Final answer:
The sexual life cycle fosters genetic variation through crossing over, fertilization, and the independent assortment of chromosomes. Tetrad alignment on the meiotic spindle is not random in the introduction of genetic variability, most animal life cycles lack a multicellular haploid stage, and a diploid cell results from fertilization.
Step-by-step explanation:
The sexual life cycle of the human species introduces genetic variations through several mechanisms. These include crossing over during meiosis, which is the exchange of genetic material between homologous chromosomes; fertilization, where the random combination of egg and sperm contributes to variation; and the independent assortment of chromosomes, which is the random distribution of maternal and paternal chromosomes into gametes. These processes ensure that each offspring is genetically unique (except in the case of identical twins).
For the questions from the document:
- The source of genetic variation in sexual reproduction that is not considered random is tetrad alignment on the meiotic spindle (d).
- Among the three types of life cycles of sexually reproducing organisms, most animal life cycles (b) do not have a multicellular haploid stage.
- The event that leads to a diploid cell in a life cycle is fertilization (b).