Final answer:
When both parents are homozygous recessive for the trait of tongue curling, all of their offspring will be homozygous recessive as well and will not be able to curl their tongues. Therefore, A) the trait for tongue curling will show up 0% of the time in their offspring.
Step-by-step explanation:
If two parents are homozygous recessive for tongue curling, it means they both carry two copies of the recessive allele for this trait. Because tongue curling is a dominant trait, and both parents lack the dominant allele for this trait, all of their offspring will inherit two recessive alleles (one from each parent). As a result, none of their children will be able to curl their tongues. Thus, the appearance of the trait for tongue curling in their offspring will be 0% of the time.
In genetics, when individuals have two identical alleles for a given gene—whether dominant or recessive—they are considered homozygous for that gene. On the other hand, a person who has one dominant allele and one recessive allele is termed heterozygous for that gene. For a child to express a dominant trait, at least one parent must contribute a dominant allele. In this scenario, both parents are homozygous recessive (rr), meaning that all offspring will also be homozygous recessive (rr) and will not exhibit the dominant trait of tongue curling.