Genetics Quiz: Lethal Alleles and Phenotype Expression
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Questions and Answers

What happens when two copies of a lethal allele are inherited?

  • The organism will exhibit a new phenotype.
  • None of the offspring with that genotype will survive. (correct)
  • The organism will survive only if the allele is dominant.
  • The organism may survive with one copy of the allele.
  • How is penetrance defined?

  • The variability in phenotype expression among individuals.
  • The degree to which a phenotype is expressed.
  • The percentage of individuals with a mutation that exhibit the phenotype. (correct)
  • The interaction between multiple genes affecting a phenotype.
  • What is meant by variable expressivity?

  • Individuals show different levels of the same phenotype. (correct)
  • All individuals with a mutation show the same phenotype.
  • The phenotype is expressed only under specific environmental conditions.
  • The mutation does not affect phenotype presentation.
  • Which factors could explain why individuals with the same mutation do not show the same phenotype?

    <p>Environmental influences and genetic background.</p> Signup and view all the answers

    What did Beadle and Tatum's experiments reveal about mutant strains?

    <p>Each mutation behaved as a single gene.</p> Signup and view all the answers

    What is the primary purpose of conducting a complementation test?

    <p>To identify the number of genes that have been mutated.</p> Signup and view all the answers

    What outcome is expected from a double mutant line in gene interaction studies?

    <p>An understanding of how mutations affect phenotype presentation.</p> Signup and view all the answers

    When multiple genes are involved in the presentation of a phenotype, what happens if one gene in the pathway is mutated?

    <p>The phenotype is altered from the wild type.</p> Signup and view all the answers

    What does it mean when two mutants fail to complement each other?

    <p>They have mutations in the same gene.</p> Signup and view all the answers

    In double mutant interactions, what does a 9:3:3:1 ratio indicate?

    <p>There is no interaction between the genes.</p> Signup and view all the answers

    What is the result of recessive epistasis in double mutant interactions?

    <p>A 9:3:4 ratio of offspring.</p> Signup and view all the answers

    What does a 15:1 ratio in synthetic mutations indicate?

    <p>Each mutation has no effect when considered separately.</p> Signup and view all the answers

    What leads to chromosomal rearrangements?

    <p>Aberrant crossing over.</p> Signup and view all the answers

    What is suggested by karyotypes remaining constant within a species?

    <p>Most genetic changes lead to a selective disadvantage.</p> Signup and view all the answers

    What indicates that closely related species differ by few rearrangements?

    <p>Their karyotypes are almost identical.</p> Signup and view all the answers

    What defines a dominant epistasis interaction?

    <p>One mutant allele masks another phenotype.</p> Signup and view all the answers

    What is a characteristic of short tandem repeats (STRs) compared to single nucleotide polymorphisms (SNPs)?

    <p>STRs can have multiple different alleles, often exceeding 20.</p> Signup and view all the answers

    In gel electrophoresis, which statement correctly describes the movement of DNA molecules?

    <p>Small DNA fragments travel quickly and further than larger ones.</p> Signup and view all the answers

    Which of the following is a key difference between Sanger sequencing and traditional PCR?

    <p>Sanger sequencing needs only one primer.</p> Signup and view all the answers

    What purpose does a molecular ladder serve in gel electrophoresis?

    <p>It helps determine the sizes of DNA samples.</p> Signup and view all the answers

    Which statement best describes the addition of ddNTPs in Sanger sequencing?

    <p>They terminate DNA synthesis when incorporated.</p> Signup and view all the answers

    What is the primary effect of chromosomal rearrangements on an organism?

    <p>They may impact gene balance affecting phenotypes or viability.</p> Signup and view all the answers

    What occurs during recombination in deletion heterozygotes?

    <p>Recombination happens only at homologous regions.</p> Signup and view all the answers

    Which type of inversion affects gene function leading to nonviable zygotes?

    <p>Paracentric inversion heterozygotes.</p> Signup and view all the answers

    What is characteristic of reciprocal translocations?

    <p>They involve the exchange of chromosome fragments between two chromosomes.</p> Signup and view all the answers

    How do translocation heterozygotes produce gametes?

    <p>They produce mostly unbalanced gametes.</p> Signup and view all the answers

    What defines a prototroph in bacteria?

    <p>A bacterium that can survive on minimal medium without supplements.</p> Signup and view all the answers

    Which statement is true regarding paracentric inversion heterozygotes?

    <p>One recombinant chromatid may lack a centromere.</p> Signup and view all the answers

    Which one describes the effect of translocations on genetic mapping?

    <p>Translocations lead to altered genetic map distances due to pseudolinkage.</p> Signup and view all the answers

    What is the outcome of genetic modification at the level of entire genomes?

