With major gene effects seedlings are grouped into several discrete categories. However, even simple traits like color are more complicated because a fruit may be red or yellow, but other genes influence the intensity and distribution of color, while additional genes are responsible for striped versus blushed overcolor. Characteristics are said to be quantitatively inherited, or complex, if they are controlled by many genes. Enough genes are involved to blur the categories so that seedlings show a continual gradient. Examples of characteristics in apple under quantitative control include productivity, flavor, firmness, texture, fruit size, and many aspects of tree structure.
Plant breeding can change both simple and complex traits but varietal identity is lost.
Biotechnology maintains varietal identity but can only improve traits under simple control. The emphasis is on obtaining a hybrid that combines the desirable attributes of its parents, but minimizes any of the faults they might have. When we cross two parents to combine favorable characteristics, we can never get the original variety back because the hybrid seedling contains genes from both parents. Similar to what is seen in human families, seedlings may resemble one or both parents or have a unique combination of parental characteristics. Although seedlings may closely resemble a parent, they are never identical to either parent so varietal identity is lost.