Chapter 5b

Working with Mutations in Genes

Overview

• Focus on mutations affecting genes in pathways controlling specific characteristics.

• Important for understanding genetic interactions and the identification of gene functions.

Study Questions Overview

12th Grade Questions

• Questions: 24, 25, 40, 46, 47, 62, 68, 69, 76, 83

11th Grade Questions

• Questions: 16, 17, 32, 38, 44, 54, 60, 61, 68, 75

10th Grade Questions

• Questions: 16, 17, 32, 38, 44, 54, 60, 61, 68, 75

9th Grade Questions

• Questions: 5, 6, 21, 27, 33, 43, 49, 50, 57, 64

Section 5.3 – Objective 5: Complementation Test

Definition

• A complementation test is an experimental approach used to determine if mutations causing similar phenotypes are in the same gene or in different genes.

Example of a Complementation Test

• Examined the biosynthetic pathway for the amino acid tryptophan.

• Notation:

  • Normal alleles: $trpA^+$, $trpB^+$, $trpC^+$

• Experiment:

  • Mutants identified: two recessive, auxotrophic mutants for tryptophan in Arabidopsis thaliana.

  • Wild type required for biosynthesis of tryptophan.

  • Cross between the trp auxotroph and wild type yields:

  • All wild type F1.

  • F2 shows a ratio of 3:1 (wild type:auxotroph).

• Results:

  • Each mutant line has only one mutated gene responsible for the trp auxotroph phenotype.

Possibility Outcomes in Complementation Tests

Possibility #1: Different Genes

Example

• Two auxotroph mutants:

  • Mutant 1: $trpA^{-1}$

  • Mutant 2: $trpB^{-1}$

Experiment Design

• Cross: $trp$ x $trp'$

• Parental Cross:

  • $trpA^{-1}/trpA^{-1}; TRPB^+/TRPB^+$

  • $TRPA^+/TRPA^+; trpB^{-1}/trpB^{-1}$

• F1 Progeny:

  • Each progeny shows the wild type phenotype due to wild type alleles at both genes.

  • These mutations complement each other as they are in different genes.

Possibility #2: Same Gene

Example

• Two auxotroph mutations:

  • Mutant 1: $trpA^{-1}$

  • Mutant 2: $trpA^{-2}$

Experiment Design

• Cross: $trp$ x $trp'$

• Parental Cross:

  • $trpA^{-1}/trpA^{-1}; TRPB^+/TRPB^+$

  • $trpA^{-2}/trpA^{-2}; TRPB^+/TRPB^+$

• F1 Progeny:

  • The progenies possess no wild type copies of trpA, resulting in mutual auxotrophy for tryptophan.

  • Therefore, they do not complement one another and represent different alleles of the same gene.

Case Study: Campanula Plant Genetics

Phenotype Analysis

• Wild type flowers are blue; observations of three mutant plants with white flowers (denoted White $, White £ and White ¥).

Experimental Steps

Step 1: Cross Mutants with Wild Type

• Cross between:

  • White $ x Blue

  • Result: All blue flowers in F1

  • F2 ratio: 3:1 (blue:white)

• Cross between:

  • White £ x Blue

  • Result: All blue flowers in F1

  • F2 ratio: 3:1 (blue:white)

• Cross between:

  • White ¥ x Blue

  • Result: All blue flowers in F1

  • F2 ratio: 3:1 (blue:white)

Conclusion

• All white phenotypes are recessive, each arising from a single gene mutation.

Further Investigation

• Determining the number of mutated genes: cross mutants with one another to check for complementation.

Complementation Crosses

• Crosses:

  • White $ x White £: neither provides a wild type allele, producing all white in F1.

  • White $ x White ¥: F1 all blue phenotype shows complementation.

  • White £ x White ¥: provides wild type copies of alleles for complementing.

Rules for a Complementation Test

1. A complementation test can only be performed with recessive mutations.

2. If mutations are in different genes, they will complement each other resulting in wild type phenotypes in progeny.

3. If mutations are alleles of the same gene, they will not complement, resulting in a mutant phenotype in progeny.

Example from Drosophila Genetics

Background

• Three homozygous recessive lines (f1, f2, f3) demonstrate a frizzled bristle phenotype.

• Intercross results yield F1 phenotypic expressions:

  • (+ = wild type phenotype; - = frizzled phenotype)

Phenotype Analysis

• Crosses among lines produce certain genotypes.

Conclusion

• Identify the number of mutated genes from analyzing F1 results.

Heterokaryons

Definition

• Heterokaryon: variation of a complementation test where cells possess two nuclei in a shared cytoplasm, enabling products from two nuclear genomes to act cooperatively.

Example

• Cell Fusion between different auxotrophs leads to a prototrophic phenotype, indicating gene functionalities.

• Example: $ARG1$ and $ARG2$ mutants complement each other when in the same cytoplasm.

Inferring Gene Interactions

Concept

• Phenotypes of double mutants can provide insights into the relationships and positions of genes in metabolic pathways.

Pigment Synthesis Schemes

• Scheme 1: $Colorless o Red o Blue$

• Scheme 2: Aster flowers typically appear purple.

Gene Interaction Analysis

• Genes segregate independently but also exhibit interactions such as:

  • Complementary gene action

  • Epistasis

  • Suppression

  • Redundancy

Identifying Gene Interaction Types

1. Identify the phenotype of double mutants.

2. Determine modifications to expected 9:3:3:1 ratios based on gene interactions.

Example from Corn Snakes

Skin Color Genes

• Genes $o^+$ (orange pigment) and $b^+$ (black pigment).

• Their interaction leads to camouflage or different phenotypic representations based on allele combinations.

Ratios Summary

• Modified dihybrid ratios resulting from gene interactions yield valuable genetic information.

Summary of Gene Interaction Ratios

Importance

• Understanding these ratios helps elucidate the underlying mechanisms of gene action within biological pathways, facilitating exploration of genetic functions.