Genetics Lecture 3: Lethal Genes
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Questions and Answers

What temperature is lethal for the Kidney-eyed mutant of the wasp Bracon hebetor?

  • 8°C
  • 25°C
  • 30°C (correct)
  • 19°C

    • In a balanced lethal system, what proportion of the offspring will survive?

  • 25%
  • 50% (correct)
  • 75%
  • 100%

    • What type of genes lead to the inviability of gametes?

  • Gametic lethals (correct)
  • Semilethal genes
  • Balanced lethal genes
  • Conditional lethal genes

    • Which of the following plants is known to have a chlorophyll mutant that can produce albino seedlings?

    <p>Barley</p> Signup and view all the answers

    In a balanced lethal system, what alleles are linked together on the same chromosome?

    <p>Recessive lethal alleles and normal alleles</p> Signup and view all the answers

    What happens to 25% of the zygotes in a balanced lethal system?

    <p>They are homozygous for one of the lethal alleles and do not survive</p> Signup and view all the answers

    Semilethal genes cause the death of what percentage of individuals that carry them?

    <p>More than 90%</p> Signup and view all the answers

    What effect does a conditional lethal gene exert based on temperature?

    <p>Requires specific conditions to be lethal</p> Signup and view all the answers

    In terms of gametic lethals, what is the outcome commonly referred to when there is a complete absence in expected ratios?

    <p>Segregation distortion</p> Signup and view all the answers

    What characterizes a lethal gene?

    <p>It causes the death of individuals with the proper genotype before adulthood.</p> Signup and view all the answers

    In what state does a recessive lethal gene express its effects?

    <p>In homozygous state only.</p> Signup and view all the answers

    Which of the following is not a category of lethal genes?

    <p>Semi-lethal genes</p> Signup and view all the answers

    What genotype results in the yellow coat color in mice, which is lethal?

    <p>Yy</p> Signup and view all the answers

    Which type of lethal gene is most common?

    <p>Recessive lethals</p> Signup and view all the answers

    What percentage of embryos from yellow mice is typically unviable?

    <p>25%</p> Signup and view all the answers

    What is the proper genotype for a recessive lethal gene that affects survival?

    <p>aa</p> Signup and view all the answers

    What occurs during the embryonic development of homozygous YY mice?

    <p>They die at an early embryonic stage.</p> Signup and view all the answers

    What is a defining property of dominant lethal genes?

    <p>They are expressed in both homozygous and heterozygous states.</p> Signup and view all the answers

    Which of the following statements is true regarding lethal genes?

    <p>Recessive lethals affect survival only in homozygous states.</p> Signup and view all the answers

    What are subvital genes?

    <p>Genes that reduce viability but kill less than 90% of individuals.</p> Signup and view all the answers

    Which of the following is an example of a supervital gene?

    <p>Genes for disease resistance</p> Signup and view all the answers

    Which genes are considered vital genes?

    <p>All wild type alleles of an organism's genes.</p> Signup and view all the answers

    What is a characteristic of supervital genes?

    <p>They may confer resistance to abiotic stresses.</p> Signup and view all the answers

    How do subvital genes primarily affect individuals?

    <p>They reduce viability but do not completely eliminate the individuals.</p> Signup and view all the answers

    Which of the following traits is NOT associated with vital genes?

    <p>They can increase the likelihood of disease resistance.</p> Signup and view all the answers

    What can be inferred about mutant genes in general?

    <p>Most mutant genes tend to be subvital.</p> Signup and view all the answers

    What distinguishes supervital genes from other genes?

    <p>They enhance survival under specific conditions.</p> Signup and view all the answers

    In what condition are xantha mutants described?

    <p>They are semilethal in the homozygous state.</p> Signup and view all the answers

    What does the term 'homozygous state' refer to?

    <p>Having identical alleles for a gene.</p> Signup and view all the answers

    What is a characteristic of recessive genes in relation to their phenotypic effects?

    <p>They cause normal phenotype and survival in heterozygous individuals.</p> Signup and view all the answers

    How do albino seedlings in barley behave in terms of phenotype?

