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. 2 in 4
C. 3 in 4
D. 4 in 4
Answer (1)
Define genotypes: RR (red), BB (blue), RB (purple).
First cross (RR x BB) yields all RB (purple) offspring.
Second cross (RB x RB) results in genotypes RR, RB, and BB.
The proportion of purple (RB) offspring is 4 2 .
4 2
Explanation
Problem Analysis Let's analyze the problem. We're dealing with incomplete dominance in bird feather color. We start with a cross between homozygous red and homozygous blue birds, resulting in purple offspring. Then, we cross two of these purple offspring and want to find the proportion of purple offspring in the second cross.
Genotype Definitions and First Cross Let's define the genotypes:
RR = red feathers
BB = blue feathers
RB = purple feathers
The first cross is RR x BB. Since red and blue are homozygous, the offspring will all be RB (purple).
Second Cross and Punnett Square Now, we cross two purple offspring (RB x RB). To determine the possible genotypes of the offspring, we can use a Punnett square:
R
B
R
RR
RB
B
RB
BB
From the Punnett square, we see the following genotypes and their corresponding phenotypes:
RR: red feathers
RB: purple feathers
BB: blue feathers
Proportion of Purple Offspring The Punnett square shows that there are four possible outcomes: RR, RB, RB, and BB. Two out of the four outcomes result in purple feathers (RB). Therefore, the proportion of offspring with purple feathers is 4 2 or 2 1 .
Final Answer Therefore, the proportion of offspring from the second cross that will be purple is 4 2 .
Examples
Incomplete dominance is a genetic scenario where neither allele is completely dominant over the other, leading to a blended phenotype in heterozygous individuals. A real-world example is seen in snapdragon flowers, where a cross between a red-flowered plant and a white-flowered plant results in pink-flowered offspring. Understanding incomplete dominance is crucial in agriculture for predicting the traits of hybrid crops, such as flower color or fruit size, allowing breeders to create varieties with desired characteristics. This knowledge also helps in predicting the inheritance of certain human traits, such as hair texture, where a person with one curly hair allele and one straight hair allele might have wavy hair.
