Hierarchy (Family Tree) Domain Model

Question

I'm wanting to model a family tree in the F# type system. The basic requirements:

• A family tree is made up of family members
• A family member has a required ancestor
• A family member has an optional partner

My struggle is what to do with the "root" family member. I thought about making it an option:

type FamilyMember = { Name:string; Ancestor:FamilyMember option; ... }

but it seems all family members are required to come from somewhere.

So, that lead me to think an ancestor union type, where either it is an empty root or it is a descendant. Is there a cleaner way?

Here's what I have:

type Person = string
type FamilyMember = {
    Person: Person
    Partner: Person option
    Ancestor: Ancestor
}
and Ancestor = 
    | Root
    | Descendent of FamilyMember

let joe = { Person = "joe"; Partner = Some("joyce"); Ancestor = Root; }
let steve = { Person = "steve"; Partner = Some("pat"); Ancestor = Descendent(joe) }
let megan = { Person = "megan"; Partner = None; Ancestor = Descendent(steve)}
megan