What caused Mendel to get different ratio results in first and second experiments with crosses?

Gregor Mendel, the father of modern genetics, conducted his famous experiments on inheritance using pea plants. In his experiments, he focused on the inheritance of certain traits, such as height and flower color.

Mendel's first set of experiments involved crossing true-breeding parental plants with contrasting traits, such as tall plants with short plants. True-breeding means that the parents consistently produce offspring with the same traits as themselves when self-fertilized. Through this initial experiment, Mendel observed that all the first-generation (F1) progeny had the same trait as one of the parents, while the other trait seemed to have disappeared. For example, all F1 plants from crossing tall and short parents were tall.

To understand why this happened, Mendel went on to perform a second set of experiments. In this case, he self-fertilized the F1 plants to obtain the second-generation (F2) progeny. Surprisingly, he found that the "disappeared" trait reappeared in the F2 generation, although in a 3:1 ratio. For instance, in the case of height, approximately 75% of the F2 plants were tall, and 25% were short.

The reason behind the difference in ratios between the F1 and F2 generations lies in the fact that traits are determined by discrete units, which Mendel called "factors" or "alleles." To explain this, Mendel proposed what is now known as the Law of Segregation. According to this law, individuals possess two copies of each factor, and during gamete formation (egg and sperm), these copies separate and only one is passed on to the offspring. Therefore, in the F1 generation, all the offspring obtained one copy of the dominant allele from the dominant parent, which masked the expression of the recessive allele. This resulted in a consistent phenotype, such as tall plants.

In the F2 generation, however, the segregation of alleles occurred again. Some offspring obtained two copies of the dominant allele, some got two copies of the recessive allele, and some got one copy of each. As a result, the trait with the dominant allele (tall) was expressed in approximately 75% of the plants, whereas the trait with the recessive allele (short) was expressed in approximately 25% of the plants. This 3:1 ratio is an expected outcome based on Mendel's Law of Segregation.

Thus, the different ratio results between Mendel's first and second experiments with crosses were due to the segregation and independent assortment of alleles during gamete formation, which determined the expression of specific traits in subsequent generations.