The Book of Mormon says nothing about Indians. It is about a family of Jews from Jerusalem who migrated by the direction of God to the American Continent. This happened about 600 years before Christ just before Jerusalem was destroyed and the Jews taken captive to Babylon. The Book of Mormon is about their journey and beliefs. They became a mighty nation on the American continent and split into two groups. One that believed in God and one that did not. According to the Book of Mormon this nation was later destroyed and their remnants scattered. They possibly could have been the forbears of the American Indians. That is the widely held belief of many Mormons, but the truth is we do not really know what came of the people mentioned in the Book of Mormon. It is a matter of much speculation, but no one knows for sure.
Lamanite is a generic term for all the indigenous people of the American continent. This includes Indians, as well as those that occupied central and South America. It also includes Polynesians as well. Now there are thousands of tribes and different sorts of peoples that populated this continent over the last couple of millennia. They all have different genetic markings. Not all of them came from the Book of Mormon people. In the Book of Mormon, when the Lamanites destroyed the Nephites they were scattered and of course mixed with all the other indigenous people. They assimilated into the various cultures depending where they were scattered. Now in the Book of Mormon it talks about the Niphites being of lighter skin and the Laminates being of darker skin. The Nephites were lighter because they did not mix into the various tribes that existed on this continent. They kept to themselves and were somewhat isolated. The Lamanites however mixed heavily with the darker skinned natives. They began to take on the beliefs and customs of the other native people and turned away from God. The curse was not that they had darker skin. It was because they were separated and set apart from God's people. Towards the end of the Book of Mormon it is the Lamanites that turned back to God and the remnants of the Nephites turned against God. That is the reason the Nephites were ultimately destroyed. The genetic markings of the original family that migrated from Jerusalem were destroyed along with them. Eventually even the few Lamanites that had turned to God died out and were not able to continue their traditions and beliefs and pass them onto the next generations. The final prophet Mormon compiled the history and beliefs of his people and wrote them upon gold plates. His son Moroni traveled all the way to what is now upstate New York and buried the plates. In 1820 the resurrected Moroni appeared to Joseph Smith and showed him were to find the plates and translate them to English by the power of God so we would have The Book Of Mormon Today.
This is why they cannot find genetic markings among American Indians today. Anti-Mormons have tried to use that fact to try and disprove The Book of Mormon. The thing is, we do not know exactly were the Nephites lived, And we do not know absolutely for sure which Native American's today are descendants of the original lamanites. Any possible trace of a genetic marking would be stretched extremely thin after two thousand years.