About 300 million years ago, chromosomes X and Y were true homologues, similar in size and gene content. Over time, deletions in the Y chromosome resulted in its size reduction to approximately 60 Mb. Significant homology in sequence with the X chromosome is still present. Y chromosome contains the fewest number of genes of any chromosome and is mostly composed of heterochromatin. The genes that are present on the Y chromosome are critically important in sexual development (sex-determining region on the Y gene, SRY, which only determines male sex). Y chromosome contains two pseudoautosomal regions at both ends of the chromosome, where possible recombination with the X chromosome occurs during spermatogenesis. Euchromatin contains functional genes and transcription inert heterochromatin forming a non-recombining region on Y chromosome, which comprises 95% of the chromosome. The same is only present in male, is inherited unchanged from father to son and is rich in polymorphic repetitive elements, microsatellite and minisatellite DNA. Short tandem repeat (STR) loci are located on the non-recombining region of the Y chromosome and are inherited as a block of linked haplotypes. Y-STR haplotyping is of great importance for forensic applications, such as identification of unknown persons, paternity testing, detecting male DNA profile in mixtures and of azoospermic individuals, and verification of amelogenin deficient males.