Mendel's law of inheritance describes what happens when the male and female genotypes combine and form a gamete. The alleles of a given trait pair up and separate from other traits. One allele from each member combine and form a pair. The phenotype of the trait is determined by the type of allele combination.
An allele can either be dominant or recessive. A dominant allele will express itself. A recessive allele will be suppressed by a dominant allele.
An allelic pair can be homozygotic or heterozygotic. A homozygotic allelic pair is made up of either 2 dominant alleles or 2 recessive alleles. A heterozygotic allelic pair is made up of 1 dominant allele and 1 recessive allele.
For example, suppose F represents a given trait. Each gamate provides a genotype of trait F. Capital letters stand for dominant alleles and lower case letters stand for recessive alleles. Therefore, according to Mendel's law of inheritance, the possible genotype combinations for this trait are:
FF - both dominant, trait is expressed
Ff - one dominant, one recessive, trait is expressed
fF - one dominant, one recessive, trait is expressed
ff - both recessive, trait is suppressed
According to this pattern, a dominant trait will be expressed 3 times as often as a recessive trait. The combination of genotypes is random.
I have attached a link to a website on Mendelian Genetics.