Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

What is the probability that another puppy from a pair of dogs without narcolepsy will have narcolepsy, given the alleles involved?

1. 50%, because narcolepsy is a dominant trait

2. 25%, because narcolepsy is a recessive trait

3. 75%, because narcolepsy is a dominant trait

4. 100%, as all offspring will inherit narcolepsy

The correct answer is: 25%, because narcolepsy is a recessive trait

The scenario describes a genetic condition, narcolepsy, which is determined by recessive alleles. In this context, for a puppy to express narcolepsy, it must inherit two recessive alleles, one from each parent. If both parent dogs do not have narcolepsy, it suggests that they either possess dominant alleles (which do not manifest the condition) or are carriers of the recessive allele for narcolepsy. When considering two dogs that do not exhibit the traits of narcolepsy, and if we assume they are both homozygous dominant (having two dominant alleles), the offspring will have no chance of inheriting narcolepsy. However, if one or both parents are carriers (heterozygous) for the recessive condition, each puppy would have a 25% probability of inheriting two recessive alleles (and therefore the condition). Thus the calculation leads to the conclusion that the likelihood of a puppy having narcolepsy in this situation, given the genetics of the parents, is 25% because narcolepsy is a recessive trait. Understanding the inheritance patterns—dominant as masking the condition and recessive requiring both alleles—clarifies how the probability is determined based on the