Alma 3:6 - 3:10

Alma 3:6 And the skins of the Lamanites were dark, according to the mark which was set upon their fathers, which was a curse upon them because of their transgression and their rebellion against their brethren, who consisted of Nephi, Jacob, and Joseph, and Sam, who were just and holy men.

The “mark” of the dark skin was placed on the Laman and Lemuel and their descendants on account of their rebellion and refusal to listen to Nephi, Jacob, Joseph and Sam, holy men, when Lehi and the family arrived in the promise land.

Alma 3:7 And their brethren sought to destroy them, therefore they were cursed; and the Lord God set a mark upon them, yea, upon Laman and Lemuel, and also the sons of Ishmael, and Ishmaelitish women.

This was in turn on account of Laman, Lemuel, sons of Ishmael and the Ishmaelitish women who all wanted to destroy the descendants of Nephi and those loyal to the gospel.
It is interesting that the daughters of Ishmael are mentioned here not just the boys. The girls must have been just as angry to be so proximately mentioned at being taken from Jerusalem and their treasures as were Laman, Lemuel and Ishmael sons. However at least three of the sisters married Nephi, Sam and the servant of Laban.

Alma 3:8 And this was done that their seed might be distinguished from the seed of their brethren, that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions which would prove their destruction.

The curse or mark was placed on the Lamanites just so make sure that the Nephites would know who the Lamanites were and not mix with them. They would be able to tell each other apart. The reason of not mixing the two groups was an effort to make sure as far as possible that the false traditions of the Lamanites did not corrupt the correct traditions of the Nephites.

Alma 3:9 And it came to pass that whosoever did mingle his seed with that of the Lamanites did bring the same curse upon his seed.

However, if a Nephite should marry a Lamanite, they would also find the mark on their children.

Alma 3:10 Therefore, whosoever suffered himself to be led away by the Lamanites was called under that head, and there was a mark set upon him.

So anyone who united with the Lamanites would also find themselves carrying the same mark.