P: o+o+ cc X oo c+c+
F1: o+o c+c
| o+c+ | o+c |
oc+ | oc | |
| o+c+ | Arg |
Arg | Arg | Arg |
| o+c |
Arg | Cit |
Arg | Cit |
| oc+ |
Arg | Arg | Orn |
Orn |
| oc |
Arg | Cit |
Orn |
Orn |
Recessive Epistasis in the
diploid Arginine biosynthesis pathway
Beadle
& Tatum analyzed the genetics of Arginine
biosynthesis in the haploid bread mold Neurospora.
The same pathway occurs in diploid organisms:
genetic analysis of mutant alleles affecting metabolic
intermediates of Arginine production provides a general
model for the behavior of multiple gene loci in a single
pathway.
In this pathway, Ornithine is converted to Citrulline by the action of enzyme Oase encoded by Gene O: the functional wildtype allele o+ is dominant to the mutant non-functional allele o. Similarly, Citrulline is converted to Arginine by the action of enzyme Case encoded by Gene C, where c+ is dominant to c. Pure-breeding auxotrophic parental lines of diploid c- and o- yeast are crossed: the F1 dihybrid offspring are all heterotrophs. A cross of these dihybrids produces the results shown as the end-point of synthesis in the pathway. Note that any individual with both an o+ and a c+ allele is able to synthesize Arginine, and any individual with an oo genotype accumulates Ornithine because Oase is not produced. Any individual with a cc genotype accumulates citrulline due to the lack of Case, except that the oocc genotype accumulates Ornithine, the previous intermediate in the pathway, because the oo genotype blocked the pathway prior to the production of Citrulline. That is, a recessive genotype at Gene O suppresses expression of the next gene in the pathway, Gene C.
Homework: Predict and/or construct Punnet squares to show the consequences of PpOo and PpCc dihybrid crosses, and PpOoCc tri-hybrid crosses, assuming complete dominance of the p+, o+, and c+ alleles.
Homework: Recall the growth / no-growth phenotypes of Neurospora as a consequence of synthesis or non-synthesis of Arginine. In the yeast example, note that both the cc and oo genotypes fail to produce Arginine, and in consequence are auxotrophic. How does the resultant ratio compare with other forms of epistatic interaction between loci?
In this pathway, Ornithine is converted to Citrulline by the action of enzyme Oase encoded by Gene O: the functional wildtype allele o+ is dominant to the mutant non-functional allele o. Similarly, Citrulline is converted to Arginine by the action of enzyme Case encoded by Gene C, where c+ is dominant to c. Pure-breeding auxotrophic parental lines of diploid c- and o- yeast are crossed: the F1 dihybrid offspring are all heterotrophs. A cross of these dihybrids produces the results shown as the end-point of synthesis in the pathway. Note that any individual with both an o+ and a c+ allele is able to synthesize Arginine, and any individual with an oo genotype accumulates Ornithine because Oase is not produced. Any individual with a cc genotype accumulates citrulline due to the lack of Case, except that the oocc genotype accumulates Ornithine, the previous intermediate in the pathway, because the oo genotype blocked the pathway prior to the production of Citrulline. That is, a recessive genotype at Gene O suppresses expression of the next gene in the pathway, Gene C.
Homework: Predict and/or construct Punnet squares to show the consequences of PpOo and PpCc dihybrid crosses, and PpOoCc tri-hybrid crosses, assuming complete dominance of the p+, o+, and c+ alleles.
Homework: Recall the growth / no-growth phenotypes of Neurospora as a consequence of synthesis or non-synthesis of Arginine. In the yeast example, note that both the cc and oo genotypes fail to produce Arginine, and in consequence are auxotrophic. How does the resultant ratio compare with other forms of epistatic interaction between loci?