The domestic horse karyotype is characterized by the diploid number of chromosomes that equals 64 (13 metacentric and submetacentric pairs, 18 pairs of acrocentric autosomes and a pair of heterochromosomes). Satellites on the first pair of chromosomes as well as secondary restrictions on acrocentric chromosomes 18 and 31 are visible and make it easier to identify them without banding. The aim of the study was to describe the horse karyotype in respect of the size and location of constitutive heterochromatin as well as the location and number of nucleolar organizer regions. C bands in horse chromosomes have been found in centromeric regions of all the examined chromosomes, excluding the 11. pair. Apart form a proximal band C, an additional band in the interstitial part of the q arm was found on the X sex chromosome. Forty-four per cent of the Y chromosome was made up of heterochromatin. Active NORs were identified in the terminal parts of p arm of the 1. chromosome as well as in the subcentromeric part of q arm of pairs 28 and 31. The average NOR number in a cell was 3.61±1.03. The most often observed cells had 3 or 4 active NORs.

41-54