Allele & Gene Interactions Notes

Allele & Gene Interactions - Learning Objectives

Introduction to Non-Mendelian Inheritance

  • Patterns of Inheritance:

    • Different inheritance patterns modify normal Mendelian ratios.

    • Key modifications include:

    • Incomplete dominance

    • Codominance

    • Penetrance

    • Expressivity

    • Lethal alleles

    • Temperature-sensitive alleles

Inheritance Patterns Explained

  • Incomplete Dominance:

    • Phenotype of heterozygote is intermediate between the phenotypes of the two homozygotes.

    • Example: If red flowered plant (RR) is crossed with a white flowered plant (rr), the result is a pink flowered plant (Rr).

  • Codominance:

    • Both alleles in a heterozygote are fully expressed, leading to a phenotype that is neither dominant nor recessive.

    • Example: In AB blood type, both A and B alleles are expressed.

  • Penetrance:

    • Refers to the proportion of individuals with a genotype that express the expected phenotype.

    • Can be complete (100% of the individuals express the phenotype) or incomplete (less than 100%).

  • Expressivity:

    • Variability in the degree to which a genotype is expressed in phenotype.

    • E.g., individuals with the same genotype might display different degrees of a trait.

  • Lethal Alleles:

    • Alleles that can cause death when present in certain combinations, often leading to specific ratios in the offspring.

    • Example: A homozygous recessive condition may be lethal.

  • Temperature-sensitive Alleles:

    • Alleles that exhibit different phenotypes depending on environmental temperature.

    • E.g., in Siamese cats, the darker colors appear on the cooler parts of the body.

Analyzing Genetic Crosses

  • Hierarchy of Dominance in Allele Series:

    • Determine the order of dominance among alleles in a series and predict phenotypes and genotypes of offspring.

    • Method involves performing crosses and evaluating the resulting offspring.

Haploinsufficiency

  • Haploinsufficiency:

    • A condition in which a diploid organism has only one functioning copy of a gene that does not produce enough gene product to display a normal phenotype.

    • Important in determining genotypes and phenotypes in genetic disorders.

Mutations: Gain vs. Loss of Function

  • Gain-of-Function Mutations:

    • Mutations that enhance the function of a gene or produce a new function.

    • Often associated with dominant phenotypes.

  • Loss-of-Function Mutations:

    • Mutations that reduce or eliminate gene function.

    • Often associated with recessive phenotypes.

Complementation Testing

  • Complementation Test:

    • A method used to determine whether two mutations that produce the same phenotype are in the same or different genes.

    • Experimental results help identify genetic interactions between alleles.

Complementary Gene Action

  • Complementary Gene Action:

    • Occurs when two genes work together to produce a single phenotype.

    • Example: In the case of flower color, two different genes may contribute to pigment production; absence of either results in no color.

Epistasis

  • Epistasis:

    • A phenomenon where the expression of one gene is masked by another gene.

    • Example: In Labrador retrievers, the presence of a specific allele can determine whether the pigment will be deposited in the fur regardless of the coat color gene.

Evaluating Genetic Pathways

  • Genetic Pathway Evaluation:

    • Predict the outcomes of different genotype combinations involving genes in a pathway.

    • Important in understanding complex traits and gene interactions within biological systems.