Intragenic Recombination. Random-Sample Analysis
It seemed obvious to Morgan and his collaborators that the gene is an indivisible unit of transmission of genetic information — a recombination unit. The gene was regarded as a discrete structure (molecule) in the chromosome capable of separation from other genes by crossing-over. The idea never even occurred that crossing-over can take place within the gene. Paradoxical as this may seem, the first indications that the gene may have a complex intragenic structure, which followed the discovery of multiple allelism strengthened this point of view still further. The allele was initially regarded as one of the two alternative states of the gene. It was soon discovered (Cuénot, 1904), however, that the gene determining coat color in the albino mouse can exist in several allelic states with different phenotypic expressions. The existence of similar series of multiple alleles has been demonstrated for many Drosophila genes. For example, the series of alleles of the white gene contains, besides the normal allele (w +) determining the red color of the eyes, the mutant alleles blood (w b ), cherry (w ch ), eosin (w e ), apricot (w a ), and white (w). If flies carrying different alleles of the same series mated, the progeny (F1) was found to be mutant or, in other words, the alleles belonging to the same series were noncomplcmentary.
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- The term “locus” in modern genetics no loger has its classical meaning. It now implies not only the site of the gene on the chromosome, but also the region where phenotypically similar mutants between which the functional relationships are known, not understood, or of no significance, are mapped. A synonym of this term is “series” (3.4).Google Scholar
- As we shall see later (Chapter 3) this is incorrect. The value of rf in interallelic crosses is now gerenally expressed in frequencies of appearance of wild-type recombinants or tetrads containing recombinant spores (w); in the latter case rf = 0.25w.Google Scholar
- A calculation based on more up-to-date results gives the same value: the whole chromosome of the phage (2 × 105 nucleotide pairs) is genetically active and its length is 2000 map units.Google Scholar
- Since we shall use the flank systems in future as the source of other information that site order, this is a convenient point to introduce the standard flank system (Fig. 26), in which P1 denotes the flank configuration of the parent with the proximal allele a, P2 the configuration of the parent with the distal (b) allele, R1 the flank configuration arising by crossing-over in the region between the alleles, and R2 the configuration arising as a result of additional exchanges on both flanks.Google Scholar
- In these cases the order can still be established on the basis of two additional criteria (Jessop and Catcheside, 1965): 1) if, in a + b + recombinants, only proximal flank alleles (M and m) are taken into account, if m > M the order of the markers is Mab, while if m > M the order of the markers is Mba; 2) if distal alleles are considered if N > n the order of the markers is abN, and if n > N the order is baN; if the criteria give consistent results the reliability of determination of the order can be guaranteed.Google Scholar
- The fed - mutants of phage λ, which synthesis products of the Red-pathway in excess, recombine with increased frequency. Consequently, rf is limited by the enzyme concentration rather than by the probability of contacts (Franklin, 1971b).Google Scholar
- The term “proximal” means nearer to the centromere. In this case, tryp-4. If the locus is not oriented relative to the centromere, its left end is regarded as proximal.Google Scholar
- Mutant alleles on the same chromosome (ab × a + b +).Google Scholar