evolution acts on the phenotype

evolution acts on the phenotype

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.


Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa, for example, it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele, meaning that the a allele could be passed down to offspring. People who are carriers do not express the recessive phenotype, as they have a dominant allele. This allele is said to be kept in the populations gene pool. The gene pool is the complete set of genes and alleles within a population. For example, Tay-Sachs disease is a recessive human genetic disorder. That means only individuals with the homozygous recessive genotype, rr will be affected. Affected individuals usually die from complications of the disease in early childhood, at an age too young to reproduce. The two parents are each heterozygous (Rr) for the Tay-Sachs gene; they will not die in childhood and will be carriers of the disease gene. This deadly allele is kept in the gene pool even though it does not help humans adapt to their environment. This happens because evolution acts on the phenotype, not the genotype ( Figure 1.1). Tay-Sachs disease is inherited in the au- tosomal recessive pattern. Each parent is an unaffected carrier of the lethal allele.


instructional diagrams

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you can tell the phenotype of a dog by looking at it.

-->  a. true

b. false

natural selection cannot distinguish between dominant and recessive phenotypes.

a. true

-->  b. false

if the recessive allele r is lethal, what genotypes will natural selection benefit?

a) r only

b) rr only

-->  c) rr and rr

d) rr and rr

natural selection acts on _______.

a) heterozygous individuals

b) homozygous individuals

-->  c) the phenotype

d) the genotype

why dont individuals with tay-sachs pass on the tay-sachs allele?

a) because tay-sachs disease is a recessive human genetic disorder.

b) because carriers are not affected.

-->  c) because affected individuals do not have children.

d) all of the above for questions 9 and 10, use the following information. in the fictional hobbit, there exist 5 alleles of the foot size gene: f1 - f5. each allele is dominant over an allele with a higher number. for example f3 is dominant over f4, and f4 is dominant over f5. f5 is recessive to all other f alleles, and results in a hobbled hobbit. this is usually a lethal phenotype.

how does the f5 allele stay in the gene pool?

-->  a) all heterozygous genotypes with an f5 allele are carriers.

b) all f5 homozygous individuals are selected for by natural selection.

c) the f5f5 hobbit is selected for by natural selection.

d) all of the above

what alleles of the foot size gene are part of the gene pool?

a) only the dominant alleles

b) f1 and f2 only

c) f1, f2, f3 and f4

-->  d) all 5 f alleles

diagram questions

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