Observation - both parents and children have just one pair of each gene, and children get one of their pair from each parent. Explanation - chromosomes duplicate and split up in mitosis so that gametes each get one of each chromosome.
Chromosomes come in pairs.
Observation - most genes act like they are inherited indpendently. The allele one gets for one gene from one parent seems independent of the alleles for any other genes. Genes on the same chromosome should be inherited all together. The answer to this paradox is recombination. Pairs of chromosomes line up during the cell division events that create gametes and switch pieces with each other.