"We often hear people say that family traits seem to skip a generation. According to Mendel, what explains this phenomenon?"

The notion that family traits seem to skip a generation can be explained using the principles of inheritance discovered by Gregor Mendel, who is known as the father of genetics. Mendel's experiments with pea plants led him to formulate two main principles: the Law of Segregation and the Law of Independent Assortment. The phenomenon where traits seem to skip a generation primarily relates to the Law of Segregation.

Mendel's Law of Segregation states that during the formation of reproductive cells (gametes), the pairs of hereditary traits (alleles) separate (segregate) so that each gamete carries only one allele for each trait. Each parent contributes one of these alleles to the offspring.

Here's a simplified example showing how traits can appear to skip a generation:

1. Generation 1 (Grandparents): Consider a trait governed by a single gene with two alleles: a dominant allele (A) and a recessive allele (a). If both grandparents are heterozygous (Aa), they show the dominant trait.

2. Generation 2 (Parents): When the grandparents produce gametes, the alleles segregate, and each gamete carries either A or a. There's a chance that two carriers (both Aa) mate and produce offspring. In a Mendelian cross between two heterozygous parents (Aa x Aa), the expected offspring genotype ratio is 1 AA : 2 Aa : 1 aa.

3. Generation 3 (Children): Even if both parents show the dominant trait (they might be AA or Aa), there is a 25% chance that they can produce a child with the genotype aa (if both pass on the 'a' allele). This child would exhibit the recessive trait, which was not visible in the parents but was carried by them.

Therefore, the "skipping" of a generation occurs when the recessive trait is unexpressed (hidden) in heterozygous individuals (carriers) of the parents' generation but appears in the next generation when two copies of the recessive allele come together.