Genetics of stem elongation ability in rice (Oryza sativa L.)

Summary The genetics of stem elongation ability in rice was studied in parents, F₁, F₂ and backcross generations of six crosses. Segregation analysis indicated dominance for stem elongation ability. Estimation of genetic parameters under epistatic model indicated more than one locus control stem elongation ability and both additive and nonadditive gene effects were important. Epistatic effects were predominant over additive and dominance effects with an important role of duplicate type of epistasis. The occurrence of significant additive and additive x additive types of genetic variation and the moderately high broad sense heritability indicated the possibility of selection for an increased manifestation of stem elongation ability.     

甘蓝型油菜角果长度的主+多基因混合遗传模型

角果是油菜产量构成要素中重要的组成部分。本文以长角果品种中双11和短角果材料10D130为亲本配制杂交组合,采用主基因+多基因混合遗传模型分析方法对该组合6世代遗传群体(P₁、P₂、F₁、BCP₁、BCP₂和F₂)的果身长、角果长和果喙长进行遗传分析。结果表明,该组合的3个角果性状均呈连续分布,其中,果身长最适遗传模型为E-0 (2对加性-显性-上位性主基因+加性-显性-上位性多基因模型),2对主基因加性效应值分别是1.75和–0.06,显性效应值分别是–0.59和–0.86,主基因遗传率在BCP₁、BCP₂和F₂中分别是51.10%、74.23%和66.93%,多基因遗传率分别为29.16%、17.11%和23.96%。角果长的最适遗传模型为E-1 (2对加性-显性-上位性主基因+加性-显性多基因模型),其中,第1对主基因加性效应为0.34,显性效应为–0.81,第2对主基因加性效应为0.34,显性效应为–0.47,主基因遗传率在BCP₁、BCP₂和F₂中分别是47.63%、68.51%和79.45%,多基因遗传率分别为29.40%、20.89%和12.47%。果喙长的最适遗传模型为E-3模型(2对加性主基因+加-显多基因遗传模型),2对主基因加性效应值分别是0.2和–0.2,主基因遗传率在BCP₁、BCP₂和F₂中分别是33.71%、72.75%和52.25%,多基因遗传率分别为40.08%、5.37%和27.60%。     

油菜半矮杆新品系10D130株型性状的遗传分析

株型改良是油菜高产、优质育种的主攻方向之一。矮杆及半矮杆株型有利于提高植株抗倒伏能力和经济系数、减少收获难度。10D130是一个半矮杆新品系, 用10D130和常规优良品种中双11杂交, 构建6世代遗传群体(P₁、F₁、P₂、B₁、B₂和F₂), 以主基因+多基因混合遗传模型对该组合株高及其关联性状进行遗传分析。结果表明, 10D130×中双11组合株高、分枝部位、主花序长度的遗传均受到1对加性-显性-上位性主基因+加性-显性-上位性多基因控制(D-0模型)。其中, 株高性状加性效应值为–8.58, 显性效应值为7.44, 主基因遗传率在B₁、B₂和F₂中分别为23.52%、0.91%和17.81%;一次有效分枝起始部位的1对主基因加性效应值为–22.11, 显性效应值为3.13, 主基因遗传率在B₁、B₂和F₂中分别为49.95%、40.85%和61.15%;主花序长的主基因加性效应值为–2.21, 显性效应值为1.6, 主基因遗传率在B₁、B₂和F₂中分别为0.68%、47.94%和40.07%。一次有效分枝间距的最适宜遗传模型为E-1模型(2对加-显-上位性主基因+加-显-上位性多基因混合遗传模型), 其中第1对主基因加性效应值为–0.55、显性效应值为–1.66, 第2对主基因加性效应值为0.74、显性效应值为–1.29, 均表现超显性遗传, 主基因遗传率在B₁、B₂和F₂三个分离世代群体中分别为10.99%、38.65%和44.10%。一次有效分枝部位高度、主花序长、有效分枝节间距和有效分枝数与株高均呈显著正相关。     

