from Anne-Sophie Rondeau
Children of roses more or less reproduce the characteristics of their parents, but each child is different, resulting from a particular combination of genetic characteristics. Everything happens on the level of the reproductive nucleus that contains the basic elements for reproduction, the chromosomes. The chromosome is in the form of sticks of complex organic acids, genes.
Each gene carries a single hereditary character. So-called modern rose-trees, that is to say after 1920, have tetraploid chromosome structure, that is to say in each nucleus there is a pile of 4 genes, two male and two female. Because chromosomes go in pairs, uneven numbers being a sign of sterility.
Rose-trees described as old roses are difficult to cross because the chromosome structure is variable, diploid, tetraploid or even pentaploid. The cross-pollination reorganises the chromosomes during sexual reproduction. Similar chromosomes in the father and the mother pair off, they separate then stick together again, differently, a female element hanging on to a male element.
When a crossing is made, either one tries to introduce new genes that will combine with others, different ones, to give a new combination that does not exist in nature, or one seeks to concentrate the genes that exist on a given level to obtain a well defined characteristic.
It sometimes happens, by chance, that a genetic link near the chromosome is broken and a new link, a favourable one, is made. Then the breeder is pleased that he has succeeded in dissociating undesirable characteristics from desirable characteristics, like sensitivity to black spot and perfume. In roses, if one seeks to intensify a characteristic it is necessary to add the right genes. Take an example of the resistance to cold of a rose bush: if the breeder's aim is to have a rose that flowers under northern latitudes, he will try to accumulate the genes of resistance to cold.
(Anne-Sophie Rondeau, The Grand Rose Family)