Which of the following explanations could account for the fact that dominant alleles that cause fatal disorders are less common than recessive alleles that cause fatal disorders?
A. Every person carrying a single fatal dominant allele dies, whereas most individuals who carry a single recessive lethal allele live and reproduce.
B. Recessive fatal alleles must cause sterility.
C. Dominant alleles that cause fatal disorders are more serious than recessive lethal disorders.
D. Dominant alleles are generally less common than recessive alleles.
Choose A, B, C or D as your solution.
The correct answer is A. This question relies on a knowledge of dominant and recessive patterns of inheritance. Dominant alleles are expressed, even in the heterozygous individual. In this case, our example is a dominant allele that causes a fatal condition. Since every individual carrying a single allele of this type would likely not survive, we would expect the frequency of the allele to decrease over time. In contrast, disorders that follow recessive patterns of inheritance require two copies of the allele for expression. Those who are heterozygous do not express the disorder and thus would survive.