An ordinary scientist conducted a cross of

问题：

An ordinary scientist conducted a cross of shmoos double heterozygote with the same genotype AaBb. The phenotypic segregation rate of the filial generation he detected is 9:7. He was sure this is not a phenomenon of double recessive epistasis and it should be involved in a segregation rate of 9:6:1, and however, aabb is lethal. He thinks it would not be realizable to distinguish 9:7 and 9:6 by statistic method. but he cannot solve this problem. Can you help him