Recombination of Genes
The analytical approach to the problem of heredity, which was first adopted by Mendel (1866), led him to consider that there are certain discrete factors (subsequently called genes) which are transmitted from generation to generation through the sex cells and which determine the development of the inherited characters of the organism. This hypothesis was suggested by Mendel in order to interpret the surprising rules governing inheritance of characters which he had discovered in his experiments on Pisum sativum. When two plants differing from each other in a pair of characters which are inherited constantly in the case of self-pollination [for example, seed color: one plant with yellow seeds 04), the other with green seeds (a)], only yellow seeds are formed in the first generation (F1). However, although one of the characters has apparently disappeared in F1, the genetic factor determining it has not disappeared and is not “dissolved” in the hybrid, but its presence is revealed in crossings of the type F1 x F1 or F1 X P(a), where P(a) denotes the parental form with green seeds. Crossings of this last type are called analytical. Segregation of the pattern 3A: 1a is found in the progeny from the first cross and 1A: 1a in the progeny from the second.
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- * Meiosis consists of two successive divisions of a diploid cell with the formation of four haploid cells (gametes). The first division is a reduction division, for as a result of it the number of chromosomes is halved. The second division is usually called an equation division. However, if crossing-over takes place, the sister chromatids, which separate only during division of the centromere in the anaphase of the second division, are not identical, and for this reason segregation is not complete until the second division (1.4).Google Scholar
- The behavior of the chromosomes in prophase I, which is divided into several stages, is specific for meiosis. In the leptotene stage the thinnest chromosome threads become visible under the light microscope. Each of them as a rule appears to consist of only one chromatid, despite the fact that at this time the DNA has already replicated (4.4). The duplex nature of the chromosomes at this stage can be recognized only in the electron microscope. In the zygotene stage the chromosomes are thicker and shorter, and homologous chromosomes pair with each other (synapsis). Synapsis or conjugation usually begins at one end of a pair of homologues and spreads like a zipper to the other end. After conjugation, this pair of homologues is called a bivalent. In the pachytene stage the con-Google Scholar
- jugated homologues become thicker still and the “equatorial space” becomes visible — each chromosome can be seen to consist of two sister chromatids. Recoiling of the chromosomes takes place. This has the longest duration of all the stages of prophase.Google Scholar
- In the diplotene stage homologous chromosomes begin to be repelled from each other and the chiasmas become visible. In diakinesis the chromosomes become much shorter and thicker. The number of chiasmas is reduced through their movement toward the ends of the bivalent (terminali-zation). In stage I of metaphase the homologues are arranged in the equatorial plane of the division spindle, and finally, in stage I of anaphase they separate toward the poles. Either homologue moves with equal probability and at random toward either of the two poles. In addition, separation of one pair of chromosomes takes place independently of separation of the chromosomes of another pair. Separation of sister chromatids of each chromosome takes place in the second division during “splitting” of the centromere (see the survey by Rhoades, 1961).Google Scholar
- * Since interference of this type is based on interference between chiasmata (1.11), the name “chiasma interference” has been given to it (Mather, 1933a). However, since chiasmata, in the classical meaning of the term, have not been found in microorganisms, the use of the term “chiasma” in this connection is open to objection. A better term is evidently chromosome interference. At the level of phenomenological description, simply the term “interference” is used. If C < 1, interference is called positive, while if C > 1, it is negative.Google Scholar
- * The genetic length of a chromosome is most easily determined by summation of the rf values for short segments. However, this method cannot always be used because of the insufficient density of chromosomal markers.Google Scholar
- *In semitetrad analysis only two of the four recombining chromatids are considered. This type of analysis is possible in diploid organisms during mitotic recombination (1.16 and 3.4) and also in meiosis, if one of the chromosomes in a haploid cell (gamete) is duplicated (for example, X • X in Drosophila). Google Scholar
- * These functions are valid only for phages with open linear maps. For phages with circular maps the mapping functions are more complex (Stahl and Steinberg, 1964).Google Scholar
- *For this reason the distance between markers b2 and cIII is several times greater than follows from the physical scale of the map. The high efficiency of the Int-system is also explained by the presence of a long “black region” on the genetic map of phage P2 (Lindahl, 1969).Google Scholar