Which statement is true about lethal alleles?
they cannot be maintained in a gene pool because the individuals with them never survive.
they can be maintained in a gene pool by heterozygous carriers.
they cannot be maintained in a gene pool because they are always dominant.
they can be maintained in a gene pool if they are expressed before
reproductive age.

They can be maintained in a gene pool if they are expressed before 
reproductive age.

For example:
Sickle cell anemia is caused by an abnormal hemoglobin in red blood cells. hemoglobin is the red pigment found in red blood cells for carrying oxygen.The abnormality arises from a genetic mutation in the DNA gene that codes for the  beta chain of the protein called globin of which hemoglobin is made of.In the beta chain, the sixth amino acid called glutamine is replaced by another one called valine.This one change in the amino acids cause the hemoglobin protein  to behave abnormally, causing  red blood cells to lose their normal spherical shape and become bent like a sickle, hence the name "sickle cell" anemia