Tetrad (genetics)

For other uses, see Tetrad (disambiguation).

The tetrad is the four spores of a yeast or Chlamydomonas, that separate after mating. If the two parents have a mutation in two different genes, the tetrad can segregate as the parental ditype (PD), the non-parental ditype (NPD) or as the tetratype (TT).

Saccharomyces cerevisiae tetrad

Parental ditype

A tetrad type containing two different genotypes, both of which are parental. A spore arrangement in Ascomycetes that contains only the two non-recombinant-type ascospores.

Non-parental ditype

A non-parental ditype (NPD) is a spore arrangement in that contains only the two recombinant-type ascospores (assuming two segregating loci). A tetrad type containing two different genotypes, both of which are recombinant.

Tetratype

A Tetratype is a tetrad containing four different genotypes, two parental and two recombinant. A spore arrangement in Ascomycetes that consists of two parental and two recombinant spores indicates a single crossover between two linked loci.

Linkage analysis

The ratio between the different types arising after the sporulation is a measure of the linkage between the two genes.

Tetrad Dissection

Tetrad Dissection has become a powerful tool of yeast geneticists, and is used in conjunction with the many established procedures utilizing the versatility of yeasts as model organisms. Use of modern microscopy and micromanipulation techniques allows the four haploid spores of a yeast tetrad to be separated and germinated individually to form isolated spore colonies.

Uses

Tetrad analysis can be used to confirm whether a phenotype is caused by a specific mutation, construction of strains, and for investigating gene interaction.

General Procedure

Crosses are performed between haploid MATa and MATα mating strains, then the resulting diploids are transferred to sporulation media to form a tetrad containing four haploid spores. Tetrads can then be prepared with Zymolyase, or another enzyme, to digest the wall of the ascus. The spores are then separated with a micromanipulator needle and deposited in separate positions on a petri dish.

Tools

Traditionally, tetrad dissection has a reputation as "dark art" ^[1]. However, instruments have since been developed specifically for tetrad dissection; the most advanced allow easy and semi-automated separation of tetrads[1] . Most micromanipulators use a fiberglass needle to which the spores adhere due to the formation of a water meniscus between the agar and the needle.

See also

• Yeast
• Chlamydomonas
• Model organism
• Synthetic genetic array

External links

• Tetrad Dissection Training Video
  • Part One
  • Part Two

• http://www-rcf.usc.edu/~forsburg/diploids.html

References

1. ^ Yeast Protocols: Methods in Cell and Molecular Biology, Ivor Howell Evan, Published by Humana Press, 1996, ISBN 0896033198, 9780896033191