Establishing Relationships In cases of disputed paternity, how does our justice system determine the rightful father of a child? DNA fingerprinting makes it easy to find alleles carried by the child that do not match those of the mother. Any such alleles must come from the child's biological father, and they will show up in his DNA fingerprint. The probability that those alleles will show up in a randomly picked male is less than 1 in 100,000. This means the likelihood that a given male is the child's father must be higher than 99.99 percent to confirm his paternity.

When genes are passed from parent to child, genetic recombination scrambles the molecular markers used for DNA finger-printing, so ancestry can be difficult to trace. There are two ways to solve this problem. The Y chromosome never undergoes crossing over, and only males carry it. Therefore, Y chromosomes pass directly from father to son with few changes. The same is true of the small DNA molecules found in mitochondriThese are passed, with very few changes, from mother to child in the cytoplasm of the egg cell.

Because mitochondrial DNA (mtDNA) is passed directly from mother to child, your mtDNA is the same as your mother's mtDNA, which is the same as her mother's mtDNA. This means that if two people have an exact match in their mtDNA, then there is a very good chance that they share a common maternal ancestor. Y-chromosome analysis has been used in the same way and has helped researchers settle longstanding historical questions. One such question—did President Thomas Jefferson father the child of a slave?—may have been answered in 1998. DNA testing showed that descendants of the son of Sally Hemings, a slave on Jefferson's Virginia estate, carried his Y chromo-some. This result suggests Jefferson was the child's father, although the Thomas Jefferson Foundation continues to challenge that conclusion.

15.3 Assessment

Review Key Concepts

1. 1. Review Give two practical applications for transgenic plants and two for transgenic animals.

   2. Infer What might happen if genetically modified fish were introduced into an aquaculture facility?

2. 1. Review Name three uses for recombinant-DNA technology.

   2. Apply Concepts Medicines in the body interact with the body's proteins. How might normal variations in your genes affect your response to different medicines?

3. 1. Review List the steps in DNA fingerprinting.

   2. Infer Why is DNA fingerprinting more accurate if the samples are cut with more than one restriction enzyme?

PRACTICE PROBLEM

4. Using restriction enzymes and gel electrophoresis, write the steps of a protocol in which you test for the allele of a gene that causes a genetic disorder.

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