recessive

Examples of recessive in the following topics:

• 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.
  • One allele can be dominant to a second allele, recessive to a third allele, and codominant to a fourth.
  • If a genetic trait is recessive, a person needs to inherit two copies of the gene for the trait to be expressed.
  • Thus, both parents have to be carriers of a recessive trait in order for a child to express that trait .
  • Recessive traits are only visible if an individual inherits two copies of the recessive allele

• Lethal Inheritance Patterns

  • In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele.
  • For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed.
  • In other instances, the recessive lethal allele might also exhibit a dominant (but not lethal) phenotype in the heterozygote.
  • For instance, the recessive lethal Curly allele in Drosophila affects wing shape in the heterozygote form, but is lethal in the homozygote.
  • 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.

• Alternatives to Dominance and Recessiveness

  • Therefore, recessive alleles can be "carried" and not expressed by individuals.
  • However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses.
  • Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene.
  • The variant may be recessive or dominant to the wild-type allele.
  • Discuss incomplete dominance, codominance, and multiple alleles as alternatives to dominance and recessiveness

• Sex-Linked Traits

  • Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males because each male only has one copy of the gene.
  • In an X-linked cross, the genotypes of F1 and F2 offspring depend on whether the recessive trait was expressed by the male or the female in the P1 generation.
  • Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males.
  • Females must inherit recessive X-linked alleles from both of their parents in order to express the trait.
  • The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected.

• Garden Pea Characteristics Revealed the Basics of Heredity

  • Mendel's experiments with peas revealed the presence of dominant and recessive traits in the filial generations.
  • He called these, respectively, dominant and recessive traits.
  • Recessive traits become latent, or disappear, in the offspring of a hybridization.
  • The recessive trait does, however, reappear in the progeny of the hybrid offspring.
  • For this same characteristic (flower color), white-colored flowers are a recessive trait.

• Epistasis

  • The recessive yellow genotype is epistatic to the B gene: mating two heterozygotes (BbEe) results in a 9:3:4 ratio of black (B_E_) to brown (bbE_) to yellow (__ee) offspring.
  • A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A.
  • Finally, epistasis can be reciprocal: either gene, when present in the dominant (or recessive) form, expresses the same phenotype.
  • When the genes A and B are both homozygous recessive (aabb), the seeds are ovoid.
  • The recessive c allele does not produce pigmentnand a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus.

• 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.
  • In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic.
  • Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes.
  • In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype.

• Mendel's Law of Independent Assortment

  • 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 instance, for a tetrahybrid cross between individuals that are heterozygotes for all four genes, and in which all four genes are sorting independently in a dominant and recessive pattern, what proportion of the offspring will be expected to be homozygous recessive for all four alleles?
  • We know that for each gene the fraction of homozygous recessive offspring will be 1/4.
  • Therefore, multiplying this fraction for each of the four genes, (1/4) × (1/4) × (1/4) × (1/4), we determine that 1/256 of the offspring will be quadruply homozygous recessive.

• Hardy-Weinberg Principle of Equilibrium

  • The variable q represents the frequency of the recessive allele, y, for green pea pods.
  • In our example, the possible genotypes are homozygous dominant (YY), heterozygous (Yy), and homozygous recessive (yy).
  • If we can only observe the phenotypes in the population, then we know only the recessive phenotype (yy).
  • We do not know how many are homozygous dominant (Yy) or heterozygous (Yy), but we do know that 16 of them are homozygous recessive (yy).
  • If q2 represents the frequency of homozygous recessive plants, then q2 = 0.16.

• Mendel's Law of Segregation

  • Observing that true-breeding pea plants with contrasting traits gave rise to F1 generations that all expressed the dominant trait and F2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation.
  • For the F2 generation of a monohybrid cross, the following three possible combinations of genotypes could result: homozygous dominant, heterozygous, or homozygous recessive.
  • Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel's observed 3:1 phenotypic ratio.