Two kinds of traits
When Mendel crossed pea plants, every trait he tracked fell into tidy categories: tall or short, round or wrinkled, yellow or green. There was no in-between. We call such traits discrete or qualitative — you can sort every individual into a small number of bins. Mendelian inheritance explains them beautifully because each is governed by a single gene with a couple of alleles.
Now line up a hundred adults by height. You won't see two groups — you'll see a smooth gradient from short to tall, with most people clustered near the middle. This is continuous variation, and the trait behind it is a quantitative trait: something you measure on a scale rather than sort into types. Weight, blood pressure, milk yield, and grain size all behave this way.
Why the gradient appears
The smooth spread isn't magic — it's arithmetic. A continuous trait is usually polygenic: many genes each nudge the value up or down by a small amount, and the trait you see is roughly the sum of all those nudges (plus the environment). With one gene you get two or three categories. With twenty genes, the number of possible totals is enormous, the gaps between them shrink, and the histogram fills in into a smooth curve.
One gene (A/a), 'A' adds 1 unit:
aa = 0 Aa = 1 AA = 2 -> 3 height classes
Three genes (A,B,C), each capital adds 1:
count of capital letters can be 0,1,2,3,4,5,6
number of genotypes giving each total:
0: 1 (aabbcc)
1: 6
2: 15
3: 20 <- most common
4: 15
5: 6
6: 1
-> 7 classes, already bell-shaped
Add environment (noise) and the steps blur into a smooth curve.Traits whose final value depends on many genes plus the environment get a special name: multifactorial traits, or more broadly complex traits. They are the rule, not the exception — most of what makes individuals differ in everyday life is complex, not single-gene.