allele

Examples of allele in the following topics:

• Population Genetics

  • Although each organism can only carry two alleles, more than those two alleles may be present in the larger population.
  • For example, in a population of fifty people where all the blood types are represented, there may be more IA alleles than i alleles.
  • The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population.
  • Natural selection also affects allele frequency.
  • However, a detrimental recessive allele can linger for generations in a population, hidden by the dominant allele in heterozygotes.

• Alternatives to Dominance and Recessiveness

  • With the inclusion of incomplete dominance, codominance, multiple alleles, and mutant alleles, the inheritance of traits is complex process.
  • Mendel's experiments with pea plants suggested that: (1) two "units" or alleles exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype.
  • The allele for red flowers is incompletely dominant over the allele for white flowers.
  • Homozygotes (LMLM and LNLN) express either the M or the N allele, and heterozygotes (LMLN) express both alleles equally.
  • Here, four alleles exist for the c gene.

• Lethal Inheritance Patterns

  • Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy.
  • The wild-type allele functions at a capacity sufficient to sustain life and is, therefore, considered to be dominant over the nonfunctional allele.
  • The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age.
  • Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted.
  • However, just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles also might not be expressed until adulthood.

• Mendel's Law of Dominance

  • In a heterozygote, the allele which masks the other is referred to as dominant, while the allele that is masked is referred to as recessive.
  • Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively.
  • The recessive allele will remain "latent," but will be transmitted to offspring by the same manner in which the dominant allele is transmitted.
  • It is sometimes convenient to talk about the trait corresponding to the dominant allele as the dominant trait and the trait corresponding to the hidden allele as the recessive trait.
  • One allele can be dominant to a second allele, recessive to a third allele, and codominant to a fourth.

• Mendel's Law of Segregation

  • Mendel's Law of Segregation states that a diploid organism passes a randomly selected allele for a trait to its offspring, such that the offspring receives one allele from each parent.
  • The allele that contains the dominant trait determines the phenotype of the offspring.
  • As chromosomes separate into different gametes during meiosis, the two different alleles for a particular gene also segregate so that each gamete acquires one of the two alleles.
  • The Law of Segregation states that alleles segregate randomly into gametes
  • When gametes are formed, each allele of one parent segregates randomly into the gametes, such that half of the parent's gametes carry each allele.

• 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.
  • When a neutral allele is linked to beneficial allele, consequently meaning that it has a selective advantage, the allele frequency can increase in the population through genetic hitchhiking (also called genetic draft).
  • Any given individual may carry some beneficial alleles and some unfavorable alleles.
  • As a result, good alleles can be lost if they are carried by individuals that also have several overwhelmingly bad alleles; similarly, bad alleles can be kept if they are carried by individuals that have enough good alleles to result in an overall fitness benefit.

• Hardy-Weinberg Principle of Equilibrium

  • The Hardy-Weinberg principle can be used to estimate the frequency of alleles and genotypes in a population.
  • For example, assume that p represents the frequency of the dominant allele, Y, for yellow pea pods.
  • The variable q represents the frequency of the recessive allele, y, for green pea pods.
  • We can also write this as p + q = 1.If the frequency of the Y allele in the population is 0.6, then we know that the frequency of the y allele is 0.4.
  • When populations are in the Hardy-Weinberg equilibrium, the allelic frequency is stable from generation to generation and the distribution of alleles can be determined.If the allelic frequency measured in the field differs from the predicted value, scientists can make inferences about what evolutionary forces are at play.

• Genetic Drift

  • Because the random sampling can remove, but not replace, an allele, and because random declines or increases in allele frequency influence expected allele distributions for the next generation, genetic drift drives a population towards genetic uniformity over time.
  • When an allele reaches a frequency of 1 (100%) it is said to be "fixed" in the population and when an allele reaches a frequency of 0 (0%) it is lost.
  • Once an allele becomes fixed, genetic drift for that allele comes to a halt, and the allele frequency cannot change unless a new allele is introduced in the population via mutation or gene flow.
  • In this example, the brown coat color allele (B) is dominant over the white coat color allele (b).
  • As a result, in the third generation the recessive b allele is lost.

• Genetic Variation

  • The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others.
  • Some new alleles increase an organism's ability to survive and reproduce, which then ensures the survival of the allele in the population.
  • Other new alleles may be immediately detrimental (such as a malformed oxygen-carrying protein) and organisms carrying these new mutations will die out.
  • Neutral alleles are neither selected for nor against and usually remain in the population.
  • Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation.

• Genetic Linkage and Violation of the Law of Independent Assortment

  • The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not.
  • Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles.
  • If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes.
  • But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles.
  • Here, the alleles for gene C were exchanged.