: (1) Variation is the difference in traits between parents and offspring and between individuals of offspring. Parents of peas are all purple flowers, while white flowers appear in offspring, which reflects the difference in traits between parents and offspring. Therefore, this phenomenon is called variation in biology.

(2) In the genetic process of a pair of relative traits, offspring individuals have traits that parents do not have, so the new traits are The genetic composition of parents is heterozygous. If B represents dominant genes, B represents recessive genes. Parents of peas are all purple flowers, and white flowers appear in their offspring, indicating that white flowers are recessive traits, and their genomes are Bb. The genetic diagram is as follows:

(3) From the diagram, it can be seen that if B represents dominant trait genes and B represents recessive trait genes, the parents are purple flowers. The probability of white flowers in offspring is 25%. < P > (4) Chromosomes in pea somatic cells exist in pairs. When forming germ cells, the paired chromosomes should be separated, so the number of chromosomes in germ cells should be reduced by half. < P > Therefore, the answer is: (1) variation;

(2) white flowers

(3) bb; 25%．