Chapter 5 Gene Interaction - Section 1 Notes PDF

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Notes for chapter 5 gene interaction. Interactions between alleles of a single gene and variations in dominance. Professor is Dr. John Stanga....

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Chapter 5: Gene Interaction - Section 1 Notes

❖ Section 1: Interactions Between the Alleles of a Single Gene: Variations on Dominance ➢ L.O. 1 - distinguish between the various types of dominance, based on the phenotypes of heterozygotes ➢ L.O. 2 - recognize phenotypic ratios diagnostic of the presence of a lethal allele ➢ L.O. 3 - give some possible reasons why incomplete penetrance and variable expressivity may occur in a population of individuals with identical genotypes at a locus under study ➢ Complete Dominance & recessiveness ■

Dominant: ●

Phenotype of homozygote cannot be distinguished from heterozygote

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How can we explain fully dominant mutations? ◆ The wild-type allele is haploinsufficient - one allele is not enough to achieve normal levels of function

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Dominant Negative: ●

Act as “spoilers” or “rogues” - mutant binds to the wild-type polypeptide and distorts it or interferes with its function ◆ If gene product is a monomer - mutant protein binds the substrate & prevents wild-type from binding

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Recessive: ●

One copy of the wild-type allele produces enough of its product - making the defective allele recessive ◆ Wilt-type is halposufficient

➢ Incomplete Dominance ■

Phenotype of heterozygote is an intermediate between the two homozygotes

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How can we explain incomplete dominance at the molecular level? ●

Each wild-type allele produces a set amount of its protein product ◆ # of doses determined the concentration of the chemical made by the protein ◆ Example: flower color - red (2dose), pink (1dose), white (0dose)

➢ Codominance ■

Expression of both alleles of a heterozygote

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Example: Blood type AB ●

Genotype IA/IB - each of the alleles produces its own form of the cell-surface molecule & so the A and B alleles are codominant

➢ Recessive Lethal Alleles ■

Lethal allele = allele that is capable of causing the death of an organism

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Can identify if a gene is essential to the organism’s operation

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Useful in determining the developmental stage at which the gene normally acts ●

See of death by the lethal allele occurs early or late in the development of a zygote

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What is the diagnostic test for lethality? ●

Cross mutant with a homozygous wild-type ◆ Get a 1:1 ratio of mutant to wild-type ◆ Suggest mutant is always heterozygous & is dominant over wild type

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Cross mutant with mutant ◆ 2:1 ratio of yellow to wild-type ◆ How can the 2:1 ratio be explained? ➢ The expected monohybrid ratio of 1:2:1 would be found among the zygotes but is altered to the 2:1 in the progeny seen at birth because the homozygous mutants do not survive to be counted

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Pleiotropic because it produces effects on 2 characters: phenotype & survival

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Most recessive lethals are silent in the heterozygote! ●

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Diagnosed by observing death in 25% of the progeny at some stage of development

Whether an allele is lethal or not often depends on the environment in which the organism develops ●

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Example: cystic fibrosis & sickle-cell anemia would be lethal without treatment!

Sublethal: ●

Lethality is expressed in only some but not all of the homozygous individuals

➢ Penetrance & Expressivity ■

Penetrance = % of individuals with a given allele who exhibit the phenotype associated with that allele

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Why would an organism have a particular genotype but not express the corresponding phenotype? ●

Several possible reasons: ◆ (1). The influence of the environment ◆ (2). Influence of other interacting genes ◆ (3). The subtlety of the mutant phenotype

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Expressivity = degree to which a given allele is expressed at the phenotypic level ●

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Measures the intensity of the phenotype

KEY CONCEPT: penetrance & expressivity quantify the modification of a gene’s effects by varying environment and genetic background; they measure the % of cases in which the phenotype is observed & its severity...