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

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Hint: A process of cell division called meiosis is performed by all species that reproduce by sexual reproduction, i.e. by creating gametes or germ cells and then fertilizing the female gamete with males. Meiosis is the cell reduction division in which the cell's chromosome number is reduced to half in the germ cell, which restores the chromosome number when merged with the other germ cell.

Complete answer:
Germ cells use meiosis to produce normal two copies of the genes using a single set and this process is called reduction division. Meiosis is of two types-
Meiosis I- Meiosis is a slow process, mainly because of the time that the cell spends in prophase I, compared to mitosis, takes place in a matter of minutes. The pairs of homologous chromosomes come together during prophase I to form a tetrad or bivalent, comprising four chromatids. Within this tetrad structure, recombination can arise between any two chromatids. In structures known as chiasmata, which occur late in prophase I, crossovers between homologous chromatids can be visualised. Chiasmata is essential for precise meiosis. Cells that do not form chiasmata can not necessarily be able to properly segregate their chromosomes during anaphase. Meiotic cells join metaphase I at the termination of prometaphase I. Next, the pairs of homologous chromosomes divide into different daughter cells during Anaphase I. However, the crossovers between chromosomes must be overcome before the pairs can split, and meiosis-specific cohesins must be released from the sister chromatids' arms.
Meiosis II- The daughter cells enter meiosis II after meiosis I without transiting via interphase or replicating their DNA. A mitotic division resembles Meiosis II, except that the number of chromosomes has been decreased by half. Thus, four haploid cells that contain a single copy of each chromosome are the results of meiosis II. In mammals, males and females vary in the amount of viable gametes obtained from meiosis. From each spermatogonium, four haploid spermatids of similar size are produced in males. The cytoplasmic divisions that occur during meiosis are rather asymmetric in females.
The descendant cells or species receive the genetic material of their parents during sexual reproduction. The male and female gametes combine during fertilisation to form the zygote, which then evolves to form the embryo. The gametes inherit either parent's haploid genome and these characters present in the genome move on to the offspring during fusion to form a diploid zygote. 22 pairs of chromosomes (autosomes) and a pair of sex chromosomes (allosomes) are present.

Therefore, the originally created germ cells (having sex chromosomes) are diploid in nature. In order to generate haploid gametes, diploid germ cells need to undergo several rounds of cell division and build several new cells. Therefore, meiosis allows the germ cells to make a new collection of genes (haploid) from the usual two (diploid) copies.