Explain why it's unknown which of two alleles an offspring will obtain from its biological mother or father for a given characteristic.

Because alleles can be either dominant or recessive. For example eyes : brown dominant and blue recessive. The mom can have Bb which is dominant and recessive and the dad can have bb only recessive. There's going to be a combination of alleles which leaves the outcome unknown. The offspring can inherit either: Bb or bb

Each parent organism contributes one copy of its alleles in a seemingly random fashion to its offspring, as meiosis to create gametes (egg or sperm cells) reduces the number of genes by half. Therefore, each gamete contributes only one allele.