Mendel's first law, states the law of segregation. This law states that every individual possesses a pair of alleles, for a particular trait. And each parent passes a randomly selected copy of only one of these to its offspring. The offspring receives its own pair of alleles, for that trait.
The behavior of chromosomes during meiosis explains Mendel's law of equal segregation. If we consider a heterozygote of general type A/a, one homolog carries A and one carries a. - replication occurs (one dyad is AA other one is aa) pairing occurs (tetrad is A/A/a/a) first division products (one cell is AA other cell is aa) second division occurs results in four cells (Two cells of type A and two of type a).
This can be explained in the case of corn pollen:
The iodine dye is used to color the starch producing corn pollen. Since, pollen is haploid, there will be meiotic division. As per Mendel's law of equal segregation, the alleles divide into gametes. So, it will be 1:1 ratio of starch producing versus non-starch producing pollen grains from a parent, which is heterozygous.
The pollens can be colored easily and ratio can be observed.
Hence, the products of meiosis from a heterozygous meiocyte A/a are 1/2 A and 1/2 a, which is exactly equal ratio required to explain Mendel's law.