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Mendel's postulates are a set of principles that form the basis of the modern understanding of genetics. They are derived from the work of Gregor Mendel, who performed breeding experiments with pea plants. The four postulates are: 1. The Law of Segregation: Each inherited characteristic is determined by a pair of factors (genes), which are passed on to offspring as separate units. In the formation of gametes, the factors segregate from each other. 2. The Law of Dominance: In a pair of factors, one factor may mask the expression of the other. The factor that masks the other is called dominant, while the masked factor is called recessive. 3. The Law of Independent Assortment: Factors that determine different traits are inherited independently of each other, as long as they are not on the same chromosome. 4. The Law of Unit Characters: Each trait is determined by a distinct pair of factors (genes).

We need to find which of these postulates can only be demonstrated in crosses involving at least two pairs of traits. The Law of Independent Assortment concerns the inheritance of multiple pairs of traits. Thus, the correct postulate is: The Law of Independent Assortment: Factors that determine different traits are inherited independently of each other, as long as they are not on the same chromosome.