Explain
What is the "principle of independent assortment" and how is this this principle demonstrated in the results of Mendel's dihybrid crosses?
The principle of independent assortment, also known as Mendel's second law, states that the inheritance of one trait is independent of the inheritance of another trait. This means that the alleles for different traits segregate independently of each other during gamete formation and combine randomly in offspring.
Mendel's dihybrid crosses demonstrated this principle. In a dihybrid cross, Mendel studied the inheritance of two different traits at the same time. For example, he crossed pea plants that had smooth and yellow seeds (SSYY genotype) with plants that had wrinkled and green seeds (ssyy genotype).
According to Mendel's observations, in the first generation (F1), all the offspring had smooth and yellow seeds (SsYy genotype) because the dominant traits (smooth seeds and yellow color) masked the recessive traits (wrinkled seeds and green color).
However, when the F1 generation was crossed with each other, the principle of independent assortment became evident. Mendel observed that the traits were inherited independently, meaning that different combinations of trait alleles occurred in the offspring. As a result, in the second generation (F2), the offspring displayed a phenotypic ratio of 9:3:3:1 for the four possible combinations of traits (smooth & yellow, smooth & green, wrinkled & yellow, wrinkled & green).
This 9:3:3:1 ratio confirmed the principle of independent assortment, as it demonstrated that the inheritance of the seed shape trait was independent of the inheritance of the seed color trait. The alleles for both traits segregated independently, contributing to the random combination of traits in the offspring.