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Isozymes are important biochemical markers in genetic and breeding research. Based on isozyme polymorphism, rice gennplasm has been classified into six varietal groups (Glaszmann 1987). In rice, 40 isozyme loci have been identified, of which 33 have been located on 10 of the 12 chromosomes (Khush and Kinoshita 1991). Isozyme loci having alleles that are no longer transcribed or that code for defective polypeptides lacking enzymatic activity are generally referred to as 'null alleles'. We analyzed 1624 genotypes of traditional rice germplasm of Philippines through starch gel electrophoresis for 16 isozyme loci. Null alleles were identified for 5 isozyme loci; namely Adh-l, Amp-3, Est-I, Est-2 and Est-5 at a frequency 0.18%, 0.99%, 4.93%, 8.13% and 0.30%, respectively. Null alleles were not reported for Cat-1 and Pgi-I earlier. We identified null alleles for these two isozyme loci; for Cat-1 in 'P-Pey Qanakqagtaq' (Accession 8123) a traditional tropical japonica cultivar and for Pgi-I in 'Balit'(Accession 8145), a traditional indica rice cultivar. Cat-1 is located on chromosome 6 having three alleles 1, 2 and 3 (Fig. 1). Our analysis showed that alleles 1 and 2 are common, their frequency being 39.2% and 60.6%, respectively, in the Philippine germplasm whereas allele 3 is very rare and only two genotypes (0.12%) showed this allele. The null allele was identified only in one of the 1624 cultivars analyzed. Pgi-1 located on chromosome 7 has 4 alleles 1, 2, 3 and 4. Amongst 1624 cultivars of Philippines, allele 1 was observed in 20% and allele 2 in 80% while alleles 3 and 4 were absent. Only one cultivar showed null allele for this isozyme (Fig, 2),

Glaszmann, J. C., 1987. Isozymes and classification of Asian rice varieties. Theor. Appl. Genet. 74: 21-30.
Khush, G. S. and T. Kinoshita, 1991. Rice karyotype, marker genes and linkage groups, pp.83-108 In: Rice
Biotechnology. G. S. Khush and G. H. Toenniessen (eds.). CAB International, Oxford, UK.