12.3: Homologous Chromosomes and Sexual Reproduction

It’s very important to keep chromosome number in context as we discuss genetics and inheritance. As discussed in the last section, the somatic cells of a human are diploid (2n; contain two copies of each chromosome, creating homologous pairs). In Figure \(\PageIndex{1}\), both the adult humans are diploid for chromosome 15, along with the other 22 pairs of chromosomes in humans, which are not shown for simplicity. When gametes (eggs or sperm) are created, those cells are haploid (1n) and contain only one copy of each chromosome. During fertilization, the male and female gametes fuse together creating a fertilized egg, or zygote, which again contains two copies of each chromosome and is therefore diploid. The zygote goes through millions of cell divisions to eventually become a baby and then an adult.

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