The most likely outcome for an allele that causes a disorder in a mammal species, leading to death soon after birth when two matching alleles are inherited, is:
A. It will disappear completely from the population because it decreases fitness.
When an allele causes such a severe disorder that affected individuals do not survive to reproduce, it significantly reduces the fitness of those individuals carrying the allele. As a result, natural selection will work against the allele, leading to its gradual removal from the population over time. Since affected individuals do not pass on the allele to the next generation due to their early demise, the allele will decrease in frequency and may eventually disappear from the population.
