<P> During conception, the father's sperm cell and the mother's egg cell, each containing half the amount of DNA found in other body cells, meet and fuse to form a fertilized egg, called a zygote . The zygote contains a complete set of DNA molecules, a unique combination of DNA from both parents . This zygote divides and multiplies into an embryo and later, a full human being . </P> <P> At each stage of development, all the cells forming the body contain the same DNA--half from the father and half from the mother . This fact allows the relationship testing to use all types of all samples including loose cells from the cheeks collected using buccal swabs, blood or other types of samples . </P> <P> There are predictable inheritance patterns at certain locations (called loci) in the human genome, which have been found to be useful in determining identity and biological relationships . These loci contain specific DNA markers that scientists use to identify individuals . In a routine DNA paternity test, the markers used are short tandem repeats (STRs), short pieces of DNA that occur in highly differential repeat patterns among individuals . </P> <P> Each person's DNA contains two copies of these markers--one copy inherited from the father and one from the mother . Within a population, the markers at each person's DNA location could differ in length and sometimes sequence, depending on the markers inherited from the parents . </P>

When was dna testing first used in criminal cases