Positive Alleles Research Articles

Diallel Analyses of Grain Yield, Percent Protein, and Protein Yield in Grain Sorghum1

A six‐parent diallel cross, including reciprocals, of grain sorghum [Sorghum bicalor (L.) Moench] was analyzed for grain yield, percent protein, and protein yield. Highly significant differences were observed among parents and F1 hybrids for each trait. Reciprocal differences were not significant, but a partial failure of assumptions for the Jinks‐Hayman analysis was noted for all traits.Differences among the parents indicated an additive component of genetic variation for all traits. The estimates for non‐additive components suggested that dominance was important for all traits. The degree of dominance was estimated to be in the overdominance range, particularly for the yield traits. Dominance was in the direction of higher yields and lower percent protein. The positive and negative alleles were estimated to be equally frequent for yield; or percent protein, the negative alleles were estimated to be more frequent. These results suggest that improvement of percent protein requires selection for recessive alleles that have a low frequency; improvement of yield requires a method to cope with overdominance.Highly significant GCA and SCA effects were observed for all traits. A comparison of their relative magnitudes indicated SCA effects were much more important than GCA. Specified comparisons among the parents were made for GCA and among the parents and hybrids for SCA.

A Quantitative Genetic Study of Verticillium Wilt Resistance Among Selected Lines of Upland Cotton1

A quantitative genetic study of the inheritance of resistance to verticillium wilt (Verticillium albo‐atrum Reinke and Berth.) was conducted among 10 selected lines of upland cotton (Gossypium hirsutum L.) utilizing the Jinks‐Hayman diallel analysis. Parents and F1's were studied in 1968 and 1969 at two locations in Oklahoma with the inclusion of F2 progenies in the second year.General tests of the diallel assumptions revealed a partial noncompliance with those assumptions in this material. Epistasis did not appear to be a complicating factor herein, but significant genotype by environment interactions were found among parents from year to year for their additive and dominance components of variation. A significant portion of the dominance interactions could not be attributed to either years or locations.All estimates of environmental variance and of additive genetic variance were significant, as were most estimates of dominance genetic variance. With one exception, estimates of additive variance were larger than the other parameters estimated in the same test. Wilt resistance displayed partial dominance except in one environment where overdominance was found to occur. Direction of dominance was toward greater susceptibility. The frequency of negative versus positive alleles in the parents was likely unequal and biased towards greater susceptibility. Narrow‐sense heritability estimates suggested that rapid genetic advances through selection could be made in most environments.

Inefficiency of genetic recombination in hybrids between Escherichia coli and Salmonella typhosa.

An Escherichia coli Hfr strain in which three negative chromosomal alleles (leu(-), arg(-), and mtl(-)) were closely linked to three positive alleles (ara(+), rha(+), and xyl(+), respectively) was employed in matings with a Salmonella typhosa recipient. The detected expression of the negative E. coli alleles in S. typhosa hybrids selected for receipt of an associated positive E. coli marker was used to determine the occurrence of haploid S. typhosa recombinants, as distinguished from stable partial diploid hybrids. At the same time, the inheritance patterns and segregation behavior of the positive alleles provided indicators of the occurrence of partial diploid hybrids. Examination of both positive and negative markers inherited by ara(+), rha(+), and xyl(-) selected S. typhosa hybrid classes indicated that relatively short E. coli chromosomal segments (generally about 4 min or less in length) were involved in recombination (haploidy), whereas rather extensive E. coli genetic segments were conserved in the diploid state. S. typhosa hybrids selected for receipt of the ara(+) marker showed a 52% incidence of leu(-) haploidy, which is probably close to being an accurate measure of recombination at the site of the ara(+) allele. S. typhosa hybrids selected for receipt of the rha(+) or xyl(+) markers showed only a 20% incidence of arg(-) or mtl(-) haploidy, respectively, but both of these hybrid classes exhibited a higher incidence of conservation of extensive E. coli diploid segments than did the ara(+) selected class. Remating of haploid S. typhosa hybrids with recombinant xyl(+)mtl(-) or rha(+)arg(-) regions resulted in higher frequencies of hybrid recovery than were observed in the initial matings. However, there was a higher incidence of partial diploidy and a lower incidence of haploidy among the hybrids obtained from these rematings.

