BIOL 2500 - Topic 5 (part 6)

Beadle and Tatums experiment process

1.) They irradiated the WT neurospora to create mutations

2.) They then transferred the irradiated fungus to a growth medium with everything in it

3.) Then, they transferred the fungi to a minimal medium, that basically had only agar and water

Why did they transfer the fungi from the vitamin-rich medium to the minimal medium?

1.) To identify the prototrophs and auxotrophs.

2.) Those that could grow in means they had all the genes required to grow on the media (i.e. prototrophs)

3.) Those that could not grow means they are lacking the genes that makes the nutrients they cannot get from the medium

What did they do with the auxotrophs that could not grow on the minimal media?

1.) They transferred the auxotroph to various supplemented media to see what it could and could not grow on (i.e. minimal media + AA or vitamins, etc)

2.) If it grew then, then it meant that the auxotroph was missing the gene to make that specific nutrient (i.e. AA or vitamins, etc)

How do they find the exact nutrient that the Auxotroph needs?

They can do a secondary transfer, where they grow them on supplemented media that has only one specific amino acid, or vitamin, or nucleic acid, etc and the one it grows on is the one it was missing a gene for

How to identify gene interactions in a particular phenotype

1.) Obtain single-gene mutants and test for dominance

2.) Test the mutants for allelism (i.e. are they at one or more loci)

3.) Combine mutants into pairs to form double mutants, to see if the genes interact

Double mutants

When two individuals with two different mutations are paired, to see how the genes may or may not interact with one another

How are genetic interactions inferred

1.) They are inferred based on the phenotype of the double mutant

2.) The phenotype usually differs from a simple combination of both single-gene mutant phenotypes, resulting in a modified dihybrid phenotypic ratio that is not 9:3:3:1

If the mutant alleles from different genes interact, we assume…

We assume that the WT genes also interact

How do we determine if two mutants are on the same gene or different genes?

1.) Can map the chromosomes and see where they lie

2.) Or perform a complementation test

Complementation tests

Intercross two diploid individuals that are homozygous for different recessive mutants and see if the progeny have the WT phenotype

Complementation tests = progeny have WT phenotype

If they have WT phenotype, that means the two recessive mutants are on different chromosomes (i.e. complemented)

Complementation tests = progeny don’t have WT phenotype

If they do not have the WT phenotype, then the two recessive mutants are on the same gene

Why do the mutant flowers turn white when the two recessive mutants are not complemented (i.e. when they’re found on the same chromosome)

1.) Them being found on the same chromosome means that neither of them have the WT gene on that chromosome, therefore neither will they progeny

2.) Therefore, their progeny do not have the WT gene to make the protein that makes them blue

How are gene interactions identified?

They have phenotypic ratios that deviate from the normal 9:3:3:1 ratio