Chapter 5 - Gene Interaction

• Dominance is a manifestation of how the alleles of a single gene interact in a heterozygote.
• Several types of dominance have been discovered, each representing a different type of interaction between a pair of alleles.
• Complete dominance and recessiveness are the simplest types of dominance.
• The phenotype of a fully dominant allele will be displayed when only one copy is present, such as in a heterozygote individual; in a heterozygote, the other allele whose phenotype is not displayed is the fully recessive allele.
• In full dominance, the homozygous dominant cannot be distinguished from the heterozygote.
• Other single-gene diseases such as pseudoachondroplasia result from alleles that are fully dominant, whereas, in those cases, the wild-type allele is recessive.
• PKU is a good general model for recessive mutations.
• The complementation test can be used to determine if the mutants complement one another.
• When two independently derived recessive mutant alleles producing similar recessive phenotypes fail to complement, they must be alleles of the same gene.
• Complementation is a result of the cooperative interaction of the wild-type alleles of the two genes.
• A polypeptide is the simplest type of protein, a single chain of amino acids.
• A signal-transduction pathway is a chain of complex signals, from the environment to the internal components of the cell, that result in activation of cellular responses.
• Mutations that cause lethality even when present in a single dose are lethal.
• Null alleles for genes identified through genomic sequencing can be made by using a variety of “reverse genetic” procedures that specifically knock out the function of that gene.
• To see if a gene is essential, a null allele is tested for lethality.
• Penetrance is defined as the percentage of individuals with a given allele who exhibit the phenotype associated with that allele.
• Incomplete penetrance means that not every individual with the genotype expresses the corresponding phenotype.
• Penetrance and expressivity are important measures in the analysis of single-gene inheritance.
• Gene interaction occurs in any cellular pathway, such as biosynthetic, signal transduction, and developmental.
• The presence of two mutations in the same gene can be determined using progeny ratios or using complementation tests.
• The interaction of two genes can be inferred based on modified Mendelian ratios.
• Gene interaction can be revealed through the phenotype of the double mutant, as if the genes interact, the phenotype differs from the simple combination of both single-gene mutant phenotypes.
• If mutant alleles from different genes interact, a modified 9 : 3 : 3 : 1 Mendelian ratio will often result.
• Before testing interactions, it is necessary to determine whether each mutation is of a different locus.
• The complementation test is a standard way of determining whether or not two recessive mutations are in the same gene.
• In a diploid, the complementation test is performed by intercrossing two individuals that are homozygous for different recessive mutations.
• The complementation test is defined as the production of a wild-type phenotype when two haploid genomes bearing different recessive mutations are united in the same cell.
• From a mutant hunt, three white-petaled mutants can be obtained and they can be available as homozygous pure-breeding strains.
• The wild-type allele P is haplosufficient, meaning it produces enough functional protein to produce the wild-type phenotype with a single dose.
• In the heterozygote, even though the mutated copy of the gene produces nonfunctional protein, the wild-type copy generates enough functional protein to produce the wild-type phenotype.
• A mutation at a particular gene is assumed to interfere with the production of a single enzyme, creating a block in some biosynthetic pathway.
• The block can be circumvented by supplying to the cells any compound that normally comes after the block in the pathway.
• One advantage of having duplicate genes is to provide backups in case of null mutations.
• A leaky mutation in one step of a pathway may cause the pathway to slow down, but leave enough function for life.
• If double mutants combine, each with a leaky mutation in a different step, the whole pathway grinds to a halt.
• Two interacting proteins perform some essential function on some substrate such as DNA but must first bind to it.
• Reduced binding of either protein allows some functions to remain, but reduced binding of both is lethal.
• In the earlier discussions of modified Mendelian ratios, all the crosses were dihybrid selfs.
• Two mutations that are individually benign can become lethal when united in the same genotype.
• Genetic analysis of gene interaction makes use of mutant alleles, but the gene interaction revealed is one that is taking place normally in the wild type.