phenotype

Examples of phenotype in the following topics:

• Complementation

  • If, when these strains are crossed with each other, some offspring show recovery of the wild-type phenotype, they are said to show "genetic complementation".
  • Since the mutations are recessive, the offspring will display the wild-type phenotype.
  • Complementation arises because loss of function in genes responsible for different steps in the same metabolic pathway can give rise to the same phenotype.
  • Because the mutations are recessive, there is a recovery of function in that pathway, so offspring recover the wild-type phenotype.
  • Thus, the test is used to decide if two independently derived recessive mutant phenotypes are caused by mutations in the same gene or in two different genes.

• Phenotypes and Genotypes

  • The observable traits expressed by an organism are referred to as its phenotype and its underlying genetic makeup is called its genotype.
  • The observable traits expressed by an organism are referred to as its phenotype.
  • Johann Gregor Mendel's (1822–1884) hybridization experiments demonstrate the difference between phenotype and genotype.
  • That is, the hybrid offspring were phenotypically identical to the true-breeding parent with violet flowers.
  • In his 1865 publication, Mendel reported the results of his crosses involving seven different phenotypes, each with two contrasting traits.

• Alternatives to Dominance and Recessiveness

  • However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents.
  • The chinchilla phenotype, cchcch, is expressed as black-tipped white fur.
  • The Himalayan phenotype, chch, has black fur on the extremities and white fur elsewhere.
  • Finally, the albino, or "colorless" phenotype, cc, is expressed as white fur.
  • Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type.

• Stabilizing, Directional, and Diversifying Selection

  • The result of this type of selection is a shift in the population's genetic variance toward the new, fit phenotype.
  • Sometimes natural selection can select for two or more distinct phenotypes that each have their advantages.
  • In these cases, the intermediate phenotypes are often less fit than their extreme counterparts.
  • Diversifying selection can also occur when environmental changes favor individuals on either end of the phenotypic spectrum.
  • Different types of natural selection can impact the distribution of phenotypes within a population.In (a) stabilizing selection, an average phenotype is favored.In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed.In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

• Epistasis

  • In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting.
  • Therefore, the genotypes AAcc, Aacc, and aacc all produce the same albino phenotype.
  • Finally, epistasis can be reciprocal: either gene, when present in the dominant (or recessive) form, expresses the same phenotype.
  • Keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction.
  • Recall the phenotypic inheritance pattern for Mendel's dihybrid cross, which considered two non-interacting genes: 9:3:3:1.

• Frequency-Dependent Selection

  • In frequency-dependent selection, phenotypes that are either common or rare are favored through natural selection.
  • As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes.
  • Negative frequency-dependent selection serves to increase the population's genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.
  • Frequency-dependent selection allows for both common and rare phenotypes of the population to appear in a frequency-aided cycle.
  • Positive frequency-dependent selection reinforces the common phenotype because predators avoid the distinct coloration.

• Mendel's Law of Dominance

  • Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively.
  • By definition, the terms dominant and recessive refer to the genotypic interaction of alleles in producing the phenotype of the heterozygote.
  • However, this can easily lead to confusion in understanding the concept as phenotypic.
  • For example, to say that "green peas" dominate "yellow peas" confuses inherited genotypes and expressed phenotypes.
  • This will subsequently confuse discussion of the molecular basis of the phenotypic difference.

• No Perfect Organism

  • Natural selection is also limited because it acts on the phenotypes of individuals, not alleles.
  • Some alleles may be more likely to be passed on with alleles that confer a beneficial phenotype because of their physical proximity on the chromosomes.
  • Natural selection acts on the net effect of these alleles and corresponding fitness of the phenotype.
  • However, the intermediate phenotype of a medium-colored coat is very bad for the mice: these cannot blend in with either the sand or the rock and will more vulnerable to predators.
  • It is simply the sum of various forces and their influence on the genetic and phenotypic variance of a population.

• The Punnett Square Approach for a Monohybrid Cross

  • If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well.
  • Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents.
  • Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green.
  • In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype.
  • This cross produces F1 heterozygotes with a yellow phenotype.

• Mendel's Law of Independent Assortment

  • Independent assortment allows the calculation of genotypic and phenotypic ratios based on the probability of individual gene combinations.
  • From these genotypes, we infer a phenotypic ratio of 9 round/yellow:3 round/green:3 wrinkled/yellow:1 wrinkled/green .
  • Because of independent assortment and dominance, the 9:3:3:1 dihybrid phenotypic ratio can be collapsed into two 3:1 ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern.
  • Round/green and wrinkled/yellow offspring can also be calculated using the product rule as each of these genotypes includes one dominant and one recessive phenotype.
  • For a trihybrid cross, the F2 phenotypic ratio is 27:9:9:9:3:3:3:1.