Genetics Chapter on Alleles and Gene Interactions

Wild Type vs. Mutant Alleles

• Wild-type allele: Functional
• Mutant allele: Non-functional
• Recessive allele: Only expresses phenotypic effect when homozygous (both alleles are recessive)
• Dominant allele: Expresses phenotypic effect even when heterozygous

Gene Interactions

• Epistasis: Interaction of separate non-allelic genes, where one gene influences or interferes with the expression of another gene
• Genetic complementation: When two organisms with mutations that produce the same mutant phenotype, produce offspring with the wild-type phenotype. This occurs because the other strain's genome supplies the wild-type allele to "complement" the mutated allele

Biosynthesis of Adenine

• ADE1 and ADE2 genes: Crucial for adenine biosynthesis
• Genotype and Phenotype:
• ADE1, ADE2 genotype: White phenotype
• ADE1, ade2 genotype: Red phenotype
• Ade1, ADE2 genotype: Pink phenotype
• Ade1, ade2 genotype: Red phenotype

Yeast Strains

• HA1: Mating type A, missing ADE1, has ADE2. Representation: ade1/ADE2
• HB1: Mating type a(alpha), representation: ade1/ADE2

Yeast Respiration

• Two factors of respiration:*
• Aerobic respiration: Sugar is converted to carbon dioxide
• Anaerobic respiration: Sugar is converted to ethanol

Yeast Life Cycle

• Vegetative reproduction: Haploid to haploid or diploid to diploid division
• Cell division: Occurs through budding (Mitosis)
• Sexual reproduction: Mating between two haploid cells, one being mating type A and the other being mating type a/alpha
  • Pheromone: Produced by the opposite mating type, triggering cells to stop dividing and change shape
  • "Shmoos": Cells fuse to form a zygote, which is diploid
• Sporulation: Diploid cells undergo meiosis during starvation or nutrient deficient environment
  • Diploid to haploid: Conversion from diploid to haploid cells, producing stress-resistant ascospores
• Germination: Occurs during nutrient-rich environment, ascospores divide mitotically, resulting in four haploid (1N) segregants.

Basic Genetic Concepts

• Gene: Unit of genetic information specifying the composition of a protein or functional RNA molecule
• Genotype: Genetic makeup of an organism
• Phenotype: Observable characteristics of an individual, resulting from the interaction of its genotype with the environment
• Allele: Alternative forms of a gene
• Wild type: Most observed phenotype in natural populations, used as a benchmark
• Controlled Experiment: One variable is changed at a time to isolate the effect on the outcome

Yeast Respiration

• Yeast can perform both aerobic and anaerobic respiration, which are two different pathways for energy production.
• Aerobic respiration in yeast involves converting sugar to carbon dioxide, while anaerobic respiration results in the conversion of sugar to ethanol.

Yeast Reproduction

• Yeast reproduce through both vegetative reproduction (mitosis) and sexual reproduction (meiosis).
• Vegetative reproduction can be either haploid to haploid or diploid to diploid.
• During vegetative reproduction, cells divide by budding (mitosis).
• Sexual reproduction involves mating between two haploid cells of opposite mating types: A and a/alpha.
• Opposite mating types produce pheromones that cause the cells to stop dividing, change shape, and form "shmoos."
• Cell fusion occurs, leading to the formation of a diploid zygote.

Yeast Life Cycle

• Yeast can undergo either sporulation or germination during sexual reproduction.
• Sporulation occurs in nutrient-deficient or starvation environments.
• Diploid cells switch from mitotic growth to meiosis, resulting in the formation of four haploid, stress-resistant ascospores.
• Germination occurs in nutrient-rich environments where ascospores divide mitotically and grow into a stable haploid phase, creating four haploid (1N) cells.

