Segregation
Now that we know what alleles are, we need to see how they are transmitted from parents to progeny. In this animation, two heterozygous brown cats are mated. Mendel's Law of Segregation states that during meiosis each of the two alleles (B or b) segregates, or separates, into different gametes. In this case, the B allele will be found in half of the gametes, and the b allele will segregate to the other half.
During fertilization, any gamete from one parent is equally likely to fertilize any gamete from the other parent. As you can see, this leads to a 1BB:2Bb:1bb genotypic ratio in the progeny. First generation progeny are often referred to as the F1, or first filial, generation.