# The math behind DNA matching

To the editor:

Among its many mistakes: The searches in the Arizona database that showed matches at nine or more loci do not provide "realistic odds" of a coincidental match for a typical DNA identification. A match at nine loci out the 13 searched means that there are four loci that are different. This excludes someone as a suspect. Police need a perfect match at all the loci that are typed to produce incriminatory evidence.

But let's pretend that there is a case with only nine loci that are typeable and that all nine match. If I buy a lottery ticket identified by nine digits generated at random, the chance that it will match another such ticket is only one in a billion. This is the realistic probability of a coincidental match between the two tickets.

On the other hand, if I look though 10,000 of these tickets to see how many match, I have an immensely better chance of finding matches. The probability has increased to about one in 20 (the 50 million comparisons I am making times the one in a billion chance that each pair of tickets match).

The Arizona database search compared all DNA profiles on record to each other -- just like the trawl through all the tickets. It would be surprising if some low-probability matches did not materialize in the process. Such phenomena might -- or might not -- explain the seemingly high number of matches at nine or more loci. Geneticists are studying the issue. But one thing is certain: It is a very safe bet that no unrelated person in Las Vegas (or even your fraternal twin) has the same DNA you do at exactly nine out of nine identifying loci.

David H. Kaye

TEMPE, ARIZ.

THE WRITER IS A PROFESSOR OF LIFE SCIENCES AT ARIZONA STATE UNIVERSITY.

Reverse mortgages

To the editor:

Hubble Smith's Sunday article, "Reverse mortgage leaves borrower stunned and stuck," was misleading because he relied on sources who don't understand how reverse mortgages work.

William Lancaster, the borrower, received a federally insured reverse mortgage, which enabled him to pay off an existing mortgage of \$54,000 and to establish a \$60,000 line of credit, which he has used to fund retirement needs.

The property does not, as the article misstates, "essentially belong to the lender." The lender has a lien, as in any other loan. This is no different in a reverse mortgage than in a traditional mortgage. Mr. Lancaster now owes the sum of those funds (\$114,000), plus accrued interest and mortgage insurance totaling \$170,000 -- but his home is worth only \$130,000.

Under the federal program, if Mr. Lancaster sells the home to pay back the \$170,000, he will only be responsible for paying up to the sales price, whether it's \$130,000 or \$120,000. The lender would recoup its loss from the Federal Housing Administration.

That's a benefit of the \$2,600 in mortgage insurance premium that he paid to FHA. The Realtor quoted in the story states that Mr. Lancaster paid interest of \$50,000 to \$60,000 in just two years, which he labels "insane," yet the lender who made the loan said he's paying 5.5 percent, which amounts to \$7,000 a year. Obviously, the Realtor has no idea what he's talking about, because he's not in the business of making reverse mortgages.

It would be helpful if the reporter would follow up with knowledgeable individuals who could help analyze the case study presented so the correct facts could be reported.

Darryl Hicks

WASHINGTON, D.C.

THE WRITER IS VICE PRESIDENT OF COMMUNICATIONS FOR THE NATIONAL REVERSE MORTGAGE LENDERS ASSOCIATION.

A new weight fee?

To the editor:

The airline industry has rightfully increased the fees for checked baggage as well as excess baggage. This is all predicated upon an increase in fuel cost.

What about humans who are overweight? Ought they to be charged with an "excess body weight fee" when their weight exceeds a pre-determined maximum limit based on height?

On my flight to Chicago last week, I sat next to a morbidly obese person who must have weighed 400 pounds.

He asked me to raise the armrest between his seat and mine so he could share some of his adipose tissue with me. I turned him down politely. Had I given in to his request, I would have turned into a pancake.

By implementing an "excess body weight fee," an auspicious side effect would be a reduction in health care expenditures because people would be more vigilant to reduce their body weight.

Osnof Iut

LAS VEGAS

Enough said

To the editor:

To all of the people who are complaining about term limits and claiming that it should be the voters who determine who should no longer by in office: It was the voters who voted for term limits! It passed. End of story.

W.J. Park

LAS VEGAS