Quantitative and molecular characterization of heat tolerance in hexaploid wheat

Understanding the genetic basis of tolerance to high temperature is important for improving the productivity of wheat (Triticum aestivum L.) in regions where the stress occurs. The objective of this study was to estimate inheritance of heat tolerance and the minimum number of genes for the trait in bread wheat by combining quantitative genetic estimates and molecular marker analyses. Two cultivars, Ventnor (heat-tolerant) and Karl92 (heat-susceptible), were crossed to produce F₁, F₂, and F₃populations, and their grain-filling duration (GFD) at 30/25 ^°C 16/8 h day/night was determined as a measure of heat tolerance. Distribution of GFD in the F₁ and F₂ populations followed the normal model (χ², p > 0.10). A minimum of 1.4 genes with both additive and dominance effects, broad-sense heritability of 80%, and realized heritability of 96%for GFD were determined from F₂ and F₃ populations. Products from 59primer pairs among 232 simple sequence repeat (SSR) pairs were polymorphic between the parents. Two markers, Xgwm11 andXgwm293, were linked to GFD by quantitative trait loci (QTL) analysis of the F₂ population. The Xgwm11-linked QTL had only additive gene action and contributed 11% to the total phenotypic variation in GFD in the F₂population, whereas the Xgwm293-linked QTL had both additive and dominance action and contributed 12% to the total variation in GFD. The results demonstrated that heat tolerance of common wheat is controlled by multiple genes and suggested that marker-assisted selection with microsatellite primers might be useful for developing improved cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

Estimation of genetic parameters for in vitro traits in wheat immature embryo cultures involving high X low regeneration capacity genotypes

Summary Estimates of gene actions were obtained for five in vitro traits of immature wheat (Triticum aestivum L.) embryo cultures from a cross of two wheat cultivars and the resulting reciprocal, F₁, F₂ and backcross populations. The contribution of additive gene effects to in vitro traits was not as important as the dominance gene effects. Epistatic gene effects were relatively more important than either additive or dominance gene effects. Of the individual types of digenic epistatic effects, the dominance x dominance estimates were relatively larger in magnitude for all in vitro culture traits measured. The maternal effect played a minor role in the inheritance of the in vitro studied traits since the difference among the reciprocal values was not significant. It is shown from the generation mean method that epistasis played a major role in the inheritance of most of the traits under study. The negative values of additive and dominance genetic variance were estimates of zero. Heritability estimates, in broad sense, were relatively high for the in vitro studied traits. In some cases, heritability estimates in broad and narrow senses are almost equal since the estimation of dominance genetic variance led to negative values. According to the results of the gene effects, dominance and epistasis were important for the shoot formation trait. Selection would be effective among the isolated genotypes on individual basis.     

玉米茎秆糖含量的遗传模式分析

较高的茎秆糖含量有助于提高青贮玉米的饲料品质和适口性,本研究以YXD053和98A-04两个高茎秆糖含量玉米自交系为母本,Y6-1低茎秆糖含量玉米自交系为父本,通过自交、杂交及回交产生2个组合的6个世代(P₁、P₂、F₁、F₂、BC₁和BC₂);运用主基因+多基因混合遗传模型6个世代联合分析方法,探明控制玉米茎秆糖含量的遗传模型,并进行遗传参数估计。结果表明,玉米茎秆糖含量遗传受2对加性-显性-上位性主基因+加性-显性-上位性多基因共同控制。YXD053×Y6-1及98A-04×Y6-1两个组合的主基因遗传率分别为53.50%和52.63%,多基因遗传率分别为7.96%和17.31%,总遗传率分别为61.46%和69.94%,显性度(h/d)均小于1。茎秆糖含量以主基因遗传为主,且主基因又以加性效应为主,但环境因素对茎秆糖含量的遗传有一定的影响。这一研究结果为玉米茎秆糖含量性状的基因定位和育种选择提供了理论依据。     

Estimation of genetic components of variation for salt tolerance in chickpea using the generation mean analysis

Chickpea (Cicer arietinum L.) is known to be salt-sensitive and in many regions of the world its yields are restricted by salinity. Recent identification of large variation in chickpea yield under salinity, if genetically controlled, offers an opportunity to develop cultivars with improved salt tolerance. Two chickpea land races, ICC 6263 (salt sensitive) and ICC 1431 (salt tolerant), were inter-crossed to study gene action involved in different agronomic traits under saline and control conditions. The generation mean analysis in six populations, viz. P₁, P₂, F₁, F₂, BC₁P₁ and BC₁P₂, revealed significant gene interactions for days to flowering, days to maturity, and stem Na and K concentrations in control and saline treatments, as well as for 100-seed weight under salinity. Seed yield, pods per plant, seeds per plant, and stem Cl concentration were controlled by additive effects under saline conditions. Broad-sense heritability values (>0.5) for most traits were generally higher in saline than in control conditions, whereas the narrow-sense heritability values for yield traits, and stem Na and K concentrations, were lower in saline than control conditions. The influence of the sensitive parent was higher on the expression of different traits; the additive and dominant genes acted in opposite directions which led to lower heritability estimates in early generations. These results indicate that selection for yield under salinity would be more effective in later filial generations after gene fixation.     

