Juan Jose Rozon lives here and looks after his 69-year-old mother, Milagro.
Three of their relatives have died from late-onset Alzheimer's. Milagro is now showing the first signs of the disease.
Dr Medrano takes Milagro's blood pressure, and examines her feet and her hands.
He then starts asking her simple questions, to test her memory. Milagro doesn't perform well. She can't remember what year she was born, or her home address.
Dr Medrano takes a sample of Milagro's blood. Along with the results of her memory test, this will go towards the library of genetic information he and others have been painstakingly building up.
The gene hunters are also at work more than 2,000km (1,242 miles) away, in the Washington Heights district of New York, where hundreds of thousands of Dominican immigrants have settled.
This is where Bilisia Rozon lives. She is Milagro's first cousin and she is also showing signs of the disease.
Researchers are looking at the Rozon family's medical history
In the memory test, she is not sure which season of the year it is.
When she is asked what day of the week it is, she smiles, looks embarrassed and says she can't remember.
The Rozon family is happy to help with the study, so Bilisia also gives a sample of her blood.
The information gathered about Bilisia will be added to an extensive Rozon family tree that is being put together in the offices of the Taub Institute at Columbia University in New York.
The tree shows which family members are affected by the disease, which are alive and which are dead. For some there is autopsy confirmation that they died of Alzheimer's.
For those hunting the Alzheimer's genes, such a wealth of information about one family is invaluable.
Dr Richard Mayeux, co-director of the Taub Institute, discovered more than 10 years ago that elderly members of New York's Dominican community were three times as likely to suffer from Alzheimer's as other ethnic groups.
He wanted to find out the genetic reason why. So he set up his study, which has become one of the largest in the world.
"Our main objective is to identify as many of the genes as we can that predispose someone to getting Alzheimer's disease.
"I think that if we know who's at risk, by studying their genes and their risk profile, then I could say okay, I've done your annual check-up, and you have a 78% chance of developing Alzheimer's. So, I think you should go on this therapy. Or, I could say I think your risk is very low - all you need to do is go on anti- oxidants or something."
Needle and haystack
Dr Martin Medrano visiting families in the study
To help reach this objective, the blood taken from Milagro, Bilisia, and all the other families in New York and the Dominican Republic comes to the Taub Institute laboratory to be analysed.
The cream-coloured, wispy strands of DNA are extracted. To stop them decaying, hundreds of samples are stored in vats of liquid nitrogen, so that the DNA from those long dead can be looked at again for clues.
All this data helped to lead to the discovery of one of the genes which determine late-onset Alzheimer's.
But along with many other scientists, Dr Mayeux believes that there may be several more.
"It's a little bit like looking for a needle in a haystack in which you have a lot of haystacks and you don't know which one the needle's in," he says.
He believes that the world is a long way from seeing a cure for Alzheimer's, but he is optimistic.
"I think the pace will get better. There will be more genetic libraries like the ones I am involved in, and scientists will explore them in ways we hadn't thought about before. We're going to get better, but it's going to take a while."
• You can watch World News America's series on Alzheimer's on BBC America weeknights at 1900 and 2200 ET and on BBC World News at 0000 BST (for viewers outside the UK only).
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