Simple Mendelian genetics in humans
Mendelian traits behave according to the model of monogenic or simple gene inheritance in which one gene corresponds to one trait. Discrete traits with simple Mendelian inheritance patterns are relatively rare in nature, and many of the clearest examples in humans cause disorders. Discrete traits found in humans are common examples for teaching genetics.
Mendelian model
According to the model of Mendelian inheritance, alleles may be dominant or recessive, one allele is inherited from each parent, and only those who inherit a recessive allele from each parent exhibit the recessive phenotype. Offspring with either one or two copies of the dominant allele will display the dominant phenotype.Very few phenotypes are purely Mendelian traits. Common violations of the Mendelian model include incomplete dominance, codominance, genetic linkage, environmental effects, and quantitative contributions from a number of genes.
OMIM is a comprehensive database of human genotype–phenotype links. Many visible human traits that exhibit high heritability were included in the older McKusick's Mendelian Inheritance in Man. Before the discovery of genotyping, they were used as genetic markers in medicolegal practice, including in cases of disputed paternity.
Human traits with probable or uncertain simple inheritance patterns
| Dominant | Recessive | References |
| Low heart rate | High heart rate | |
| Widow's peak | Straight hair line | |
| Facial dimples | No facial dimples | |
| Ability to taste PTC, "Taster" | Unable to taste PTC, "Nontaster" | |
| Unattached earlobe | Attached earlobe | |
| Clockwise hair direction (left to right) | Counter-Clockwise hair direction | |
| Cleft chin | Smooth chin | |
| Freckles | No freckles | |
| Wet-type earwax | Dry-type earwax | |
| Roman nose | No prominent bridge | |
| Marfan syndrome | Average body proportions and connective tissue | |
| Huntington's disease | No nerve damage | |
| Normal mucous lining | Cystic fibrosis | |
| Photic sneeze reflex | No light-induced sneeze reflex | |
| Forged chin | Receding chin | |
| White forelock present | No white forelock | |
| Ligamentous angustus | Ligamentous Laxity | |
| Ability to eat sugar | Galactosemia | |
| Total leukonychia and Bart pumphrey syndrome | Partial leukonychia | |
| Absence of fish-like body odour | Trimethylaminuria | |
| Primary hyperhidrosis | Little sweating in hands | |
| Lactase persistence | Lactose intolerance | |
| Prominent chin | Less prominent chin | |
| Acne prone | Clear complexion |