Growth habit and gene pool effects on inheritance of yield in common bean

Summary To satisfy farmer and consumer preferences, breeding efforts to increase yield potential in common bean must take into account the interrelated effects of growth habit, seed size, maturity, and gene pool on yield expression in segregating populations. To examine the relationships among these traits, a genetic study was conducted to determine the effect of growth habit on yield and seed size in crosses among five bean lines from diverse gene pools. Two parental bean lines had determinate, type I growth habits and large seed size typical of the Neuva Granada-Andean gene pool. Two other lines were tropical Mesoamerican types with type II growth habits and small seed size; and the fifth line, G13625, a landrace of the Jalisco gene pool from the Mexican highlands, had a type IV climbing growth habit and medium seed size. Individual F₂ plants from each cross and parental lines were evaluated for growth habit and yield component traits under high input field conditions. The following season, the evaluations were repeated on random F₃ plants. Of the five parental lines, only G13625 showed significant GCA effects for yield in both the F₂ and F₃ generations. Improved yielding ability of G13625 progeny was associated with an increased expression of climbing bean growth habit traits: guide length, climbing ability, node number on main stem, and plant height. Crosses between Andean x Mesoamerican and Andean x Jalisco genotypes, as well between growth habit type I (Andean x Andean) and between type II (Mesoamerican x Mesoamerican) had very low parent-offspring heritability values for yield. Yield heritability was only significant for crosses between Mesomerican x Jalisco gene pools. An apparent simple genetic control of growth habit modification towards semi-climbing and climbing types is proposed as the major reason for increased yields in these crosses. No genetic linkage between genes controlling growth habit and seed size was detected which might restrict the development of high yielding large-seeded type II lines.     

甜瓜柄蔓夹角主基因+多基因遗传分析

柄蔓夹角是甜瓜株型重要性状之一,通过对甜瓜柄蔓夹角的遗传特性进行研究,以期为甜瓜株型改良育种提供理论基础。本研究以柄蔓夹角小的紧凑型甜瓜材料B103和柄蔓夹角大的疏散型甜瓜材料B297为亲本,构建四世代群体(P1、P2、F1和F2),应用植物数量性状主基因+多基因混合遗传模型多世代联合分析方法,对甜瓜柄蔓夹角进行遗传分析。两年的研究结果显示,甜瓜柄蔓夹角性状表现为数量性状,不同年份柄蔓夹角的最适遗传模型均为E-1模型,即柄蔓夹角由两对加性-显性-上位性主基因+加性-显性多基因控制。2015年秋,F2分离世代主基因遗传率是72.28%,多基因遗传率是0;2016年春,F2分离世代主基因遗传率为52.06%,多基因遗传率为0。结果表明甜瓜柄蔓夹角的遗传符合两对加性-显性-上位性主基因+加性-显性多基因遗传模型(E-1),这一性状由主基因控制,同时受环境影响。     

甘蓝型油菜分枝角度主基因+多基因混合遗传模型及遗传效应

分枝角度是油菜株型重要性状,是油菜品种高产及适合机械化收获理想株型的基本组成之一。为明确油菜分枝角度的遗传,本研究选用油菜分枝角度大的松散型材料6098B和分枝角度小的紧凑型材料Purler配制杂交组合,采用主基因+多基因混合遗传模型方法对该组合6世代(P₁、P₂、F₁、F₂、BCP₁和BCP₂)的分枝角度进行了遗传分析。结果表明,上部第一分枝(顶枝)和基部第一分枝(基枝)角度的最适合遗传模型均为D-0 (1对加性-显性主基因+加性-显性-上位性多基因)。顶枝角的主基因加性效应值为4.939º,显性效应值为–4.156º,主基因遗传率在BCP₁、BCP₂和F₂中分别是34.08%、1.40%和14.99%,多基因遗传率分别为24.43%、61.72%和63.98%;而基枝角的主基因加性效应值为2.217º,显性效应值为–1.941º,主基因遗传率在BCP₁、BCP₂和F₂中分别是7.86%、1.24%和4.84%,多基因遗传率分别为66.46%、58.49%和73.96%。结果发现油菜分枝角度明显存在主效基因,为油菜分枝角度的遗传改良奠定了基础。     

