Final answer:
New combinations of linked genes are typically due to the process of crossing over during meiosis. Independent assortment and fertilization also contribute to genetic variation, but to a different extent.
Step-by-step explanation:
New combinations of linked genes are primarily due to crossing over. This process occurs during prophase I of meiosis, where non-sister chromatids of homologous chromosomes exchange genetic material, leading to unique combinations of genes on each chromosome. This recombination can happen multiple times within the same homologous pair, creating a diversity of genetic outcomes.
While independent assortment also contributes to genetic variability, it operates by different mechanisms where chromosomes - and hence the genes upon them - are distributed randomly into gametes. This occurs during the metaphase stage of meiosis. Finally, the random fusion of sperm and egg during fertilization adds another level of variation.