This article by Walter Lammerts (breeder of 'Queen Elizabeth' and other fine roses) was published in 1945 and is still an important and useful essay on the topic.
The Scientific Basis of Rose Breeding
Dr. Walter E. Lammerts
Every rose breeder
has certain more or less definite ideals or objectives toward which
1 ) Vigor similar
Although the rose
which perfectly meets these ideals may never be developed, any variety
which is a step toward perfection is worthy of introduction. In my own
experience most rapid progress toward combining all the above characteristics
in one plant may be made by crossing variety A, having such desirable
characteristics as mildew resistance and large glossy leaves, with variety
B, having double dark red flowers but dull foliage, susceptibility to
mildew, and short buds (unfortunately characteristics of most red roses
so far introduced).
If species A has a paired chromosome set:
species B may be symbolically presented as having a set:
indicating relatively slight change in the genetic factors located in the seven chromosome pairs represented by the seven letters of the alphabet. Thus when species A is crossed with species B, a variable number of pairs is formed instead of 14 unpaired chromosomes, such as one would expect if no similarities existed in the sets of the two species. It has been found that occasionally egg cells and pollen grains are formed with unreduced number of chromosomes, i.e., 14 instead of 7. When these unite, plants are formed combining the diploid number of the two parental species, that is having 28 instead of 14 chromosomes, and so are called tetraploid (tetra meaning four, and ploid referring in this case to the basic number seven). There are in these plants of our commercial hybrids now four chromosomes of each kind (represented by the letters A-G), and since chromosome A, for example, can pair with A as well as A, we quitem frequently get association of four chromosomes, A A A A instead of pairs. These associations of four are called quadrivalents.
Since one oftens finds a variable number of pairs in the hybrids between the diploid species, it is not surprising that a variable number of quadrivalents is observed in the tetraploid rose varieties, though usually only three or four are seen, the rest of the chromosomes being in associations of three (trivalents), paired (bivalents), and unpaired chromosomes (univalents). The number may theoretically vary from 7IV to 14II depending on the amount of pairing in the original species crossed and the length of the chromosomes involved.
By now no doubt
you wonder what all this has to do with breeding the ideal rose. It
so happens that Captain Thomas is about the best source so far found
of large glossy leaves and resitance to mildew. Not only is it disease
resistant on the Pacific Coast, but in a letter Dr. J. Horace McFarland
states, "Captain Thomas, as a climbing hybrid tea has shown satisfactory
hardiness in this climate (Pennsylvania) and has not been at all bothersome
with respect to disease proclivities. I don't think I have ever seen
a mildewed leaf on it, nor does it seem anxious to take up blackspot
Dominant and Recessive Factors
the progress of rose breeding, several very important desirable characters
have been found to depend in their expression on the action of one,
or at most a few factors which behave as dominants. These are long symmetrical
urn-shaped buds (such as are found in Soeur Thérèse and
Eclipse), glossy leaf, double flower, and mildew resistance. Genetically
speaking, dominant characters are the ones which appear substantially
unchanged in the hybrids obtained by crossing to varieties which do
not have the characters in question or any latent or recessive factors
for them. The latent characters or the ones which disappear in the hybrids
are called recessives.
Glossy vs. Dull Foliage
Let us now consider the effect of quadrivalent formation on the inheritance and expression of glossy vs. dull foliage. Intercrossing of dull-leaved varieties always results in plants having dull foliage; hence we know that dull foliage is recessive, since dull x glossy results in glossy and dull-foliaged plants. However, there are two distinct types of behavior as exressed in the ratio of glossy to dull, depending on which varieties are selected as glossy-leaved parents. This fact may be seen by study of the following examples:
Type I -- Crosses
Total actual number of plants
231 Dull leaved
Expectation at a 15:13 ratio
246 Dull leaved
*Some plants in this population were definitely accidental selfs.
Type II -- Crosses
Total actual number of plants
58 Dull leaved
Expectation at a 3:11 ratio
may be explained by the fact that the chromosomes carrying the dominant
factor G, which causes glossy foliage are able to form quadrivalents.
Students of chromosome behavor have found that the chromosomes divide
in the early thread stage before the reduction division, so that instead
of only four chromosome threads being present there are actually eight,
and each factor is thus represented eight times instead of the original
four. Furthermore, factors sufficiently distant from the spindle fibre
attachment of the chromosome (which is the mechanism by which the chromosomes
are separated from one another) may cross over freely from one chromosome
to the other. However, the final result is that only two chromosomes
of the eight are segregated to each pollen or egg cell, thus giving
the total of 14, two of each of the seven different kinds previously
designated A to G.
