Question

Which statement is true about lethal alleles?

Answer 1

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

Answer 2

Answer:

Last option.  

Explanation:

Lethal alleles can be defined as the alleles that can cause organism's death having lethal alleles. This is because these alleles are resulted from mutations in genes, which are essential for proper growth and development.

Although lethal alleles are harmful, they can be maintained in gene pool if they express in an organism before his reproductive age as mutations can be passed from one generation to other if mutations are expressed in germline cells before reproduction.

Thus, the correct answer is last option.