 These stocks are often used to provide sources of easily traceable traits when breeding two different lines together  so that the correct progeny of each cross might be selected.
<fluency>  These stocks are often used to provide sources of easily traceable traits when breeding two different lines together  so that the correct progeny of each cross might be selected.
 These stocks are often used to provide sources of easily traceable traits when breeding two different lines together , so that the correct progeny of each cross might be selected.