Mixed Ancestry

Your character has inherited qualities from biological parents of two or more ancestries. Perhaps your parents are an elf and a dwarf, a halfling and a gnome, a dragonborn and a human, or one or both are themselves of mixed ancestry. Regardless, your ancestries provide you certain inherited characteristics. In addition, you may have traits that are unique to children who claim more than one ancestry.
Characters of mixed ancestry might look almost entirely like one parent or the other, or anywhere on the continuum between them. Thus the two children of a dwarf-tiefling couple might both look like tiefling, dwarven, some combination, or one might look tiefling and the other dwarven, even though they are siblings.
The rules below are intended to allow players to make characters that have two primary ancestries, rather than one.
The Player's Handbook Half-Elf and Half-Orc races are Human-Elf and Human-Orc mixed ancestries respectively.

Source
Ancestry & Culture by Arcanist Press

Mixed Ancestry

Ability Score Increase Varies on ancestries

Size Medium

Speed Varies on ancestries

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Ability Score Increase. You may distribute 3 ability score increases among the ability score increases available for your two selected ancestries. The maximum ability score increase you may have for a single ability score is 2. Hence, you may have either a +2 in one ability score and +1 in another, or have +1 to three ability scores. For example, a High Elf-Asmodeus Tiefling character may distribute their ability scores between Dexterity and Intelligence (from their High Elf ancestry) and Charisma (from their Asmodeus Tiefling ancestry [Intelligence omitted because it is shared between both ancestries]), to get a +2 Charisma and +1 Dexterity ability score increase.
Age. Pick two of your ancestries from the available races. Select a number in between the two ages listed for when people of those races come of age, and again between the numbers listed for their average lifespans. Write these down; these are your age at which you become an adult and your expected lifespan.
Size. Pick the listed size from your chosen ancestries and choose one of the sizes listed. If both of your ancestries are size Medium, then so are you. If both are Small, then likely so are you. If both Small and Medium sizes appear in your chosen ancestries, you may choose which size you are of those two.
Speed. Your base walking speed is 30 feet, unless both of your chosen ancestries have a base speed of 25 feet, or your size is Small, in which case your base walking speed is 25 feet.
Darkvision. As long as one of your ancestries has the Darkvision trait, you may have darkvision as well, if you choose.
Additional Traits. You may select three traits from your two chosen ancestries, though not all three may be from a single ancestry. For example, you might select Fey Ancestry and Keen Senses from an elven ancestor, and Infernal Legacy from an Asmodeus Tiefling ancestry. If you have a dragonborn ancestor, you make select Draconic Ancestry and either the Breath Weapon trait or the Damage Resistance trait, but not both. You may not have the Feat trait from Variant Human as one of the traits if you are of mixed Human ancestry.

Languages. You can read, speak and write Common and two languages from your two selected ancestries.