The sickle-cell allele is pleiotropic (that is, it affects more than one phenotypic trait). specifically, this allele affects oxygen delivery to tissues and affects one's susceptibility to malaria. heterozygous individuals who have one normal and one sickle-cell allele have a phenotype that has both costs and benefits. the cost: under conditions of low atmospheric oxygen availability, individuals heterozygous for this allele can experience life-threatening sickle-cell "crises." the benefit: such individuals are less susceptible to malaria. thus, pleiotropic genes/alleles reveal that

adaptations are compromises

Explanation:

The pleiotropic gene involved in sickle-cell anemia is an example of heterozygous advantage. This means that the heterozygous genotype is favourale over other two genotypes (recessive homozygous and dominant homozygous). Eventhough it has harmful effect (sickle cell disease carrier) heterozygous genotype enables resistance to infection (malaria) and that is why this genotype is still favourable by natural selection. This phenomenon is evidence of compromise between harmful ad benefit.

one characteristic that is shared by all is a soft body. many

of these invertebrate animals also have an outer shell to protect their

body. oysters, clams, snails, and mussels are all mollusks. so are octopuses, squids, and slugs. mollusks live on land and in freshwater.