Simple traits vs complex traits
Some traits follow simple Mendelian rules, where a single geneGeneA stretch of DNA that codes for a functional product, usually a protein. mostly decides the outcome. These often sort people into a few clear categories.
Most common traits are complex instead. Many genes each nudge the trait a little, so the result is a smooth, continuous range, what scientists call a quantitative trait. Height is the classic example: measure a class and you get a bell-shaped spread, not a handful of groups.
Where variation comes from
Variation has two main sources. First, genetic variation: everyone inherits a different mix of allelesAlleleAlternative forms of a gene at the same spot on a chromosome., reshuffled each generation during meiosisMeiosisCell division producing haploid gametes through two rounds of division.. Second, environmental variation: diet, activity, illness, and surroundings all leave a mark.
Because complex traits depend on many genes and the environment, the number of possible combinations is enormous, which is why families resemble each other but are never identical.
