The process of “grading up” is when animals of two pure breeds are crossed, and the crossbred progeny of each successive generation are bred back to animals belonging to one of the pure breeds from the initial cross. This grading up process does not automatically imply inbreeding. This process may also pertain to pure breeding lines (not necessarily “breeds” per se) in animals or in plants.
This grading up concept is illustrated in the table below with two breeds (A and B). The female progeny in one generation are the female parents in the following generation.
|Cross||Male parent||Female parent||Progeny generation|
|1||A||B||½ A ½ B - (F1, First-cross progeny)|
|2||A||½ A ½ B F1||¾ A ¼ B - (a backcross)|
|3||A||¾ A ¼ B (backcross)||Third cross progeny|
|4||A||Third cross progeny||Fourth cross progeny|
|5||A||Fourth cross progeny||Fifth cross progeny|