INTRODUCTION Wheat (Triticum aestivum L. em Thell) is an important cereal crop grown globally. Wheat production is a key component in sustaining global food security. Among various threats to wheat production, rust poses a serious problem to wheat cultivation worldwide. There are several evidences of increased yellow rust epidemics around the world which may be due to changing climatic conditions and increased adaptation of pathogen races. Yellow rust also known as stripe rust, caused by Puccinia striiformis Westend f.sp. tritici Eriks and Henn. (Pst), is an economically important foliar disease of wheat crop. In the past two decades, there has been global emergence of aggressive and genetically diverse pathogen populations which are adapted to warmer temperatures (Milus et al., 2009; Hubbard et al., 2015; Hovmøller et al., 2016). In India, yellow rust has become economically important in the recent past especially in cooler areas and is a threat in 10 mha area under Northern parts of India (Bhardwaj et al., 2019). Virulence on major seedling resistance genes including Yr2, Yr6, Yr9, Yr11, Yr12, Yr17, Yr24 and Yr27 has been reported (Wellings and McIntosh 1990; Nsabiyera et al., 2018; Gangwar et al., 2016). Only a few resistance genes are still effective against Pst races which urges the demand to develop durable resistant varieties. Deployment of resistant genes effectively and economically is important to reduce fungicide use and minimize crop losses. Yellow rust resistance genes have been identified progressively in wheat bringing the total number of catalogued genes to 70. Among all the R genes which are still effective against Pst races, Yr5 is dominant seedling- expressed yellow rust R- gene originally identified in T. aestivum subsp. spelta var. album accession (Macer, 1963) and later to be shown in a number of spelta wheats (Kema, 1992). The gene is located on the long arm of chromosome 2B (Law, 1976). This gene can be used effectively in varieties grown in north western Himalayas in India where yellow rust poses havoc to wheat cultivation. In the present study, the inheritance pattern of Yr5 gene was studied in the cross of Avocet-Yr5 with the agronomically superior variety HS 240 and a doubled haploid genotype DH 40 which are suitable for cultivation in NWH zone but susceptible to yellow rust disease. MATERIAL AND METHODS The plant material for the study comprised of wheat genotypes HS 240 (spring wheat variety), DH 40 (a doubled haploid genotype developed by Imperata cylindrica- mediated doubled haploidy breeding technique (Chaudhary et al., 2005), and Avocet-Yr5 (resistant source for Yr5 gene). DH 40 and HS240 were hybridized with Avocet-Yr5 Two sets of F2 population derived from crosses, HS 240 × Avocet-Yr5 and DH 40 × Avocet-Yr5 were tested for rust resistance and linkage with marker. The molecular marker used for the amplification was STS7/STS8 (Chen et al., 2003) (Table 1, Table 2). A. Seedling resistance test Seedling tests were conducted under controlled environment conditions. The parents and segregating generation were tested with pathotype110S119. Fully extended primary leaves were inoculated with the uredospore suspension. The seedlings were transferred to humid glass chamber for 48 hours. The inoculated seedlings were then transferred to glass house at about 15° C. The infection types were recorded 20 days after inoculation and were classified as resistant and susceptible according to Nayar et al. (1997). After phenotypic evaluation, F2 populations were screened for analysis of marker gene association and inheritance of Yr5 gene. B. DNA isolation and PCR amplification Genomic DNA was extracted from leaf samples as per CTAB method (Murray and Thompson 1980). The PCR reaction was performed in a total volume of 15µl, containing 100ng template DNA, 1× PCR Buffer, 2.5 mM MgCl2, 0.2MM dNTP, 0.75U Taq DNA polymerase and 0.3µM of each primer. STS marker STS7/STS8 was used for detection of Yr5 gene (Chen et al., 2003). Amplification were performed in thermal Cycler at 94°C for 4 minutes followed by 40 cycles at 94°C for 45 seconds, 45°C for 45 seconds and 72°C for 60 seconds. A final elongation was performed at 72°C for 10 minutes. PCR products were analyzed by electrophoresis using 3% high resolution agarose gel melting in 1× TAE followed by staining with ethidium bromide and visualized with UV light. C. Data Analysis Chi square analysis was applied to check the validity of expected ratios to that of observed ratio in the segregating generation to test goodness of fit and investigate the inheritance of stripe rust resistance gene & molecular marker. RESULTS AND DISCUSSION In F2 population derived from cross HS 240 × Avocet-Yr5, sixty three individuals exhibited resistant response and twenty four showed susceptible reaction. The segregation pattern in F2 population developed from cross DH 40 × Avocet-Yr5 revealed that sixty nine plants were resistant and twenty nine were susceptible to yellow rust (Table 3). The segregation ratio exhibited goodness of fit to 3:1 ratio in both the crosses. The segregation pattern was analogous to the ratio exhibited by single dominant gene. DNA samples from F2 plants were analyzed to determine linkage between STS marker and resistant gene Yr5. STS marker STS7/STS8 showed polymorphism in the parental genotypes. This marker was further used to analyze segregating ratio in F2 individuals. The PCR amplification showed bands of 478bp in resistant homozygous individuals, 472bp in susceptible homozygous individuals and both the bands in heterozygous genotypes (Fig. 1). The resistant gene Yr5 followed a segregation ratio of 1:3:1 with marker STS7/STS8 in segregating F2 population of crosses HS 240 × Avocet-Yr5 and DH 40 × Avocet-Yr5. These results suggested that the yellow rust resistance to Pst strain is determined by a single dominant gene Yr5. There was no recombination between molecular marker and Yr5 gene, indicating complete linkage between the two.