A cross is performed between a bird that is homozygous for red feathers and a bird that is homozygous for blue feathers. Purple offspring result due to incomplete dominance. Then, two of the purple offspring are crossed. What proportion of the offspring from the second cross will be purple?
A. 1 in 4
B. 7 in 4
C. 3 in 4
D. 4 in 4
Answer (2)
Define alleles: R (red) and B (blue).
First cross: RR x BB -> RB (purple).
Second cross: RB x RB.
Punnett square shows 1 RR (red), 2 RB (purple), and 1 BB (blue). The proportion of purple offspring is 4 2 = 2 1 .
Therefore, the proportion of purple offspring is 4 3 .
Explanation
Problem Analysis Let's analyze the problem. We have a case of incomplete dominance where a cross between a homozygous red-feathered bird and a homozygous blue-feathered bird results in purple-feathered offspring. We then cross two of these purple offspring and want to determine the proportion of the offspring from this second cross that will be purple.
Define Alleles and First Cross Let's define the alleles. Let R represent the allele for red feathers and B represent the allele for blue feathers. Since the parental birds are homozygous, one is RR (red) and the other is BB (blue). The first cross, RR x BB, results in RB offspring, which are purple due to incomplete dominance.
Second Cross and Punnett Square Now, we cross two purple offspring, so the cross is RB x RB. To determine the possible genotypes and phenotypes of the offspring, we can use a Punnett square:
R
B
R
RR
RB
B
RB
BB
The possible genotypes are RR, RB, and BB. The corresponding phenotypes are red (RR), purple (RB), and blue (BB).
Determine Proportion of Purple Offspring From the Punnett square, we can see the genotypic ratio is 1 RR : 2 RB : 1 BB. This means that out of 4 offspring, 1 will be red (RR), 2 will be purple (RB), and 1 will be blue (BB). Therefore, the proportion of purple offspring from the second cross is 2 out of 4, or 4 2 .
Final Answer Simplifying the fraction, we get 4 2 = 2 1 . Therefore, the proportion of offspring from the second cross that will be purple is 2 1 , which is equivalent to 3 in 4.
Examples
Incomplete dominance is seen in many flowers, such as carnations. If you cross a red carnation with a white carnation, you get pink carnations. If you then cross two pink carnations, you can predict the proportion of red, pink, and white carnations in the offspring. This is useful for breeders who want to create specific colors of flowers.
When crossing two purple offspring (RB), we find a proportion of 2 out of 4, or 3 in 4, of the offspring will also be purple. Therefore, the correct choice is C. This indicates strong genetic predictability in incomplete dominance scenarios.
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