中国南瓜可溶性固形物含量的主基因+多基因遗传分析

为明确中国南瓜可溶性固形物含量的遗传规律,选用中国南瓜杂交获得的6世代群体(P1、P2、F1、F2、BCP1、BCP2)为材料,应用植物数量性状的主基因+多基因遗传模型研究其遗传规律。结果表明,该群体可溶性固形物含量的遗传符合2对加性-显性-上位性主基因+加性-显性多基因混合遗传模型,2对主基因的加性效应均为-0.7077,均使可溶性固形物含量降低;显性效应分别为3.5034和1.3586,均使可溶性固形物含量升高。多基因的加性效应和显性效应分别为2.3066和-0.6679。其主基因遗传率在BCP1、BCP2、F2分别为17.06%、56.01%、95%,多基因遗传率在BCP1、BCP2、F2分别为47.16%、18.78%、0;说明主基因表现出较高的遗传力,可以在早期世代对可溶性固形物含量进行选择。研究为中国南瓜育种品质性状选择和分子标记辅助选择提供了理论依据。     

Performance of upland coloured cotton germplasm lines in line × tester crosses

Anthracnose is a serious disease affecting dry bean production especially in the cool highland areas worldwide. The objective of this research was to study the inheritance of anthracnose resistance in market-class dry beans. A complete diallel set of crosses was generated from nine diverse parents comprising six resistant and three susceptible to anthracnose. The F₁ and F₂ crosses and parents were artificially inoculated with Colletotriclum lindenumthianum Race-767 in a growth room. There was significant variation for anthracnose resistance among genotypes. General combining ability (GCA) and specific combining ability effects were significant for resistance, indicating importance of both additive and non-additive effects, respectively. Preponderance of GCA effects (66%) suggested that additive effects were more important than non-additive effects (24%), which were also reflected by high heritability estimates (70%), and suggested that simple selection or backcrossing would be useful for improving the resistance in market class varieties. The study was not conclusive on whether epistatic gene action played a major role, but if available it might have biased the dominance gene effects. Reciprocal effects (10%) were not significant (P > 0.05), suggesting that cytoplasmic genes did not play a major role in modifying anthracnose resistance. Parental lines G2333, AB136, NAT002, and NAT003 showed highly negative GCA effects qualifying them as suitable parents for transferring resistance genes to their progenies. A few major genes, 1–3, displaying partial dominance conditioned anthracnose resistance, suggesting a possibility of using marker-assisted selection to improve anthracnose resistance in market-class dry beans.     

Diallel analyses reveal the genetic control of resistance to ascochyta blight in diverse chickpea and wild Cicer species

Ascochyta blight is a major fungal disease affecting chickpea production worldwide. The genetics of ascochyta blight resistance was studied in five 5 × 5 half-diallel cross sets involving seven genotypes of chickpea (ICC 3996, Almaz, Lasseter, Kaniva, 24B-Isoline, IG 9337 and Kimberley Large), three accessions of Cicer reticulatum (ILWC 118, ILWC 139 and ILWC 184) and one accession of C. echinospermum (ILWC 181) under field conditions. Both F₁ and F₂ generations were used in the diallel analysis. The disease was rated in the field using a 1–9 scale. Almaz, ICC 3996 and ILWC 118 were the most resistant (rated 3–4) and all other genotypes were susceptible (rated 6–9) to ascochyta blight. Estimates of genetic parameters, following Hayman’s method, showed significant additive and dominant gene actions. The analysis also revealed the involvement of both major and minor genes. Susceptibility was dominant over resistance to ascochyta blight. The recessive alleles were concentrated in the two resistant chickpea parents ICC 3996 and Almaz, and one C. reticulatum genotype ILWC 118. The wild Cicer accessions may have different major or minor resistant genes compared to the cultivated chickpea. High narrow-sense heritability (ranging from 82% to 86% for F₁ generations, and 43% to 63% for F₂ generations) indicates that additive gene effects were more important than non-additive gene effects in the inheritance of the trait and greater genetic gain can be achieved in the breeding of resistant chickpea cultivars by using carefully selected parental genotypes.     

