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Some defective genes can be linked to a genetic marker – usually a SNP. The causative gene for the disorder in this case has not actually been identified, but a marker is available that can provide at least some predictive ability. Because the mutation and linked marker lie on the same chromosomal segment, they are usually inherited together, and the marker is relatively (but not absolutely) predictive of the presence of the disease mutation. During meiosis in the formation of eggs and sperm, the maternal and paternal paired chromosomes can cross over and exchange DNA segments. If a genetic cross‐over occurs between a linked marker and the defective gene, the marker will no longer be linked to the defective gene. False‐positive and false‐negative results will occur and allow the marker to be passed on to offspring and descendants. Due to this phenomenon, linked marker test results must be compared with results from other family members to determine whether they correlate with their known phenotype.