Sex cells (gametes) of the father and mother have a haploid (half) set of chromosomes. When the egg is fertilized, a diploid zygote is formed, in which the chromosome set is already doubled, since half of the chromosomes are from the father and half from the mother.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.