Showing papers on "Gene interaction published in 1971"

Measurement of fitness at the esterase-5 locus in drosophila pseudoobscura

Abstract: KNOWLEDGE of the amount of genic variation in natural populations is the first step towards understanding how such variation is maintained. The first systematic studies of the amount of genic variation in natural populations of Drosophila pseudoobscura were reported by HUBBY, LEWONTIN, and co-workers (HUBBY and LEWONTIN 1966; LEWONTIN and HUBBY 1966; PRAKASH, LEWONTIN and HUBBY 1969) who estimated that 40% of the loci are polymorphic and that on the average 12% of loci per individual are heterozygous in North American populations of the species. Similar estimates of heterozygosity have now been made for several organisms, including mice (SELANDER and YANG 1969) and humans (HARRIS 1969; LEWONTIN 1967). All these estimates indicate that the proportion of polymorphic loci in natural populations may be much greater than was previously supposed. Were these polymorphisms maintained primarily by heterosis operating independently at each locus, populations probably would not be able to tolerate the huge genetic load involved (see LEWONTIN and HUBBY 1966). SVED, REED and BODMER (1967) have attempted to solve this dilemma by postulating that there is a limit to maximum fitness because of gene interaction. Owing to the rarity of the maximally fit genotypes, the amount of reduction in the selective advantage of heterozygote over homozygote at individual loci is extremely small. KING (1967) proposed a different threshold model in which he assumed that a certain proportion of the population with the “worst combination of genes, environment, and luck” will be eliminated by natural selection. The effect of KING’S model is similar to that of SVED, REED and BODMER (1967) both in the sense that maximal fitness is approached asymptotically as heterozygosity increases, and that there are thousands of different genotypes with essentially the same fitness. According to these models a large number of polymorphic loci can be maintained without increasing genetic load to an excess. However, both hypotheses fail to explain the relatively small loss of fitness found when natural populations are inbred. The hypothesis of neutrality or very small selection coefficients of allozymes as proposed by KIMURA (1968) may be adequate to explain the maintenance of large amounts of polymorphism by assuming a rather high mutation rate, migration rate, and population size, but it does not appear to be an adequate explanation of

Analysis of the quantitative inheritance of body size differing at a small number of loci' in mice, vi: diallel crosses between lines

Abstract: THE chief problem inherent in quantitative genetic studies lies in the complexity of traits involving many genes which interact mutually and also with the environment. Such interactions are often so great that the effects of individual genes may be overshadowed, and sometimes results disagree (see JINKS 1955). In an attempt to circumvent such difficulties in our study of inheritance of body size in mice, we designed a breeding scheme dealing with a small number of genes with relatively large effects on a genetically homogeneous background. The breeding was begun by crossing strains LG (large body size) and SM (small body size), and was continued by repeated backcrossing to LG, then inbreeding from the fifth backcross generation for 20 or more generations. During backcrossing and inbreeding we concurrently selected for large and small body sizes, thus forming sublines high in homozygosity, and differing at a small number of loci with relatively large effects. Results from the backcrossing and the subline differences obtained by inbreeding were published previously (CHAI 1968). This paper concerns the results of our analyses of the F, 5 x 5 diallel crosses of the LG sublines according to the method of HAYMAN (1954) , which we chose because it is descriptive in standard genetic terms and suitable for our present breeding scheme. The estimates of genetic parameters as based on a small number of genes are more revealing than those in our previous studies involving hybrid generations from LG X SM crosses (CHAI 1956a, 1957). What may be considered sex-limited effects of genes on body size are also disclosed. As a supplementary approach to the understanding of genetic differences between the sublines, additional crosses were made with the SM strain in order to test for two alternative hypotheses, mutation us. gene interaction, with results indicating polygenic mutation as described herein. MATERIALS AND METHODS Diallel crosses: Five closely related lines of mice were used as pqrents for the F, set of dialleI crosses, LG/Ckc, B5LG/LA, B5LG/LB, BSLG/SC, and BSLG/SE, thelatter four lines being derived from LG/Ckc. The production history of these lines follows.