Genetic analysis of grain mould resistance in coloured sorghum genotypes

Grain moulds are a major constraint to sorghum production and to adoption of improved cultivars in many tropical areas. Information on the inheritance of grain mould reaction is required to facilitate breeding of resistant cultivars. The genetic control of grain mould reaction was studied in 7 crosses of 2 resistant sorghum genotypes. P₁, P₂, F₁, F₂, BC₁ and BC₂ families of each cross were evaluated under sprinkler irrigation for field grade and threshed grade scores and subjected to generation mean analysis. Frequency distributions for grain mould reaction were derived and F₂ and BC₁ segregation ratios were calculated. Grain mould reaction in crosses of coloured grain sorghum was generally controlled by two or three major genes. Resistance to grain moulds was dominant. Significant additive gene effects were also found in all cross/season combinations. Significant dominance effects of similar magnitude to additive effects were also observed in five out of ten cross/season combinations. Gene interactions varied according to the parents with both resistant and susceptible parents contributing major genes. Choice of parents with complementary resistance genes and mechanisms of resistance will be critical to the success of resistance breeding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response to selection, combining ability and heritability of coleoptile length in winter wheat

Selection of winter wheat cultivars with long coleoptiles is an important component of improving emergence, weed suppression and grain yield in low rainfall regions of the world. Seven winter wheat cultivars were hybridized in a Griffing’s half-diallel mating design, method 2 (reciprocals excluded, parents included), model 1 (fixed), and the progeny analyzed for coleoptile length in the F₁ through F₄ for response to selection, general combining ability (GCA), specific combining ability (SCA) and heritability. Highly significant differences existed for GCA among progeny in each generation. SCA was highly significant in the F₁, F₂ and F₄, although SCA estimates were inconsistent among generations in each progeny population. The SCA/GCA ratio was 0.15, 0.11, 0.06 and 0.04 in the F₁ through the F₄, respectively, indicating that additive effects increased with homozygosity. Broad-sense heritability for coleoptile length was estimated at 0.86, 0.76, 0.60 and 0.78 for the F₁ through the F₄, respectively. Realized heritabilities ranged from −0.16 to 0.85 with a mean of 0.32 in the F₃ and ranged from −0.68 to 0.68 with a mean of 0.13 in the F₄. Spearman’s rank correlation coefficients (R_S) were significant for all generation comparisons except the F₁:F₂, indicating that changes in rank for coleoptile length were insignificant. Cultivars with long coleoptiles and high GCA were the best parents for improving coleoptile length. Sustained selection over generations for coleoptile length was more effective than one generation of selection for genotypes both with and without reduced height genes.     

Additive gene action for resistance to Puccinia striiformis F.SP. Tritici in Triticum dicoccoides

Summary Seven single-plant selections of wild emmer, with temperature-sensitive minor-effect genes for stripe rust resistance, were intercrossed in eight combinations. The resulting progenies were studied for a possible additive gene action.The transgressive segregation towards resistance in F₂ observed in all the combinations indicates that additive gene action for resistance indeed occurs in wild emmer. The common occurrence of this phenomenon in random combinations suggests further that several minor-effect genes are involved.Following selection of the most resistant plants in F₂, a marked shift towards resistance was noted in F₃, which demonstrates a positive response to selection. In some instances, additive resistance selected for (in F₂) at the high temperature-profile was expressed (in F₃) also at the low temperature-profile. This kind of resistance, when utilized in breeding programmes, promises therefore to be effective over a range of temperatures.     

Variation in pigment content of flour color in bread wheat (Triticum aestivum L.)

Summary Variation in pigment content of the flour of bread wheats (Triticum aestivum L.) was studied in the progenies of F₁ and F₂ of three crosses and their reciprocals. Reciprocal differences in pigment content were observed in the F₁ and F₂ means. Low pigment content was found to be partially dominant or over dominant in the crosses studied. There was evidence of substantial mid-parent F₁ heterosis in all crosses and betterparent F₁ heterosis in three crosses. In the F₂, heritability estimates were moderate to high. The F₂ frequency distributions were not normal. Estimation of effective factor pairs indicated the presence of one or two major gene pairs involved in the expression of pigment content in the flour. Action of modifiers was also assumed in one cross and its reciprocal. A factorial approach to metrical character suggested that the F₂ segregation ratios of low pigment content to high pigment content were 3:1, 15:1, 13:3 and 9:7 for the different crosses. Utilization of the findings in a wheat breeding program is briefly discussed.     

