1.
recessive alleles sometimes remain unexpressed because

another allele masks the recessive allele

recessive alleles rarely cross over

recessive alleles are very uncommon in the population

all of the above are correct.

All of the above

I am torn between 'All of the above' and 'another allele masks the recessive allele,' because 'recessive alleles rarely cross over' may not be correct, for 'crossing over' doesn't depend on whether the allele is dominant or not, if they are there they can cross over, it's the gene's position on the chromosome that determines the probability of crossing over.

The correct answer to this question would be c