Principles of Inheritance and Variation

Map distances are obtained by adding the recombination frequencies between closely placed genes. So the total length of a map (here 66 units) can be more than 50.
However, the recombination frequency actually observed between two genes can never exceed 50 percent. This is because multiple crossovers between distant genes cancel each other's effect (a double crossover restores the parental combination), and crossing over involves only two of the four chromatids.
A recombination frequency of 50 percent means the genes behave as if they assort independently.
So between y and b, at opposite ends of the chromosome, the recombination frequency should be $\leq$ 50%.

The garden pea has 7 pairs of chromosomes (2n = 14).
Later studies showed that the genes for the seven characters studied by Mendel lie on only four different chromosomes (chromosomes 1, 4, 5 and 7).
Some of them are on the same chromosome but far apart, so crossing over is frequent and they still assort nearly independently. Mendel also did not test every pair of characters together, so he did not detect linkage.
So the genes are located on four different chromosomes.