Food chain and food web

The governing concept is the distinction between genetic dominance and biological advantage. A dominant allele is expressed in the phenotype when present in one copy, whereas a recessive allele may require two copies. Dominance describes expression, not whether a trait improves survival or reproduction. Usefulness depends on the environment and selection. Therefore option A is correct; dominance is not itself natural selection, and recessive traits can appear in suitable genotypes.

The governing concept is allele masking in a heterozygous genotype. When a recessive allele is paired with a dominant allele, the dominant phenotype may appear, so the recessive trait is not visible. However, the recessive allele remains in the DNA and can enter gametes. If two carriers reproduce, their offspring may inherit two recessive copies and express the trait. Thus option A is correct.

In biomagnification, harmful non-biodegradable chemicals such as DDT pass through food and become more concentrated at successive higher trophic levels. Therefore, top consumers may be affected the most. An increase in body size is not biomagnification. Exam tip: Link biomagnification with food chains and non-biodegradable pollutants.