Basic Genetic Concepts

• Gene - a unit of genetic information encoded in DNA (or RNA in some viruses) specifying the composition of a protein or functional RNA molecule.
• Genotype - the total genetic makeup of an organism, represented by its DNA.
• Phenotype - the observable characteristics of an individual due to the interaction of its genotype with the environment.
• Allele - one of two or more alternative forms of a gene, usually arising from mutation.
• Wild type - the most commonly observed phenotype in natural populations, chosen as a benchmark strain.
• Controlled experiment - designed to study the effect of changing only one variable at a time.
• Dominant allele - expresses its phenotypic effect even when heterozygous. The wild type is often dominant over a mutant.
• Recessive allele - expresses its phenotypic effect only when homozygous (both alleles are recessive). Dominant alleles usually override recessive alleles in a heterozygote.
• Epistasis - the interaction of separate non-allelic genes, where one gene influences or interferes with the expression of another gene.
• Genetic complementation - occurs when an organism with a mutation producing a mutant phenotype mates with an organism carrying a functional copy of the mutated gene. The offspring will produce the wild-type phenotype because the functional gene complements the mutated one.
• Auxotroph - a mutant strain requiring the addition of a nutrient to the growth media.
• Prototroph - a wild-type strain capable of producing all necessary nutrients without supplementation.

Biosynthesis of Adenine

• Wild-type alleles are functional, while mutant alleles are non-functional.
• The alleles of ADE2 are ADE2 (wild type) and ade2 (mutant).
• The alleles of ADE1 are ADE1 (wild type) and ade1 (mutant).
• The following genotypes and their corresponding phenotypes in regards to adenine biosynthesis:
  • ADE1, ADE2 - White phenotype.
  • ADE1, ade2 - Red phenotype.
  • ade1, ADE2 - Pink phenotype.
  • ade1, ade2 - Red phenotype.

Nomenclature of Yeast Strains

• HA1 - Mating type A, missing ADE1, but has ADE2. Representation: ade1/ADE2.
• HB1 - Mating type a/alpha, also missing ADE1, but has ADE2. Representation: ade1/ADE2.

Yeast Respiration

• Yeast, also known as S. Cerevisiae, has two types of respiration: aerobic and anaerobic.
• In aerobic respiration, sugar is converted to carbon dioxide.
• In anaerobic respiration, sugar is converted to ethanol.

Yeast Reproduction

• Yeast can reproduce through vegetative or sexual reproduction.
• Vegetative reproduction involves either haploid to haploid or diploid to diploid cell division through budding (Mitosis).
• Sexual reproduction involves mating between two haploid cells: one of mating type A and the other of mating type a/alpha.
• The cells respond to pheromone produced by the opposite mating type, stopping division and changing shape to form “shmoos”.
• This leads to cell fusion and the formation of a diploid zygote.

Yeast Sexual Reproduction: Sporulation and Germination

• In sexual reproduction, cells can undergo sporulation, which occurs during starvation or a nutrient-deficient environment.
• Diploid cells switch from mitotic growth to meiosis, converting from diploid to haploid cells (stress-resistant ascospores).
• Germination, the opposite of sporulation, occurs in a nutrient-rich environment, where ascospores divide mitotically and grow into a stable haploid phase.
• Germination produces four haploid (1N) segregants of meiosis.

Basic Genetic Concepts

• Gene: The unit of genetic information encoded in DNA (or RNA for some viruses) that determines the composition of a protein or functional RNA molecule.
• Genotype: The genetic makeup of an organism, represented by the total DNA.
• Phenotype: The observable characteristics of an individual resulting from the interaction between genotype and environment.
• Allele: One of two or more alternative forms of a gene, usually arising from mutation and responsible for phenotypic variation.
• Wild Type: The phenotype most commonly observed in natural populations, serving as a benchmark.
• Dominant Allele: An allele that expresses its phenotypic effect even in a heterozygous state. Wild-type alleles are often dominant over mutant alleles.
• Recessive Allele: An allele that only expresses its phenotypic effect when homozygous (both alleles are recessive).
• Epistasis: The interaction of separate non-allelic genes, where one gene influences or interferes with the expression of another gene.
• Genetic Complementation: When an organism with a mutation producing a mutant phenotype produces offspring with the wild-type phenotype when mated or crossed with an organism with a functional copy of that gene. This occurs because the other strain's genome provides the wild-type allele to “complement” the mutated allele.
• Auxotroph: A mutant strain that requires the addition of a nutrient to the media for growth.
• Prototrophic: A wild-type strain that can produce all necessary nutrients without supplementation.