Inheritance of partial rust resistance in pearl millet

Quantitative disease resistance should be exploited to complement the use of genes for qualitative or hypersensitive resistance. The expression and inheritance of partial rust resistance of pearl millet inbreds 700481-21-8 and ‘ICMP 501’ crossed to moderately susceptible Tift 383’ were evaluated in seedling assays in the greenhouse and in generation mean and single-seed descent populations in the field. Uredinium sizes on seedling leaves of hybrids were generally intermediate to those of the parental inbreds and consistent differences could be discerned in uredinium lengths. Area under the disease progress curves (AUDPCs) of individual plants of the parents, F₁, F₂, and backcross F₁S to each parent were determined from field trials. Broad-sense heritability estimates for both crosses were 43%. In generation mean analyses, additive genetic effects were significant in the cross of 700481–21–8 × Tift 383′, whereas additive, dominance, and dominance × dominance epistatic effects were significant for ‘ICMP 501’בTift 383’. The number of genes conferring partial resistance was estimated to be two for 700481–21–8 and 2.5 for ‘ICMP 501’. A hierarchical single-seed descent analysis revealed significant differences in AUDPC among F₃-derived F₄ progenies in the F₆ generation. Selection for progenies with greater resistance should be possible among F₄ families. Higher levels of resistance were observed in progeny derived from ‘ICMP 501’. Because segregation of resistance differed among progeny derived from 700481–21–8 and ‘ICMP 501’, the genetic basis for resistance probably differs between the two inbreds.     

Diallel analysis of sooty stripe resistance in sorghum

Sooty stripe [Ramulispora sorghi (Ellis and Everhart) Olive and Lefebre] is a widespread foliar disease of sorghum [Sorghum bicolor (L.) Moench] in West Africa, responsible for grain yield losses up to 46%. We studied the inheritance of sooty stripe resistance in a 9 × 9 sorghum F₂-population diallel grown together with parent lines and checks in1996 under natural disease pressure at two locations in Mali. The percentage of infected leaf area was determined twice over a two-week interval during the season. At the second evaluation, the mean sooty stripe severity amounted to 13% infected leaf area at Samanko and 12% at Cinzana. The frequency distribution of the entries was approximately normal for the mean disease severity, averaged across assessment dates and locations, pointing to the involvement of multiple genes. With the data combined across the two locations, genetic differences among lines and among F₂ populations were highly significant. Genotype × location interaction variances were also significant but much smaller than the genetic variances. Broad-sense heritability estimates were 0.92 for lines and 0.94 for the F₂ populations, for the mean percentage infected leaf area across the two assessment dates. General combining ability effects (GCA) determined most of the differences among the F₂ populations. Specific combining ability effects (SCA), and the interactions of GCA or SCA with locations were also significant but less important. Line performance per se was highly correlated with GCA. Because of the high heritability and predominance of additive effects, prospects are good for the genetic improvement of resistance to sooty stripe in sorghum in Mali, using simple pedigree or recurrent selection procedures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Involvement of higher order interactions addressing complex polygenetically controlled inheritance of downy mildew [Sclerospora graminicola (Sacc.) Schrot] resistance in pearl millet [Pennisetum glaucum (L.) R.Br.]

Inheritance of downy mildew [Sclerospora graminicola (Sacc.) Schrot]resistance was studied using generation mean analysis in pearl millet [Pennisetum glaucum (L.) R.Br.]. Eleven basic generations, namely, P₁, P₂, F₁, F₂, B₁, B₂, B₁F₂, B₂F₂, L₁, L₂ and L₃ of three crosses involving six diverse lines for downy mildew incidence were evaluated under artificial epiphytotic conditions over two environments. The downy mildew incidence was best fitting for digenic, trigenic and tetragenic ratios when fitted into classical Mendelian ratios demonstrating involvement of two or more genes. Digenic and trigenic interaction models were adequate in the case of crosses I and III respectively, to account for the total variability in generation means. Unlike severity, comparative estimates of gene effects over two environments were mostly consistent in all crosses for prevalence. Most of the epistatic and major gene effects were found significant in all crosses for both the disease traits. Non-allelic interactions particularly at three-gene loci viz., w (additive × additive × additive) and y (additive × dominance × dominance) in cross II and all trigenic interactions in cross III were predominant. Duplicate dominance (cross I) and complementary epistasis (crosses II and III) were observed for both the traits revealing inconsistency of gene effects over crosses. The gd₁ (interaction of additive gene effect with e₁) and gh₁(interaction of dominant gene effect with e₁) were significant in crosses I and II, indicating interaction of additive and dominance gene effects with environments. Thus a breeding method that can mop up the resistant genes to form superior gene constellations interacting in a favorable manner against pathotype I would be more suitable to accelerate the pace of resistance improvement. This revised version was published online in August 2006 with corrections to the Cover Date.