A rapid screening technique for the detection of repeated DNA sequences in plant tissues.

DNA sequences cloned from nuclear and mitochondrial chromosomes have been used as hybridisation probes to distinguish different plant genotypes. The probes are hybridised to squashed segments of tissue e.g. root tips. The 'squash-dot' method is rapid and suitable for screening large numbers of individual plants. One probe, specific for a rye repeated sequence family, enables rye chromosomes to be detected in wheat plants. A probe for ribosomal DNA enables plants with high or low numbers of ribosomal RNA genes to be distinguished. A maize mitochondrial DNA probe is used to distinguish plants with N, T or S cytoplasms.