The patterns of recombination and segregation in terminal regions of T4DNA molecules.

We have analysed single bursts of nineteen-factor T4 crosses in which each bacterium was coinfected with only two particles. The two T4 particles contained, in bursts of class A, each a complete chromosome, in bursts of class B, a complete and an incomplete chromosome and, in bursts of class C, each an incomplete chromosome. Under these conditions the incomplete chromosomes transmit all of their markers to progeny. Thus, their ends can be deduced from genetic analysis. Recombination near such ends is unusually frequent and is, at least partly, due to reiterated events. In bursts of class C, recombination near chromosomal ends often, but not always, shows positive interference and alleles appear with decreasing frequencies the closer they are to an end (i.e. they show a pattern resembling polarized segregation). Many ends in bursts of class B show a similar pattern; however approximately one quarter of the ends show negative interference and alleles appear with increasing frequencies the closer they are to an end, as if the direction of polarity were reversed with respect to the end. Our data also confirm the previous finding that after simultaneous infection with complete chromosomes, on the average, markers on incomplete chromosomes are transmitted to progeny with decreasing frequency the further they are in a clockwise direction from a region near gene 43. We conclude that “polarized” or “reversed polarized” segregation are consequences of two alternative patterns of recombination in which chromosomal ends can participate, and of the fact that only circular T4 DNA molecules are capable of continuous replication. Markers on incomplete chromosomes are excluded from continuous replication until they recombine into circular replicons. If the first recombination of a terminal marker yields single exchange recombinants (cf. Fig. 1C) terminal regions may show positive interference and “polarized” segregation. If the first recombination of a terminal marker yields a double exchange recombinant (cf. Fig. 1D), terminal regions may show negative interference and “reversed polarized” segregation.

Genetic effects on morphological and yield traits in cotton (Gossypium hirsutum L.)

The nature and magnitude of genetic effects on morpho-yield traits were studied in a 6 × 6 F1 and F2 diallel cross inupland cotton. An additive-dominance model was adequate for most of the traits except plant height and seed cottonyield, where the model was partially adequate. Genetic parameters were estimated following Hayman’s and Mather’smodel. Additive effects controlled lint percentage and monopodia in both generations, and plant height and sympodia in F2. Non-additive inheritance with over-dominance controlled yield in both generations, and plant height and sympodia in F1. Most traits presented an unequal proportion of positive (U) and negative (V) alleles in the loci (H2<H1)and an asymmetrical distribution of genes in the parents (H2/4H1 < 0.25 and F different to zero). The value of H2/4H1was lower than maximum value (0.25) for all of traits, which arises when U = V = 0.5 over all loci. The proportion4DH1+F/4DH1–F) confirmed by half of the traits that dominant alleles were in excess as compared to recessive alleles. Dominance effects (h2) for most of the traits suggested that substantial contribution of dominance was not due to heterogeneity of loci in these parameters. Broad and narrow sense heritabilities were high for most of the traits.Correlation coefficient between the Wr + Vr and mid parental (y) indicated that dominant genes were responsible forincreased sympodia, lint % and yield, and recessive genes increased monopodia and plant height. Genetic gain wasencouraging for most traits. Cultivar CIM-1100 was identified by genetic advancement as a promising parental cultivarto cross combinations.

Genetic studies in poultry XIII. Recessive black

A recessive form of black is described and some of its genetical relations set out. Two alternative hypotheses are suggested, of which one involves the existence of two kinds of melanic pigment while the other seeks an explanation in terms of a single pigment. In determining the loci of chromosomal genes it is suggested that the use of the ‘presence and absence’ theory is likely to be less misleading to the poultry geneticist than the assump-tion of pairs of positive alleles.