Wild-type strains of the haploid fungus Neurospora can make...

Wild-type strains of the haploid fungus Neurospora can make their own tryptophan. An abnormal allele $t d$ renders the fungus incapable of making its own tryptophan. An individual of genotype $t d$ grows only when its medium supplies tryptophan. The allele $s u$ assorts independently of $t d ;$ its only known effect is to suppress the $t d$ phenotype. Therefore, strains carrying both $t d$ and $s u$ do not require tryptophan for growth. a. If a $t d ; s u$ strain is crossed with a genotypically wildtype strain, what genotypes are expected in the progeny and in what proportions? b. What will be the ratio of tryptophan-dependent to tryptophan-independent progeny in the cross of part $a ?$

Wild-type strains of the haploid fungus Neurospora can make their own tryptophan. An abnormal allele $t d$ renders the fungus incapable of making its own tryptophan. An individual of genotype $t d$ grows only when its medium supplies tryptophan. The allele $s u$ assorts independently of $t d ;$ its only known effect is to suppress the $t d$ phenotype. Therefore, strains carrying both $t d$ and $s u$ do not require tryptophan for growth.
a. If a $t d ; s u$ strain is crossed with a genotypically wildtype strain, what genotypes are expected in the progeny and in what proportions?
b. What will be the ratio of tryptophan-dependent to tryptophan-independent progeny in the cross of part $a ?$

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Key Concepts

Independent Assortment

Independent assortment is a fundamental principle of genetics which posits that genes located on separate chromosomes, or far apart on the same chromosome, are inherited independently of one another. This concept is crucial when predicting the progeny genotype ratios in crosses where mutations and suppressors are segregating.

Mendelian Inheritance in Haploids

In haploid organisms, each gene is present as a single copy, meaning that the phenotype is directly determined by the allele at each locus without the confounding effects of dominance or heterozygosity. This simplifies the analysis of genetic crosses as seen in fungal genetics.

Auxotrophy

Auxotrophy refers to the inability of an organism to synthesize a particular organic compound required for its growth, forcing it to obtain that compound from its environment. This concept is central to understanding how mutations affecting biosynthetic pathways, like that of tryptophan, can lead to specific nutritional requirements.

Suppressor Mutation

A suppressor mutation is a genetic change that counteracts or mitigates the effect of another mutation. In this context, it allows an organism carrying a defective allele to restore a normal phenotype by compensating for the original mutation’s impact, illustrating the complexity of gene interactions.

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