Why are lethal dominant alleles so much more rare than lethal recessive alleles?

a. the lethality associated with lethal dominant alleles is much more severe than that associated with lethal recessive alleles.
b. the types of mutations that create lethal dominant alleles are much less frequent than those that create lethal recessive alleles.
c. less is known about lethal dominant disorders, so the rareness of these alleles is an artifact due to a lack of detection.
d. lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles.

The correct answer is: d. Lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles.

Lethal alleles cause the death (prenatal or after birth, but usually early in development) of the organism that carries them. There are three types of lethal alleles:  

•Recessive-both alleles must be defective,  

•Dominant-it is sufficient that one allele is lethal to cause the death,

•Conditional depending on the gene or genes involved.  

Lethal alleles are usually a result of mutations in genes essential for development of an organism.

i need an image of the diagram

helicase, i think thats it