How does only half of the genetic material of the parent get transmitted to the child?

By meiosis

Explanation:

By the process of meiosis, the diploid (2n) chromosome are halved into haploid chromosomes (1n) and The zygote receives only one of the male and female gamete (1n) to become a diploid (2n) zygote.

during the process of meiosis, in meiosis1 the sister chromatids exchange their segments and the process is known as crossing over the exchange of segments results in the recombination of genetic information and lead to mutation (changes) in the zygote or the diploid parent cells divide into haploid gametes and the gametes fuse together to form a zygote