Example research essay topic: Egg Cells Genetic Factors - 1,105 words

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Every rose breeder has certain more or less definite ideals or objectives toward which his breeding efforts are directed. Some of these ideals are shared by all rotarians, while others express the individuality of the breeder and may or may not find popular acceptance. Thus in my opinion an ideal red rose, for example, should have the following characteristics: 1. ) Vigor similar to Radiance 2. ) Long pointed or urn-shaped buds such as Lulu, Eclipse, or Charlotte Armstrong. 3. ) Glossy or semi-glossy, hard, leathery foliage. 4. ) High degree of mildew and rust resistance. 5. ) Ever blooming, rapid "breaking habit." 6. ) Long cutting stems. 7. ) Strong neck. 8. ) Fragrance. 9. ) Absence of blue fading reaction. World's Fair is ideal in this respect. 10. ) A high-centered open flower with 25 to 35 large petals. Breeding Plan 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).

Simultaneously variety A is crossed with variety C, having long buds, vigorous growth, and deep pink or light red flowers (hence carrying factors for red). Hybrids from cross A x B having wide glossy leaves, a high degree of mildew resistance, and deep pink or light red double flowers are then crossed with pink-flowered hybrids of A x C having the longest buds and most vigor. Large pro genies must be grown in order to get the best combination of characteristics in a very few plants. These sister seedlings with wide glossy leaves, mildew resistance, and fairly long buds, are then crossed together, and in the resulting generation deep red roses having a vigorous growth habit, a high degree of mildew resistance, large glossy leaves, very long buds, and sufficiently double flowers may be expected. In order to understand the need for this type of breeding procedure, a discussion of rose chromosome numbers and behavior as they affect the inheritance of important characteristics is necessary. Mechanics of Inheritance Biologists are agreed that the factors determining the characteristics of plants and animals are serially located in certain deeply staining bodies of the cell nucleus, called chromosomes.

At an early stage in the growth of the anthers, these chromosome are very elongated thread-like structures having a beaded appearance, and it is believed that the genetic factors are located in these small bead-like chromatin bodies called chromomere's. Basic wild rose species ordinarily have seven pairs of these chromosomes and so are called diploids; one member of each pair is contributed by the male parent and the other by the female parent. In the formation of the pollen and egg cells this chromosome number is reduced one-half, so that each such gamete (mature sex cell) possesses only seven chromosomes, or one from each of the parental pairs. At the fertilization process, however, the pollen and egg nucleus unite, thus restoring the full number of fourteen chromosomes or seven pairs.

In this manner each seed possesses a complete set of hereditary factors necessary for the normal development of the new plant. Tetraploid y Most commercial roses, however, are derived from hybrids involving several different wild species of roses. Somewhere in the history of their development the chromosome number became doubled up so that instead of having only 7 pairs or 14 chromosomes, they have about 28. Since the species of wild roses are rather similar in general characteristics, their chromosome sets are also similar, and have many regions or portions which are sufficiently similar to pair when brought together in the hybrid nucleus. If species A has a paired chromosome set: ABCDEFG ABCDEFG Species B may be symbolically presented as having a set: ABCDEFG ABCDEFG 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 paid 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 quite frequently get association of four chromosomes, A A A A instead of pairs.

These associations of four are called quadrivalent's. Since one often's finds a variable number of pairs in the hybrids between the diploid species, it is not surprising that a variable number of quadrivalent's 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 (trivalent's), paired (bivalent's), and unpaired chromosomes (univalent's). The number may theoretically vary from 7 IV to 14 II depending on the amount of pairing in the original species crossed and the length of the chromosomes involved. Practical Application 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 resistance 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 infection. " Incidentally, it is a very lovely single deep yellow rose and so is well worth planting in its own right if you like single blooms. Captain Thomas is then our variety A.

Crimson Glory is a fine red rose but in common with all other red roses is ve...

Research essay sample on Egg Cells Genetic Factors