[From Modern Garden Roses, by Peter Harkness, p. 17:] The genes of a rose, which make it what it is, are strung along its chromosomes, which occur in the plant cells of the rose in multiples of seven. Whether a rose is compatible -- i.e., capable of being crossed with another to produce useful seed -- depends on its having a number of chromosomes to match those of its partner without odd ones being left over.
[From Ibid, by Peter Harkness, p. 22:] 'Pinocchio' proved a most potent parent rose, and American breeders reaped an amazing harvest from it.

[From Modern Garden Roses, by Peter Harkness, p. 26:] Cocker and Harkness used R. persica in their breeding programs in the 1960s… 'Sweetbriar' was used by Kordes to breed in hardiness… R. bella [was] used by Ted Allen (Britain) for its qualities of vigour, hardiness, health and early bloom.

In theory, it is said, two rose parents can furnish 250 million different seedlings from combinations of their genes. Some genes are more dominant than others. This shows up in the seedlings from two different parents, where you observe a family likeness in, for example, growth or leaf or colour or proneness to disease. The breeder with a strategy in mind need not despair if at first what he seeks is missing. Less dominant genes will surface in succeeding generations, and a lucky combination of the right ones could give him what he seeks, or at least a signpost along they way.

[From Ibid, p. 30:] Tantau (Germany) used seedlings from 'General MacArthur' and 'Crimson Glory' in his breeding program.

[From The Practical Book of Outdoor Rose Growing, by George C. Thomas, p. 35:] it seems an open secret that three generations are often required before a new rose of merit is secured.

In his marvelous book about miniatures, Miniature Roses: Their Care and Cultivation, Sean McCann tells about the creation of 'Cupcake', see References for that rose for more information.

[From A Family of Roses, by Sam McGredy and Sean Jennett, p. 35:] Roses with 14 chromosomes are called diploid, that is they have 2 x 7 chromosomes. Other roses are triploid, with 3 x 7 or 21 chromosomes, others again are tetraploid, with 4 x 7 or 28 chromosomes. Occasionally one finds a hexaploid with 42 chromosomes. If a diplooid is crossed with a tetraploid the result is a triploid. [See Reference for more information.]

[From The Complete Book of Roses, edited by John Mattock, p. 74:] The first production of a new rose cultivar by deliberate hybridizing did not occur until the first half of the nineteenth century, probably about 1820, in France.

[From The Old Rose Adventurer, by Brent Dickerson, p. 385:] The French were indisputably the masters of the rose breeding world from 1815 until 1900!

[From Red Hybrid Teas, by Patrick Dickson, p. 21:] Since 1935 almost all the successful reds can be traced back to Wilhelm Kordes' 'Crimson Glory'. Until around 20 years ago, they always inherited two undesirable characteristics -- two outer petals with prominent white stripes and an unfortunate lack of vigor.

[From Growing Old-Fashioned Roses, by Trevor Nottle, p. 15:] the Dutch, Flemish and French nurseryman who raised the Gallicas [didn't use the pollen daubing techniques of modern breeders, instead, the planted] the best varieties of all sorts of roses close together and [left] the skilled work up to Chance and Nature...