Pollen is essentially a male plant germ cell .
Like all germ cells it is produced in the process of meiosis. In this process, the diploid cells (cells that have two sets of chromosomes) divide into two haploid cells (cells that have one set of chromosomes).
In our example, the diploid mother cell has a SS Tt genotype. This essentially means that the cell has two alleles of the same gene on a different chromosome in the chromosome pair.
When the cell divides, it can give only one allele to the haploid cell.
For the S gene, this cell has two identical dominant alleles( SS genotype), therefore, it can only give the dominant allele to the haploid cell.
As for the T gene, this cell has two different alleles, one dominant and one recessive (Tt genotype) and therefore can give either the dominant (T) or the recessive (t) allele to the haploid germ cell (the pollen).
So, the possible genetic combinations are
ST if the dominant T allele is passed
St if the recessive t allele is passed
