Traditional Dutch chicken breeds are marginalised breeds of ornamental and cultural-historical importance. In the last decades, miniaturising of existing breeds (so called neo-bantam) has become popular and resulted in alternatives to original large breeds. However, while backcrossing is increasing the neo-bantams homozygosity, genetic exchange between breeders may increase their genetic diversity. We use the 60K SNP array to characterise the genetic diversity, demographic history, and level of inbreeding of Dutch heritage breeds, and particularly of neo-bantams. Commercial white layers are used to contrast the impact of management strategy on genetic diversity and demography. A high proportion of alleles was found to be shared between large fowls and neo-bantams, suggesting gene flow during neo-bantams development. Population admixture analysis supports these findings, in addition to revealing introgression from neo-bantams of the same breed and of phenotypically similar breeds. The prevalence of long runs ... : Genotype data of 480 chickens from 37 traditional Dutch chicken breedsThe folder contains the information for 52,232 markers genotyped in 480 samples using the Illumina Infinium iSelect 60K BeadChip. Markers are uniformly distributed across the Gallus_gallus5.0 chicken genome, comprising 29 autosomes (Gga 1-28 and Gga 33), two sex chromosomes (W,Z), and one linkage group (LGE64).Traditional_Dutch_chicken_breeds_dataset.zip ...