Final answer:
A child inherits a combination of DNA from both parents, with each parent contributing one set of chromosomes and associated alleles. This genetic mixing leads to the unique genetic makeup of each individual, apart from identical twins.
Step-by-step explanation:
Our model for how DNA is passed on from parent to offspring in sexually reproducing organisms assumes that all the DNA that a child inherits is a combination of DNA from both parents. In sexual reproduction, a child receives 50% of their genes from the mother's egg cell and the remaining 50% from the father's sperm cell. The process of meiosis ensures that gametes (sperm and egg) contain one allele for each gene, with alleles being different versions of a gene located at the same locus on homologous chromosomes. Therefore, the child inherits one set of chromosomes and associated alleles from each parent, which leads to genetic variation.
Genetic variation is further increased by the fact that these alleles can be different, resulting in combinations such as homozygous (the same allele from both parents) or heterozygous (different alleles from each parent) for a given trait. For instance, a child may inherit the allele for dimples from one parent and the allele for smooth skin from the other parent. This genetic combination is what creates a unique genetic profile for each individual, apart from identical twins.