Types of SSRs (simple sequence repeats), SSRs are short DNA sequences consisting of a tandem repeat of a few nucleotides, typically 2-6 nucleotides in length. There are different types of SSRs based on the length and pattern of the repeated sequence, as well as the presence or absence of interruptions of non-repeated nucleotides within the repeat array. The four types of SSRs are:

1. Perfect SSR: This is the simplest type of SSR, where the same repeat motif is present adjacent to each other without any interruption of any other nucleotide. For example, a perfect SSR with the repeat motif "CAT" would be "CATCATCATCAT", where the "CAT" sequence is repeated four times.

2. Imperfect SSR: This type of SSR contains repeat motifs that are interrupted by one or a few non-repeat nucleotides. For example, an imperfect SSR with the repeat motif "CAT" would be "CATCATGGCATCATCAT", where the "CAT" sequence is repeated twice, but interrupted by "GG".

3. Compound perfect SSR: This type of SSR contains two or more repeat motifs lying adjacent to each other, separated by no or very few intervening nucleotides. For example, a compound perfect SSR with the repeat motifs "CAT" and "GTC" would be "CATCATCATGTCGTC", where the "CAT" sequence is repeated three times, followed by the "GTC" sequence repeated twice.

4. Compound imperfect SSR: This type of SSR contains two or more repeat motifs interrupted by several non-repeat nucleotides. For example, a compound imperfect SSR with the repeat motifs "CAT" and "GTC" would be "CATCATCATNNNNNNNGTCGTCGTC", where the "CAT" sequence is repeated three times, interrupted by several non-repeat nucleotides, followed by the "GTC" sequence repeated three times.