Final answer:
The 9:3:3:1 ratio in dihybrid crosses is indicative of the independent assortment of alleles for two different traits, reflecting that each trait is inherited separately from the other.
Step-by-step explanation:
The ratio 9:3:3:1 in dihybrid crosses indicates independent assortment of alleles for two different traits.
When crossing two heterozygous parents (AaBb), we see that the F2 generation exhibits a phenotypic ratio that can be deconstructed into two separate 3:1 monohybrid ratios, which represents the dominant and recessive patterns.
For example, if 'A' represents the dominant allele for seed texture (round), and 'a' the recessive (wrinkled), and 'B' represents the dominant allele for seed color (yellow), and 'b' the recessive (green), the offspring would have a phenotypic ratio of 9 round/yellow, 3 round/green, 3 wrinkled/yellow, 1 wrinkled/green.
This 9:3:3:1 ratio results from the law of independent assortment, which states that alleles for different traits are distributed to sex cells (»gametes) independently of one another. This is confirmed using a 4x4 Punnett square or the probability method, both showing that the traits sort independently to produce the 16-part ratio characteristic of dihybrid crosses assuming that genes are not linked and are assorting independently.