Mendelian traits in humans

Hitchhiker's thumbs

In Mendelian inheritance, a child receiving a dominant allele from either parent will have the dominant form of the trait. Only those that received the recessive allele from both parents present with the recessive phenotype. Those that receive a dominant allele from one parent and a recessive allele from the other parent will have the dominant form of the trait. Purely Mendelian traits are a tiny minority of all traits, since most phenotypic traits exhibit incomplete dominance, codominance, and contributions from many genes.

The recessive phenotype may theoretically skip any number of generations, lying dormant in heterozygous "carrier" individuals until they have children with someone who also has the recessive allele and both pass it on to their child.

Examples

These traits include:

• Wet (dominant) or dry (recessive) earwax - dry is found mostly in Asians and Native Americans
• Ability to taste phenylthiocarbamide (dominant) - largely determined by a single gene, TAS2R8, with two common alleles, though there are 8 possible haplotypes
• Albinism (recessive)
• Brachydactyly (shortness of fingers and toes)

Questionable traits

May be Mendelian but there is conflicting evidence:

• Ability to smell (bitter almond-like) hydrogen cyanide (recessive) http://omim.org/entry/304300?search=smell%20hydrogen%20cyanide&highlight=cyanide%20smell%20hydrogen

Traits previously believed to be Mendelian

Some traits were previously believed to be Mendelian, but their inheritance is (probably) based on more complex genetic models, possibly involving more than one gene. These include:

• Eye color
• Hair color
• Morton's toe
• Tongue rolling
• Widow's peak (allele)
• Detached (dominant) or attached (recessive) earlobes
• Hitchhiker's thumb (recessive)

See also

• Heritability
• Human genetic variation
• Human genetic clustering

External links

• OMIM Online Mendelian Inheritance in Man