Q15-8TYU - Campbell Biology

Question

Two genes of a flower, one controlling blue (B) versus white (b) petals and the other controlling round (R) versus oval (r) stamens, are linked and are 10 map units apart. You cross a homozygous blue oval plant with a homozygous white round plant. The resulting F1 progeny are crossed with homozygous white oval plants, and 1,000 offspring plants are obtained. How many plants of each of the four phenotypes do you expect?

Step-by-Step Solution

Answer

The following four types of offspring are obtained:

• 450 blue oval

• 450 white round 

• 50 blue round 

• 50 white oval 

Step 1: The genotype of F1 progeny

The genotype of the homozygous blue oval plant is BBrr, and the genotype of the homozygous white round plant is bbRR. The gametes produced from the BBrr plant are Br, and the gametes produced from the bbRR plant are bR. 

Thus, when both the plants are crossed (BBrr × bbRR), the genotype of the F1 progeny is BbRr.

Step 2: Genotype of offspring from F1 progeny and homozygous white oval plants

The genotype of homozygous white oval plants is bbrr.

Four different progenies are produced when F1 progeny (BbRr) is crossed with the homozygous white oval plant (bbrr). The phenotypes and genotypes of the offspring are blue round (BbRr), blue oval (Brrr), white round (bbRr), and white oval (bbrr).

Some of the offspring obtained are of parental types, and some are recombinants. The blue oval and white round plants are of a parental type, and the blue round and white oval are recombinants.

Step 3: Number of recombinants produced

The genes that control the color and shape of stamens are ten map units apart. This means the recombination frequency between the genes is 10%. The number of offspring produced is 1000.

The number of recombinants can be calculated using the formula for recombinant frequency. The formula: 

$Recombination frequency=\frac{Number of recombinants}{Total number of offsprings}\times 100$

The number of recombinants is calculated as: 

$10=\frac{Number of recombinants}{100}\times 100$

Thus, the number of recombinants is 100.

Step 4: Number of each type of plants

The number of recombinants is 100, which means 50 each of blue round and white oval plants is produced as these are recombinants. The remaining 900 (1000-100) offspring are of the parental type. This means 450 each of blue oval and white round plants are obtained.

Thus, 50 each type of recombinants and 450 each of parental type plants are produced.