Question

how do germ-cells make a single set of genes from the normal two copies that all other cells in the body have ?

Answer 1

In the fertilization, both parents contribute their chromosomes. Each gene is present as a independent piece in a chromosome. So from both parents, two copies of chromosomes come. Each germ cell will take one chromosome from each pair and these may be either of maternal or paternal origin. By combination of both gene cells, the number of chromosomes gets normal resulting in the stable DNA structure. 

Answer 2

Explanation:

So from both parents, two copies of chromosomes come. Each germ cell will take one chromosome from each pair and these may be either of maternal or paternal origin. By combination of both gene cells, the number of chromosomes gets normal resulting in the stable DNA structure