No two people are genetically identical, except for identical twins. The main source of genetic variation among humans is a. new mutations that occurred in the preceding generation. b. genetic drift. c. the reshuffling of alleles in sexual reproduction. d. environmental effects.

Verified step by step guidance

1. Identify the key concept: The question is asking about the main source of genetic variation among humans, which is crucial for understanding how genetic diversity is maintained in populations.
2. Eliminate incorrect options: Option 'a' focuses on new mutations in the preceding generation, which do occur but are not the main source of variation. Option 'b' refers to genetic drift, which affects allele frequencies mostly in small populations. Option 'd' mentions environmental effects, which can influence phenotype but do not directly cause genetic variation.
3. Focus on the correct concept: Option 'c' mentions the reshuffling of alleles during sexual reproduction. This process, known as recombination, occurs during meiosis when gametes are formed. Recombination and independent assortment of chromosomes lead to new combinations of alleles, which is a primary source of genetic variation.
4. Understand the biological process: During sexual reproduction, each parent contributes half of the genetic material to the offspring. The combination of these different sets of alleles from each parent leads to offspring that are genetically unique, except in the case of identical twins.
5. Conclude with the correct answer: Therefore, the main source of genetic variation among humans is 'c. the reshuffling of alleles in sexual reproduction.' This process ensures a diverse population capable of adapting to changing environments and conditions.