Several inheritable traits or congenital conditions in humans are classical examples of Mendelian inheritance: Their presence is controlled by a single gene that can either be of the autosomal-dominant or -recessive type. People that inherited at least one dominant gene from either parent usually present with the dominant form of the trait. Only those that received the recessive gene from both parents present with the recessive phenotype.
This mode of inheritance implies that the child of two parents that display the "recessive" phenotype should also display the "recessive" phenotype. Should this not be the case, that would be an indicator (but no proof) that one of the parents may not be the child's biological parent.
It also means the "recessive" phenotype may theoretically skip any number of generations, lying dormant in heterozygous individuals until they mate with someone who also has the recessive gene and both pass it on.
These traits include:
Hitchhiker's thumbs
- Oculocutaneous Albinism
- Rh Factor Blood type
- Brachydactyly, type D (Shortness of fingers and toes)
- Cheek dimples (unstudied assertion)
- Wet or dry earwax
- Face freckles
- Widow's peak (unstudied assertion)
Some traits were previously assumed to be Mendelian, but their inheritance is (probably) based on more complex genetic models, possibly involving more than one gene or dependent upon gene x environment interactions. These include:
- Ability to taste phenylthiocarbamide
- Cleft chin
- Eye color
- Free or attached earlobes
- Hair color
- Hitchhiker's thumb
- Six fingers
- Tongue rolling
- Hallux / Morton's toe