    <p>They survive on stored food as they cannot photosynthesize.</p> Signup and view all the answers

    What do zygotic lethal genes affect?

    <p>Survival rates of zygotes with specific genotypes.</p> Signup and view all the answers

    Which of the following is true about dominant lethal alleles?

    <p>They cause lethality in both homozygous and heterozygous states.</p> Signup and view all the answers

    What is Huntington's disease characterized by?

    <p>It has onset usually in a person's thirties or forties.</p> Signup and view all the answers

    What are conditional lethals?

    <p>Genes that behave normally unless certain conditions are met.</p> Signup and view all the answers

    Which gene is an example of a dominant lethal in humans?

    <p>Epiloia</p> Signup and view all the answers

    What ratio of green to albino seedlings is expected shortly after germination in barley?

    <p>3:1</p> Signup and view all the answers

    Why cannot dominant lethal alleles be maintained in a population?

    <p>Affected individuals die before reproducing.</p> Signup and view all the answers

    How does a dominant lethal gene impact heterozygous individuals?

    <p>They experience severe health conditions and early death.</p> Signup and view all the answers

    Study Notes

    Lecture 3: Lethal Genes

    • Lethal genes impact the survival of zygotes/individuals with a specific genotype.
    • Equal survival of all gametes and zygotes is a fundamental assumption of inheritance.
    • Lethal genes are grouped into five categories:
      • Recessive lethals: Effects only in the homozygous state; heterozygotes are unaffected.
      • Dominant lethals: Kill both homozygous and heterozygous individuals.
      • Conditional lethals: Lethal only under specific conditions (e.g., temperature).
      • Balanced lethals: Linkage of multiple lethal genes causes heterozygotes to survive while homozygotes die.
      • Gametic lethals: Effect is on gamete viability and results in non-typical ratios in offspring.
    • Semilethal genes: Cause death in more than 90% of individuals carrying them in the proper genotype.
    • Subvital genes: Reduce viability, but less than 90% of individuals carrying them die.
    • Vital genes: Do not impact survival in any genotype.
    • Supervital genes: Enhance the survival of individuals carrying them.

    Recessive Lethals

    • Most lethal genes are recessive lethals.
    • Effects are only expressed in the homozygous state (e.g., yellow coat color in mice).
    • Heterozygotes with only one recessive copy of the lethal gene are normal.
    • Examples include mice with yellow coat color (Yy heterozygotes survived while YY homozygotes died), and albino seedlings in barley.

    Dominant Lethals

    • Dominant lethal alleles are lethal in both homozygous and heterozygous states (e.g., Huntington's disease, epiloia).
    • Examples include epiloia, and Huntington's disease.
    • Cannot persist in the population unless continually produced by mutation.

    Conditional Lethals

    • Lethal only under specific conditions or temperatures.
    • Example (Kidney-eyed mutant): Normal development at lower temperatures, but lethal at higher temperatures. A similar example is found in chlorophyll mutants of barley.

    Balanced Lethals

    • Linkage of multiple lethal genes causes only the heterozygotes in the progeny to survive.
    • Homozygotes will not survive, therefore the only possible surviving progeny are heterozygotes.

    Gametic Lethals

    • Lethal genes affect gamete viability.
    • Result in the complete disappearance of one class of gamete or phenotype in segregating generations.
    • Example phenomenon is known as segregation distortion or meiotic drive.

    Semilethal Genes

    • Cause death in more than 90% of affected individuals.

    Subvital Genes

    • Reduce viability.
    • Cause death of less than 90% of individuals carrying them in the proper genotype.

    Vital Genes

    • Do not affect the survival of individuals. Examples are wild-type alleles in an organism.

    Supervital Genes

    • Enhance the survival of individuals.
    • Examples include genes for resistance to disease and abiotic stresses (e.g., salinity, drought). These may enhance plant fitness during stress.

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    Lecture 3: Lethal Genes (PDF)

    Description

    Explore the concept of lethal genes and their classifications in this quiz. Understand how different types of lethal genes, including recessive, dominant, and conditional lethals, impact survival and genetic inheritance. This lecture emphasizes the implications of these genes on zygotes and overall genetic viability.

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