The basis of classification of
Linnaeus’ system is sexual reproduction. The
only "natural" taxon in Linnaeus' system is the species. Species defined
as a population with viable, fertile reproduction are separated from
interbreeding with other species.
When a single copy of a disease allele doesn't result in a disease but instead is good for the person or organism that carries it, we say that allele has a heterozygote advantage. In other words this occurs when heterozyhotes have increased fitness over both homozygotes. A good example is sickcle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes.
Part of the dermal tissue that is made of hard, dead, cork cells is the periderm.