Biosynthesis of Adenine

• The key difference between a wild-type and mutant allele lies in their functionality: wild-type alleles are functional, while mutant alleles are non-functional.
• ADE2 alleles: ADE2 (wild-type) / ade2 (mutant)
• ADE1 alleles: ADE1 (wild-type) / ade1 (mutant)
• Genotype and Phenotype:
  • ADE1, ADE2 genotype: White phenotype
  • ADE1, ade2 genotype: Red phenotype
  • Ade1, ADE2 genotype: Pink phenotype
  • Ade1, ade2 genotype: Red phenotype

Nomenclature of Yeast Strains

• HA1: Mating type A, missing ADE1, has ADE2. Representation: ade1/ADE2
• HB1: Mating type a(alpha), representation: ade1/ADE2.

Yeast Respiration

• Yeast can perform both aerobic and anaerobic respiration.
• Aerobic respiration converts sugar to carbon dioxide.
• Anaerobic respiration converts sugar to ethanol.

Yeast Reproduction

• Yeast can reproduce both vegetatively and sexually.
• Vegetative reproduction can occur through either haploid to haploid or diploid to diploid cell division.
• Vegetative reproduction occurs through budding (mitosis).
• Sexual reproduction involves mating between two haploid cells of opposite mating types, A and a/alpha.
• Cells release pheromones to signal and attract the opposite mating type for sexual reproduction.
• Cells stop dividing and change shape to form "shmoos," leading to cell fusion and the formation of a diploid zygote.

Sexual Reproduction in Yeast

• Sexual reproduction can involve sporulation, which occurs during starvation or nutrient deficiency.
• During sporulation, diploid cells transition from mitotic growth to meiosis.
• Meiosis converts diploid cells to haploid cells (stress-resistant ascospores).
• Sexual reproduction also involves germination, which is the opposite of sporulation.
• Germination occurs in a nutrient-rich environment, and ascospores divide mitotically, growing in a stable haploid phase.
• Germination ultimately yields four haploid (1N) segregants from meiosis.

Basic Genetic Concepts

• Gene: The unit of genetic information encoded in DNA or RNA that determines the composition of a protein or functional RNA molecule.
• Genotype: The genetic makeup of an organism, representing the total DNA information.
• Phenotype: The observable characteristics of an individual resulting from the interaction between its genotype and the environment.
• Allele: One of two or more alternative forms of a gene, usually arising from mutations, responsible for phenotypic variation.
• Wild type: The most commonly observed phenotype in natural populations. Arbitrarily chosen as the benchmark strain.
• Controlled Experiment: An experiment where only one variable is changed at a time to isolate and identify its effect on the outcome.
• Dominant allele: An allele that expresses its phenotypic effect even when heterozygous. Wild types are often dominant over mutants.
• Recessive allele: An allele that expresses its phenotypic effect only when homozygous (both alleles are recessive). In a heterozygote, the dominant allele usually overrides the recessive allele.
• Epistasis: The interaction of separate non-allelic genes, where one gene influences or interferes with the expression of another gene.
• Genetic complementation: When an organism with a mutation producing a mutant phenotype produces offspring with the wild-type phenotype when mated with another organism with a functional copy of that gene. This happens because the other strain's genome provides the wild-type allele to "complement" the mutated allele.
• Auxotroph: A mutant strain that requires the addition of a nutrient to the media for growth.
• Prototroph: A wild-type strain that can produce necessary nutrients without supplementation.

Biosynthesis of Adenine

• The main difference between a wild-type allele and a mutant allele is that the wild type is functional, and the mutant is non-functional.
• ADE2 alleles:
  • ADE2 (wild type)
  • ade2 (mutant)
• ADE1 alleles:
  • ADE1 (wild type)
  • ade1 (mutant)
• Genotype: The combination of alleles for ADE1 and ADE2 genes determines the phenotype:
  • ADE1, ADE2: White phenotype
  • ADE1, ade2: Red phenotype
  • ade1, ADE2: Pink phenotype
  • ade1, ade2: Red phenotype

Nomenclature of Yeast Strains

• HA1: Mating type A, missing ADE1, has ADE2. Represented as ade1/ADE2.
• HB1: Mating type a/alpha, represented as ade1/ADE2.