miércoles, 9 de noviembre de 2011
Justin Bieber a Dad? How a DNA Test Can Tell
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After obtaining the sample, a lab technician duplicates the relevant parts of the DNA. "There are specific regions that are helpful," Dr. Michael Baird, chief scientist at the DNA DIsgnostic Center, told Discovery News. His lab lab has does testing for parentage -- and has done so for a number of celebrities such as Anna Nicole Smith. "The regions we look at are the same ones used in forensic analysis," he said.
The DNA is tagged with molecules that fluoresce and is drawn up through a narrow tube, where different parts separate according to their weight. When they emerge from the tube they are hit with a laser light. The way the DNA fluoresces gives a pattern of peaks on a graph. Next, a scientist will compare patterns of peaks produced from the child's DNA to the patterns produced by results from the mother's and father's DNA test. The test examines up to 18 regions of the DNA. In each region, genes from the parents are in pairs, each component of which is called an allele.
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If almost all of the 18 alleles match up, then the man is considered to the be father. If three or more do not match up, then it’s almost certain that the man is not the father. However, if a lot of other markers are the same, one might look for a close male relative, such as a father or brother (Bieber has half-siblings, but his younger brother is in grade school).
Is there anything that can mess up a DNA test? Baird said there are some very rare conditions under which a DNA test can be “wrong.” If Bieber had leukemia and had gotten a bone marrow transplant, the DNA in his blood would show the markers from the donor. "We had one guy come in and his brother was a [bone marrow] donor," he said. Even a cheek swab might pick that up, though it doesn't happen often.
It’s also possible to have mutations, in which one or more of the alleles in the child differs form the parent. Two mismatched alleles occur about one in 500 times. But labs will often simply discard a single allele that doesn't match if all the others do.
The rarest of all cases is chimerism, in which a person has two sets of DNA sequences. There was a case in 2006 in which DNA tests results showed a woman wasn’t the mother of her children, when it fact it was later determined she was. But chimerism is so rare that Baird said he has seen only one case of it in nearly a decade at DDC.