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In a study of 35 white-throated sparrows five distinct karyotypes were observed. A chromosomal polymorphism is present which involves at least two pairs of macrochromosomes. This species is phenotypically polymorphic with selective breeding occurring between morphs. Phenotype is related to chromosomal constitution, and selective breeding appears to maintain heterozygosity within the population.
H. Bruce Thorneycroft (Fri,) studied this question.