Question

can someone please explain this paragraph to me:
Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual. For many traits, the homozygous genotype, AA, for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the "aa" phenotype is lethal, the recessive a allele, will be maintained in the population through heterozygous Aa individuals. Furthermore, the mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes.

so i can answer the question: explain how a lethal recessive gene can stay in the gene pool.

Answer 1

"Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual." ----> as a quick review, remember that natural selection means that organisms with favorable genes are more likely to pass on their genes (are "selected" to pass on their genes)

when it says natural selection acts on *phenotype* not genotype, it means that an organisms phenotype (actual, physical traits) are the main determining factor of whether it gets to reproduce or not. an organism with a more "favorable" phenotype is more likely to reproduce. it's the trait that matters in the end, not the individual genotype.

"If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual" ---> for simple dominant trait, natural selection will treat Aa and AA genotypes "equally" because they present as the same phenotype (trait)

Answer 2

with that in mind, re-read the passage, and focus especially on the second half, which explains *why* this mechanism of natural selection preserves lethal recessive genes