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?
The answer is: 450 each of blue oval and white round (parentals) and 50 each of blue round and white oval (recombinants).
Here's how I solve it: BBrr (homozygous blue oval) X bbRR (homozygous white round) gives four times BbRr (heterozygous blue round). Then BbRr (the resulting F1 progeny) X bbrr (homozygous white oval) gives BbRr, bbRr, Bbrr, and bbrr. I don't get how the numbers 450 and 50 are related to the 10 map units apartness, but BbRr (blue round) and bbrr (white oval) are parentals.
Source: Campbell Biology 2017, page 313.
According to my book, a map unit is a unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.