The majority of kelpies have a shorter, or smooth, type of coat. Probably because most kelpies are predominantly homozygous for a short coat. It means both alleles at the locus are the same. However, some kelpies are heterozygous for coat length. They have a dominant allele for the short coat, but also a recessive allele for the longer coat. Therefore, occasionally a kelpie may have a longer coat.
At the beginning of the breed's development, some long coated dogs were used which brought the gene for long coat into the breed. Since then, the recessive gene was inherited but hidden during many generations until dogs mated both carrying the same recessive gene.