR'$? No, correction: Parent 1 is $RR$, Parent 2 is $R'R'$. Wait, no, original cross is $RRdd$ x $R'R'Dd$. Flower color cross: $RR$ x $R'R'$.
Gametes from $RR$: only $R$; Gametes from $R'R'$: only $R'$.
Offspring genotype: $RR'$ (pink) is 100%? No, wait no, wait the problem says $RR$ is red, $R'R'$ is white, $RR'$ is pink. Wait no, wait the cross is $RR$ (red) x $R'R'$ (white). So all offspring are $RR'$ (pink)? No, wait no, wait I misread. Wait no, the question says: cross $RRdd$ with $R'R'Dd$. Oh right! Flower color: $RR$ x $R'R'$. So all offspring get $R$ from first parent, $R'$ from second? No, $RR$ can only give $R$, $R'R'$ can only give $R'$. So all offspring are $RR'$ (pink)? No, that can't be, option C says 50% pink 50% white. Wait wait, no! Wait the problem says: "The allele for red flower color, R, in a certain plant is incomplete dominant with the allele for white flowers, R'. Thus, a plant with the genotype RR' has pink flowers." Oh! Wait the cross is $RRdd$ (so flower color RR) x $R'R'Dd$ (flower color R'R'). Wait no, that would make all RR' (pink). But option C says 50% pink 50% white. Wait I must have misread. Wait no, wait the cross is $RRdd$ x $R'R'Dd$? Wait no, maybe the first parent is $RR'$? No, the problem says: "cross of RRdd plants with R'R'Dd plants". Oh! Wait wait, no: $RR$ x $R'R'$ gives all $RR'$ (pink). But that's not matching options. Wait no, wait no! Wait the second parent is $R'R'Dd$: flower color is $R'R'$ (white). First parent is $RR$ (red). So all offspring are $RR'$ (pink). Now for height: first parent is $dd$ (dwarf), second parent is $Dd$ (tall). Cross $dd$ x $Dd$. Gametes from $dd$: $d$; gametes from $Dd$: $D$ and $d$. Offspring genotypes: $Dd$ (tall) 50%, $dd$ (dwarf) 50%. Wait that would be 50% pink tall, 50% pink dwarf. But that's not an option. Wait wait, I misread the problem! Oh no! Wait the problem says: "The allele for red flower color, R, in a certain plant is incomplete dominant with the allele for white flowers, R'. Thus, a plant with the genotype RR' has pink flowers. Tall, D, is dominant to dwarf, d. What would be the expected phenotypic ratio from a cross of RR'dd plants with R'R'Dd plants?" Oh! Oh right! I misread the first parent as RRdd, but it's RR'dd! That's the mistake. Okay, correct:

Step1: Correct flower color cross

Parent 1 (flower color): $RR'$; Parent 2 (flower color): $R'R'$
Gametes from $RR'$: $R$, $R'$ (50% each)
Gametes from $R'R'$: $R'$ (100%)
Offspring genotypes:
$R \times R' = RR'$ (pink): 50%
$R' \times R' = R'R'$ (white): 50%

Step2: Analyze height trait cross

Parent 1 (height): $dd$; Parent 2 (height): $Dd$
Gametes from $dd$: $d$ (100%)
Gametes from $Dd$: $D$, $d$ (50% each)
Offspring genotypes:
$d \times D = Dd$ (tall): 50%
$d \times d = dd$ (dwarf): 50%

Step3: Combine phenotypic ratios

Combine flower color and height:

• Pink tall: 50% × 50% = 25%
• Pink dwarf: 50% × 50% = 25%
• White tall: 50% × 50% = 25%
• White dwarf: 50% × 50% = 25%

This gives a 1:1:1:1 ratio, with 50% tall, 50% dwarf, 50% pink, 50% white.

Answer:

D. 1:1:1:1, in which 50% are tall, 50% are dwarf, 50% are pink, and 50% are white