玉米杂交种是由自交系杂交育成的。自交系又是从玉米育种群体中分离选出的。所以原始群体的状况如何,对从中所选取的自交系的优劣及是否能育成强优势的杂交种起着决定性的作用。但是在过去很长的时间里,由于忽视了群体的量要性,把由少数书干自交系组配成的亲缘关系很近的杂交种做为主要群体,从中分离选取二环系,从而导致了玉米种质的单一化。不仅影响到新的优良杂交种的选育,还使得现有生产上使用的杂交种抗逆性明显减弱,难以抵御病害的侵袭。如1971年美国因小斑病t小种的流行,当年减产玉米165亿kg。又如70 Maize hybrids are bred from inbred lines. Inbred lines are again isolated from the corn breeding population. Therefore, the status of the primary population will play a decisive role in the pros and cons of the inbred lines selected and whether it will breed strong heterosis. However, for a long time in the past, due to the neglect of the importance of the population, as a major group, the close relatives of the few inbreds from inbred lines were selected as major groups and the second ring system was separated Led to a single corn germplasm. Not only affect the selection of new hybrids, but also make the existing production of hybrids significantly reduced the resistance, it is difficult to resist the invasion of disease. As in 1971 the United States due to the prevalence of small spot disease t race, then reduce corn output 16.5 billion kg. Another example is 70