Part B: Basic Genetics Experiments

A Genetic Test for Allelism

The experiment A Dihybrid Cross uses mutants with different phenotypes to demonstrate dominant and recessive gene expression. In the experiment Two Genes/One Trait, introduces two refinements of the gene concept: 1) mutants in different genes can have the same phenotype, and there can be different forms of the same gene (alleles.) This experiment combines the two previous ones to explore the interactions of the two red mutants, ade1 and ade2 in a dihybrid cross.. The four types of gametes produced in the F1 generation of a cross heterozygous for both of the red adenine genes (ADE1/ade1 ADE2/ade2) would be ADE1 ADE2, ADE1 ade2, ade1 ADE2, and ade1 ade2. By making all possible crosses of these strains in both mating types, we get a graphic demonstration of the dominant and recessive relationships of the red color vs cream color, and adenine-dependence vs adenine independence.

This cross also illustrates a development in genetics that was essential for understanding the nature of the gene at the molecular level. This is a genetic test for allelism. It demonstrates that the two mutations ade1 and ade2 are not allelic in spite of having the same phenotype.


In this experiment you will use a set of haploid strains to produce the second (F2 ) generation of a cross between two different red mutants. This set of haploid strains represents the eight different haploid genotypes (spores or gametes) that are expected in equal frequencies from a dihybrid cross with independent assortment.

Geneticists get tired of writing the same symbols repeatedly and use a simple shorthand. The normal allele of a gene can often be represented by just a "+" without any confusion. That way the genotype ADE1 ADE2 is just + +, ADE1 ade2 is + ade2, etc. The table of genotypes and phenotypes of these strains becomes simply:

Genotype Phenotype on: YED MV
+ + cream-colored grows
ade1 + red doesn't grow
+ ade2 red doesn't grow
ade1 ade2 red doesn't grow

Of course, we need to have each of these strains in both mating types, so that makes a total of eight strains.

The two different media, YED and MV, allow you to distinguish the mutant phenotype from the normal phenotype, but does not let you distinguish among the different mutant strains.

Time Line:

1st Day: 50 min. Make mating grid and subculture haploid strains
2nd Day: 20 min. Make mating mixtures
3rd Day: 10 min. Replica plate to MV
4th Day: 50 min. Record and analyze results

Figure 1