Summarizing all the gametes having dominant G or GG and those lacking it, we have 1 GG plus 12 Gg or 13 G-bearing gametes to 15 gg gametes. Accordingly, on crossing a glossy plant of the above genetic constitution to dull, gggg, which forms only gg gametes, we expect to get a ratio of 15 dull to 12 glossy to 1 very glossy, which is substantially the result obtained in Type I crosses summarized above, though fluctuations in individual progeny results occur. Nevertheless, all fit the approximately 1:1 ratio quite closely except Sanguinaire x Night, where the excess of dull-leaved plants may possibly be due to accidental self pollination since Night was used as the female parent. When the two dominant G factors are present, the quadrivalent G1G2g3g4 divides to form G1G1, g2g2, g3g3, g4g4. Any two of these may segregate at random, giving us 6 gg gametes to 16 Gg gametes to 6 GG gametes, or as regards dull vs glossy, a ratio of 3 dull to 11 glossy, when grossed to dull-foliaged varieties. This is approximately the ratio obtained in the Type II series of crosses.
Among the glossy-leaved plants there are two grades of glossiness, 3 having the constitution of GGgg and so being much glossier than the other 8 which have the Gggg constitution. From the practical breeding point of view, the important facts are that glossy leaf is dominant and is inherited as a unit character, i.e., depends on a single factor, and so is not lost or diluted no matter how many backcrosses to dull varieties one may wish to make. Due to quadrivalent association, it, as well as other dominant factors, may vary strikingly in its degree of expression, depending on whether it is present in the simplex Gggg, duplex GGgg, triplex GGGg, or quadruplex GGGG form. Modifying factors may limit slightly the degree of expression of the glossy factor, but their importance in this case is very minor.
Mildew Resistance In the progeny of crosses between Crimson Glory, which when selfed gives only susceptible seedlings, and Captain Thomas, one gets varying degrees of mildew resistance. Letting 0 equal no infection and using grade 4 to express a high degree of susceptibility or infection, a small progeny when tested by the floating leaf method under ideal conditions of growth of mildew mycelium gave the results shown in Table I.
Classification of a much larger population under field conditions when mildew was severe on susceptible varieties gave substantially the same results. Only a few were completely immune but many could be classified as good, i.e., showing a high degree of resistance. Evidently mildew resistance is dominant and rather simple in its inheritance. It may then be handled as a unit character and combined with any other desirable characteristics.
The evidence so far collected indicates that the long urn-shaped bud is dependent on a dominant factor or factors since short-budded varieties such as Crimson Glory give only short-budded seedlings when crossed with other short-budded varieties such as Captain Thomas. Symbolizing long bud by L and short bud by l, Soeur Thérèse has the factorial composition LLll. The various grades of bud length can then be represented as follows:
Soeur Thérèse when self-pollinated gave 1 plant with extremely long buds about twice the length of Soeur Thérèse; 5 plants with very long buds; 21 plants with buds as long as Soeur Thérèse; 22 plants with medium long buds; 7 with short, and 1 with very short buds.
The best assumption seems to be that doubleness is dominant but also quantitative in its expression. The various grades of doubleness may then be set up as follows:
Dark maroon-red flower color is dependent on recessive factors. [?!] The nearest approach to red obtained when deep red varieties such as Crimson Glory or Night are crossed to yellow varieties, is a rose-red to Tyrian rose. The flowers also fade rapidly to magenta-red. Only by backcrossing to maroon, nonfading reds such as World's Fair, can one recover a large percentage of plants with dark maroon-red color.
Orange-yellow, yellow, white, and scarlets are also recessive in their inheritance, i.e., give pinks when crossed to magenta-red or maroon-red roses. In other words, magenta-pink or Tyrian rose to rose-red is the dominant kind of color in roses. Deep yellow such as that found in Goldenes Mainz is recessive to light yellow. Hence to recover deep yellow colors, one must backcross to the deep yellow varieties. Also white is recessive to cream, buff, or light yellow. Many of our so-called "white" roses, such as Sir Henry Segrave, Odine, and Alice Stern, are really creams and dominant to true white in their color expression. Hence to recover the latent white in hybrids with these varieties it is necessary to backcross to white.
The very popular "yellow and silver reverse" type of petal color such as characterizes roses like Condessa de Sastago and Contrast is recessive to self-color. It may be of interest to note that a few yellow and silver reverse plants were recovered in the hybrids of Captain Thomas and Crimson Glory, indicating that both of these varieties carry these recessive factors. In all cases of color inheritance, the expresion of the color is of course complicated by the fact that the factors determining the desired color may lie in chromosomes which are able to form quadrivalents. Hence the number of plants which need to be grown to recover a desired recessive color such as deep buttercup-yellow are much largerthan is necessary in diploid crosses.
Finally it should be mentioned that characteristics such as vigor, fragrance, thorniness, strength of neck, length of cutting stem, width of leaf, and shape of bud and open flower seem, in the present limited state of our knowledge, to be the result of the interaction of many factors and so are usually intermediate in their expression and quantitative in their type of inheritance. One should therefore always cross plants having such dominant factors as glossy leaf, long bud, double flowers, and mildew resistance to varieties having vigor, comparative freedom from thorns, strong necks, well-shaped buds and flowers, and wide leaves as the final step in the breeding program. Backcrosses will also probably be necessary before the ideal desired combination is obtained.