See? I did find a post for the letter X! I promise, though, it is relevant.
How certain traits are passed through a family is of particular interest to me, as my series is based on two families. As with any family, there are certain traits that are predominant throughout the family, and it can often skip generations or present in some but not others.
Many diseases or disorders are passed through X-linked inheritance. In The House of Crimson and Clover, the Deschanels have long possessed certain “skills,” such as telekinesis, telepathy, healing abilities, and even necromancy. Through X-linked dominant inheritance, we see that these skills manifest in quite a few of the Deschanels, where a recessive version of a trait might appear only sporadically.
This can also present through appearance. In the Sullivan family, there are a number of males who have black hair, green eyes, and dimples. There is another set that has red curly hair and a face full of freckles.
It is also not uncommon for the gene to skip generations, based on the way it is passed. Estella Broussard (of the Deschanel clan), one of the MCs in my current WIP, slightly resembles a doll, petite with tiny features and blonde hair. She gets this from her great-grandmother Blanche. They are so alike in appearance that they could be twins, but there are no other women in the generations between them that share these traits.
This is one of the great gifts of being a writer…everything in the world can be relevant to your world.