I had this idea as a gift for my wife, step-daughter and mother-in-law for a few years. I got started on it right away but never managed to finish it because of availability of the individuals, a decent camera and the materials at the same time. Finally! It's done! My project is a portrait of the mitochondrial DNA line of my wife, or for someone not that interested in DNA or genealogy, a generational ladies' portrait.
To start, I purchased three of the same frame. You can accomplish it with one frame or with none, but since I wanted to give it as a gift to three of them, I bought all three at once so they all look identical to the ones shown in the pictures.
Then, I started with the youngest generation. My step-daughter has two daughters, one son, and another daughter on the way. When we started, the second daughter was still very young and another child wasn't even on the radar. Now, the youngest daughter is three years old. Yes, that's how long it took. I took the portrait of her two young daughters. We decided on a location to take the photos so the background was consistent. The background for these photos is my fireplace. Since my step-daughter has a great digital SLR camera and a lot of experience photographing family, she took most of these photos. I appreciate her help on this.
I printed that photo out as an 8x10. You can print it or have it printed at whatever size you want to end up with as the final product. Put this photo into one of the frames you purchased and have the next generation hold this framed photo in front of the same background.
Then print this photo as the same size, remove the first photo from the frame and put the new photo into it. Have the next generation hold the framed photo in front of the same background, print it frame it etc. Repeat this for every generation.
My policy is to not have photos or personal information about living private individuals on my blog so I've blurred out the faces. You can still see how great this turned out.
You can see the photo of my granddaughters held by my step-daughter, held by my wife, held by her mother. I love it and I know all three of them will enjoy a copy of this photo. My granddaughters are now of the age they'll likely enjoy a copy of this also. Even though they all know about it, I know they're going to love it as a Christmas gift.
After this was done, I thought of a longer-term project. This would be the opposite order of generations and could be turned into an impressive photo if many coming generations continue it. You'd start with the oldest living generation, then their child holding that photo, and so on. Doing it this direction, it could end up being a true never-ending project with someone hundreds of years from now holding a photo of a photo of a photo of ... their 5x-great-grandparent. Yes, that's wishful thinking but it could happen.