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A deterministic model has been constructed to study the heterozygosity in a triploid and tetraploid parthenogenetic lineage. New alleles produced by mutation may be of two different kinds: those coding for functioning enzymes and those coding for defective enzymes. Population size is assumed to be stable. Selective processes are not considered if an individual has at least one functioning allele at a given locus. The amount of heterozygosity depends on time. Mutation pressure first increases functioning heterozygosity to a maximum value, which is a function of time, mutation rate and initial heterozygosity. Functioning heterozygosity will therefore decrease due to the increase in the frequency of nonfunctioning alleles. The life expectancy of polyploid parthenogenetic populations is, however, increased in relation to diploid parthenogenetic populations. The proportion of functioning heterozygosity is higher in a polyploid model, which, in part, explains the preponderance of polyploid forms among apomictically parthenogenetic insects.