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

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.

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