    <p>It results in an increase in the number of gene copies and gene families.</p> Signup and view all the answers

    What is the role of DNA polymerase during DNA replication?

    <p>It catalyzes DNA synthesis.</p> Signup and view all the answers

    Which statement about the base pairing in DNA is true?

    <p>There is equal pairing between A and T, and C and G.</p> Signup and view all the answers

    What does semi-conservative replication of DNA imply?

    <p>One original strand is paired with a newly synthesized strand.</p> Signup and view all the answers

    What is the main difference between cotransduction experiments and recombination frequency experiments?

    <p>Cotransduction frequency increases with the closeness of genes.</p> Signup and view all the answers

    Which of the following components is NOT found in nucleotides?

    <p>Amino group</p> Signup and view all the answers

    Why are GC-rich regions of DNA considered more stable than AT-rich regions?

    <p>GC pairs form three hydrogen bonds, providing extra stability.</p> Signup and view all the answers

    What is the function of helicase in DNA replication?

    <p>To disrupt hydrogen bonds between the DNA strands.</p> Signup and view all the answers

    What is the significance of phosphodiester linkages in DNA?

    <p>They link the 3' carbon of one sugar to the 5' carbon of another.</p> Signup and view all the answers

    Study Notes

    Lethal Alleles

    • Two copies of a mutant allele can be lethal
    • Homozygous individuals with a lethal allele will not survive

    Penetrance and Expressivity

    • Penetrance is the percentage of individuals with a mutation that show the phenotype
    • Incomplete penetrance occurs when individuals with a mutant genotype do not express the mutant phenotype
    • Expressivity refers to the varying degrees to which a phenotype is expressed
    • Variable expressivity occurs when individuals show different levels of the phenotype

    Factors Influencing Phenotype Expression

    • Environmental factors can influence phenotype
    • Other genes, known as genetic background, contribute to phenotypic expression
    • Subtlety of mutant phenotype may lead to misclassification

    Gene Interactions in Pathways

    • Multiple genes typically participate in the expression of a phenotype
    • These genes often function in a pathway, where the product of one gene activates the next
    • Mutations in any gene within the pathway can disrupt the wild-type phenotype

    Beadle and Tatum's Experiment

    • Studied the genetic control of cellular chemistry in Neurospora
    • Identified numerous mutant strains that required arginine for survival, known are arginine auxotrophs
    • Each mutation was attributed to a single gene
    • Mutations were mapped to three distinct loci

    Determining Functional Relationships between Genes

    • Mutate a population to generate mutations in genes, creating numerous mutant lines
    • Conduct complementation tests to identify the number of genes mutated
    • Create double mutant lines to investigate gene interactions

    Complementation Test

    • Used to determine if mutations are in the same gene or different genes
    • Mutations in different genes complement each other, resulting in a wild-type phenotype
    • Mutations in the same gene fail to complement, resulting in a mutant phenotype

    Double Mutant Interactions

    • No interaction: 9:3:3:1 ratio, genes are not in the same pathway
    • Same pathway: 9:7 ratio, genes are in the same pathway and collaborate towards a final product or phenotype
    • Recessive epistasis: 9:3:4 ratio, mutations in two different genes affect the same trait
    • Dominant epistasis: 12:3:1 ratio, one mutant allele masks the phenotype of another mutant allele, where the masking alleles are dominant
    • Suppressor mutations: 13:3 ratio, mutations in one gene reverse the effect of mutations in another gene, resulting in a wild-type phenotype
    • Synthetic mutations: 15:1 ratio, mutations in two different genes individually do not cause a phenotype but together result in a mutant phenotype

    Chromosomal Rearrangements

    • Generally conserved within a species
    • Related species typically have distinct karyotypes

    Types of Chromosomal Rearrangements

    • Deletions: loss of chromosomal segment
    • Duplications: duplication of chromosomal segment
    • Inversions: reversal of a segment within a chromosome
    • Translocations: exchange of segments between nonhomologous chromosomes

    Origins of Chromosomal Rearrangements

    • Chromosome breakage can lead to all types of chromosomal rearrangements
    • Aberrant crossing over at repeated sequences can also result in rearrangements

    Effects of Chromosomal Rearrangements

    • Can impact phenotypes, including viability, by altering gene balance
    • Severity of effects depends on whether the individual is homozygous or heterozygous for the rearranged chromosome
    • Can alter crossing over, impacting fertility

    Deletions

    • Deletion loop forms in deletion heterozygotes
    • Recombination is restricted to homologous regions, preventing recombination within deletion loops
    • Genetic map distances are inaccurate in deletion heterozygotes

    Duplications

    • Exons can duplicate or shuffle
    • Entire genes can duplicate, creating multigene families
    • Gene families can duplicate, producing gene superfamilies
    • Entire genomes can duplicate, doubling the number of genes and gene families

