Homozygous Recessive Alleles Research Articles

Genetics of host reactions to three races of the bacterial pustule pathogen in cowpea

Inheritance of the brown hypersensitive resistant (BHR), non-hypersensitive resistant (R) and susceptible (S) host reactions produced by three races of the bacterial pustule pathogen (Xanthomonas campestris pv. vignaeunguiculatae) was studied in 45 F1, F2 and testcross progenies, using the infiltration inoculation method BHR reaction was dominant over R and S reactions, and R was recessive to S reaction. Two genes appeared to be involved in BHR reaction; one governing BHR reaction to race 1 and the other to races 1 and 2. Both were ineffective against race 3. R reaction, effective against all the races, appeared to be controlled by one, two or three recessive genes. One cowpea line had one BHR gene and two duplicate recessive R genes. Reaction expression in the segregants was clear and as expected with races 2 and 3 but was modified with race 1, possibly due to interactions between dominant or recessive alleles of the BHR genes and the homozygous recessive allele of the R genes. Gene symbols Bp-1 and Bp-2 are proposed for the BHR genes and bp-3, bp-4 and bp-5 for the recessive R genes. The genes present in each of the differential cowpea line are suggested.

Crinkle-yellow, a new mutant in Gossypium hirsutum L.

A new mutant called crinkle-yellow and assigned the gene symbol cy is conditioned by homozygous recessive alleles at a single locus. Tests of the mutant with 41 mutant loci found no significant linkage associations. Environmental stresses reduce or eliminate the mutant expression. so that consistent performance with the use of this mutant cannot be expected.

INHERITANCE OF SEED COLOR IN TURNIP RAPE

The inheritance of seed color was investigated in one Swedish and three Canadian cultivars of turnip rape (Brassica campestris L.). Seed color was determined by two independent dominant genes, Br1 and Br3. Dominance at the Br1 locus results in brown seed, while dominance at the Br3 locus and homozygous recessive alleles at the Br1 locus produces yellow-brown seed. The homozygous recessive condition at both loci results in yellow seed. The significance of these findings in relation to other reports is discussed.

Nicotine Biosynthetic Enzyme Activities in Nicotiana tabacum L. Genotypes with Different Alkaloid Levels

Young plants of five Nicotiana tabacum L. genotypes were examined for activity of nicotine biosynthetic enzymes. Genotypes near isogenic except at two loci each with two alleles controlling nicotine level were used in a comparison of the four homozygous allelic combinations producing high, high intermediate, low intermediate, and low nicotine levels in a "Burley 21" background. Putrescine N-methyltransferase (EC 2.1.1.53) and quinolinic acid phosphoribosyltransferase (EC 2.4.2.19) activities in root tissue of these four genotypes were proportional to leaf nicotine level, whereas N-methylputrescine oxidase activity in root tissue differed in proportion and ranking. Quinolinic acid phosphoribosyltransferase activities in leaf tissue were lower than in roots, but no differences were found among the four genotypes. The homozygous recessive alleles at either locus affect levels of all three enzyme activities examined in roots. Each locus seems to be involved in regulation of nicotine metabolism, but whether directly as a regulatory locus or indirectly through the metabolic product of a structural locus is not known.No difference was observed between enzymic oxidation of putrescine and N-methylputrescine by leaf and root extracts of Burley 21 (a high nicotine, low nornicotine genotype) and a high nornicotine cultivar, "Robinson Medium Broadleaf." Putrescine was utilized as a substrate to a greater extent than N-methylputrescine by leaf extracts compared with root extracts of both cultivars. It was concluded that genetic differences in levels of nicotine and nornicotine were not due to differences in enzymic oxidation of these two precursors during alkaloid biosynthesis.

The inheritance of the red leaf reaction of grain sorghum to sugarcane mosaic virus infection

The hypothesis that the necrotic red leaf and red stripe reactions of grain sorghum to sugar-cane mosaic virus – Johnson grass strain infection are independently inherited was tested. Five F1 and 15 F2 populations and one bc1F1 population were grown under both high (26.6–23.8°C) and low (15.5°) temperature regimes following manual inoculation with the virus. The results were consistent with the hypothesis that the presence of a homozygous recessive allele, termed rlf, conditions the expression of the necrotic red leaf reaction following exposure of plants to low temperatures. Plants express only mosaic symptoms in the presence of the dominant allele RLF and the absence of the N gene. The previously reported N gene and the rlf gene were shown to be independently inherited.

Inheritance of Genic Sterility in Cultivated Safflower1

Vegetatively normal but reproductively sterile safflower (Carthamus tinctorius L.) plants were observed in segregating generations (F2, F3, BC1F2, and BC1F3) of crosses involving the cultivar ‘US‐10’ and an introduction from India, 57‐147. Seed heads appeared pinched because of shorter florets at anthesis and an absence of seeds.Different methods of classification, namely, the morphological appearance of flowering heads, seed set under self‐ and open‐pollination, and the morphology and stainability of pollen grains were used to identify the sterile plants under field and greenhouse conditions. Data strongly support the involvement of three unlinked nuclear genes in the inheritance of sterility. It is proposed that US‐10 and 57‐147, respectively, have the genotypes AAbbcc and aaBBCC. Sterility occurs when a homozygous recessive allele at the A locus interacts with a homozygous recessive allele at either the B or C locus. As expected, F2 families segregating at the three loci gave a good fit to a 57:7 (fertile:sterile) ratio in which the sterile genotypes are aabbC‐, aaB‐cc, and aabbcc.

Inheritance and Linkage Relationship of Strap Leaf Mutant in Cotton1

A new mutant in Upland cotton (Gossypium hirsutum L.) is described. The inheritance and linkage data show that the mutant expression is controlled by homozygous recessive alleles at a new locus. The mutant was designated “strap leaf” and assigned the gene symbols ss for the homozygous recessive character. Strap leaf is linked to veins‐fused (vf) and Green lint (Lg) by 3.7 and 11.1 crossover units, respectively.

Cytogenetical Studies on Ecological Races of <i>Mecardonia dianthera</i> (SW) Pennell

Two, morphologically different forms of Mecardonia dianthera viz. erect and repent were found to be growing in nature. By breeding experiments it was found that the erect character (R) is dominant over repent character (r) and the allelic difference seems to lie at one locus. While the erect plants are either homozygous (RR) or heterozygous (Rr), the repent plants possess only the homozygous recessive alleles (rr). Since the two forms differ in ecological distribution, in habitat preference (the erect form being facultative calcicole and repent form an obligate calcifuge) and genetically, these appear to be the two edaphic ecotypes of M. dianthera. Their probable origin is also discussed.

Analysis of a Genetic Double Recessive Completely Male‐Sterile Cotton1

Genetic analysis of a male‐sterile character in cotton (Gossypium hirsutum L.) showed that the male‐sterile expression was conditioned by two pairs of homozygous recessive alleles. Tests of six varieties of cotton showed that they had a homozygous dominant constitution at the two loci. The varieties 'Lankart 57' and 'Gregg' apparently have homozygous dominant genes at only one of the two loci. It is proposed that the appearance of a single male‐sterile plant from an F2 population of a cross involving Lankart 57 occurred as a gene mutation. Tests indicated that the genes at these two loci are independent of the previously reported ms2 gene. The new genes have been assigned the symbol ms5 and ms6. This is the second published report of a complete male‐sterile that is inherited as a recessive character and the first to be inherited as a double recessive.