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... ry susceptible to mildew and has rather small dull foliage. This will be our variety B in the hybridizing scheme outlined above. Charlotte Armstrong, derived from a cross of Soeur These and Crimson Glory, has a long bud, vigorous growth, and carries factors for red, i.
e. , will be variety C. All three of these varieties as well as many others of value in breeding better roses are tetraploid's, that is, they show a variable number of quadrivalent's. Hence even characters dependent on a single genetic factor and accordingly very simple in their inheritance in diploid crosses, show a much more complex segregation and variability in degree of expression as seen in the hybrid plants. Dominant and Recessive Factors Fortunately for 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 These 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. Climbing habit is also dominant to dwarf bush habit and dependent on the action of a single factor, so that by backcrossing an otherwise desirable climber or pillar type to the commercially desirable bush type, one can always recover the recessive dwarf bush habit in about one half of the progeny; and among these dwarf bush types a rather small percentage will have the desirable features of the climber. 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-foliage plants. However, there are two distinct types of behavior as expressed 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 Glossy-leaved Variety (Gggg) X Dull-leaved Variety (gggg) Progeny; Dull Progeny; Glossy Sanguinaire X Night 50 20 Mrs.
Sam Mcgredy X Herbert Hoover (35038) 13 14 Mrs. Sam Mcgredy X Herbert Hoover (37148) 18 28 Mrs. Sam Mcgredy X Soeur These 92 114 Sanguinaire X C. P. Kilham 37 34 Sanguinaire X Joanna Hill 21 19 Total actual number of plants 231 Dull leaved individuals 229 Glossy leaved individuals Expectation at a 15: 13 ratio 246 Dull leaved individuals 213 Glossy leaved individuals Some plants in this population were definitely accidental self's. Type II -- Crosses Glossy-leaved Variety GGgg X Dull-leaved Variety gggg Progeny: Dull Progeny: Glossy Captain Thomas X Radiance 7 12 Captain Thomas X Crimson Glory 3 21 Captain Thomas X LuLu 6 28 Captain Thomas X Soeur These 28 67 Golden Maine's X Soeur These 14 33 Total actual number of plants 58 Dull leaved individuals 161 Glossy leaved individuals Expectation at a 3: 11 ratio 47 Dull leaved individuals 172 Glossy leaved individuals These results may be explained by the fact that the chromosomes carrying the dominant factor G, which causes glossy foliage are able to form quadrivalent's.
Students of chromosome behavior 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. In the varieties giving the Type I ratio only one of the quadrivalent chromosomes carries the dominant factor G; the others have the recessive factor g.
Hence before the division, the quadrivalent may be represented be numbers referring to the four members of the quadrivalent. These then divide into eight and may be represented as follows Any two of these may be segregated at random to the gametes, giving the following possible combinations: 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 G 1 G 2 g 3 g 4 divides to for 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-foliage 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.
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Research essay sample on Herbert Hoover Type Ii