    Inversions

    • Chromosome breakage can produce inversions
    • Inversions can disrupt genes
    • In inversion heterozygotes, an inversion loop forms to allow maximum alignment of homologous regions
    • Crossing over within the inversion loop produces aberrant recombinant chromatids

    Pericentric Inversions

    • Pericentric inversions include the centromere
    • Recombinant chromatids are genetically unbalanced, leading to nonviable zygotes

    Paracentric Inversions

    • Paracentric inversions do not include the centromere
    • One recombinant chromatid lacks a centromere, while the other has two centromeres
    • Nonviable zygotes result from the union of normal gametes with gametes carrying these abnormal chromatids

    Translocations

    • Reciprocal translocations: exchange of segments between two nonhomologous chromosomes, resulting in two reciprocal translocations
    • Robertsonian translocations: breaks near centromeres of two acrocentric chromosomes, generating one large metacentric chromosome and one small chromosome, which is often lost

    Phenotypic Effects of Reciprocal Translocations

    • Usually do not affect phenotype because they do not add or remove DNA
    • Can disrupt a gene or alter gene expression
    • In somatic cells, can lead to oncogene activation
    • Defects in translocation heterozygotes include:
      • Unbalanced gametes, leading to reduced fertility
      • Altered genetic map distances due to pseudolinkage

    Translocation Homozygotes

    • Chromosomes segregate normally during meiosis I if breakpoints do not impact gene function

    Translocation Heterozygotes

    • Chromosome pairing in translocation heterozygotes forms a cruciform structure during prophase I of meiosis

    Semi-Sterility

    • Occurs in corn plants heterozygous for a reciprocal translocation or inversion
    • Slightly less than 50% of gametes are viable due to alternate segregation
    • Unbalanced ovules resulting from adjacent-1 or adjacent-2 segregation are aborted

    Bacteria

    • Prokaryotes
    • Reproduce through binary fission, not meiosis or mitosis

    Bacterial Genetic Traits

    • Prototroph: Wild-type bacterium that can synthesize essential nutrients
    • Auxotroph: Mutant bacterium that cannot synthesize certain nutrients and requires them for growth
    • Ability to use a carbon source: Wild-type bacteria can utilize specific carbon sources, while mutants may lack this ability

    Cotransduction and Recombination Frequency

    • Cotransduction experiments determine the proximity of genes
    • Recombination frequency experiments determine the distance between genes

    DNA Structure

    • Must be replicated accurately
    • Must contain information
    • Must be able to change (rarely)

    DNA Building Blocks

    • Purines: adenine (A) and guanine (G)
    • Pyrimidines: cytosine (C), uracil (U), and thymine (T)
    • RNA contains two hydroxyl groups, while DNA contains one

    Base Pairing

    • Equal amounts of purine and pyrimidine nucleotides
    • Equal amounts of adenine (A) and thymine (T), and cytosine (C) and guanine (G)
    • A+T does not necessarily equal G+C
    • GC-rich DNA is generally more stable than AT-rich DNA

    Double Helix

    • Nucleotides consist of a phosphate group, a sugar, and a base
    • Nucleotides form DNA strands through phosphodiester linkages
    • The two sugar-phosphate backbones are antiparallel
    • DNA strands are held together by hydrogen bonds between purine and pyrimidine bases
    • A forms two hydrogen bonds with T, while G forms three hydrogen bonds with C

    Semi-Conservative Replication

    • Two DNA strands unwind
    • Each strand acts as a template for the synthesis of a new complementary strand
    • Results in two identical double helices

    DNA Replication

    • Synthesis occurs in the 5' to 3' direction
    • DNA polymerase catalyzes the process
    • Cleavage of pyrophosphate provides energy for DNA synthesis

    DNA Replication Proteins

    • Single-strand DNA-binding proteins stabilize unwound DNA
    • Topoisomerase relaxes supercoiled DNA and rejoins DNA strands
    • Helicase disrupts hydrogen bonds between DNA strands
    • DNA polymerase III catalyzes DNA synthesis

    Gel Electrophoresis

    • Used to separate DNA molecules based on size
    • An electrical field moves molecules through a gel matrix, with smaller DNA molecules traveling further
    • A molecular ladder provides known DNA sizes for comparison

    Sanger Sequencing

    • Different from PCR because:
      • Not exponential amplification
      • Requires only one primer
      • Uses ddNTPs along with dNTPs
    • Original method used four separate reactions
    • Automated Sanger sequencing utilizes all ddNTPs in one reaction with different colored fluorescent markers

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    Description

    Test your knowledge on lethal alleles, penetrance, and expressivity in genetics. Explore how various factors influence phenotype expression and the role of gene interactions in pathways. Understand the complexities of genetic mutations and their effects on survival and phenotype.

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