A zoo blot or garden blot is a type of Southern blot that demonstrates the similarity between specific, usually protein-coding, DNA sequences of different species. A zoo blot compares animal species while a garden blot compares plant species. The purpose of the zoo blot is to detect the conservation of the gene(s) of interest throughout the evolution of different species.
In order to understand the degree to which a particular gene is similar from species to species, DNA preparations from a set of species is isolated and spread over a surface. The sequence of interest is labeled and allowed to hybridize to the prepared DNA. Usually, the labeled DNA is marked with a radioactive isotope of phosphorus. The hybridization is a process that happens spontaneously: DNA pairs with complementary strands. The hybridization, however, is not perfect.
The hybridization of two strands will happen even when the strands are similar but not identical. This procedure is used to detect similar or exact relationships between the DNA in question and other organisms, so the technique takes advantage of non-exact hybridization. It also allows you judge the locations of introns and exons as the latter will be far more conserved than the former.