Streptomycin resistance in Chlamydomonas may result from a mutation in either a chloroplast gene or a nuclear gene. What phenotypic results would occur in a cross between a member of an mt⁺ strain resistant in both genes and a member of a strain sensitive to the antibiotic? What results would occur in the reciprocal cross?

Verified step by step guidance

1. Understand the genetic basis of streptomycin resistance: Resistance can arise from mutations in either a chloroplast gene (which is maternally inherited in Chlamydomonas) or a nuclear gene (which follows Mendelian inheritance).
2. Analyze the first cross: In the cross between an mt⁺ strain resistant in both genes (chloroplast and nuclear) and an mt⁻ strain sensitive to the antibiotic, the chloroplast genes will be inherited from the mt⁺ parent, while nuclear genes will segregate according to Mendelian inheritance.
3. Predict the phenotypes for the first cross: Since the mt⁺ parent contributes the chloroplast genome, all offspring will inherit the resistant chloroplast gene. For the nuclear gene, offspring will inherit one resistant allele from the mt⁺ parent and one sensitive allele from the mt⁻ parent, resulting in a heterozygous nuclear genotype.
4. Analyze the reciprocal cross: In the reciprocal cross, where the mt⁻ strain (sensitive) is the chloroplast donor and the mt⁺ strain (resistant) is the nuclear donor, the chloroplast genome will be inherited from the mt⁻ parent, making all offspring sensitive in terms of chloroplast resistance. The nuclear gene will segregate as before, resulting in heterozygous nuclear genotypes.
5. Summarize the phenotypic results: In the first cross, all offspring will be resistant due to the resistant chloroplast gene, regardless of the nuclear genotype. In the reciprocal cross, all offspring will be sensitive due to the sensitive chloroplast gene, even though they carry one resistant nuclear allele.

Key Concepts

Chloroplast and Nuclear Genes

Chloroplast genes are inherited maternally and are located in the chloroplasts, while nuclear genes are found in the cell nucleus and follow Mendelian inheritance patterns. In Chlamydomonas, mutations in either type of gene can lead to phenotypic changes, such as antibiotic resistance. Understanding the distinction between these gene types is crucial for predicting inheritance patterns in crosses.

Phenotypic Expression

Phenotypic expression refers to the observable traits or characteristics of an organism resulting from the interaction of its genotype with the environment. In the context of the question, the phenotypic results of the crosses will depend on whether the resistance alleles are dominant or recessive and how they interact in the offspring. This concept is essential for predicting the outcomes of genetic crosses.

Reciprocal Crosses

A reciprocal cross involves swapping the roles of the parents in a genetic cross to determine if the inheritance pattern changes. In this case, crossing an mt⁺ strain resistant in both genes with a sensitive strain and then performing the reverse cross can reveal insights into the genetic basis of resistance. Analyzing the results of both crosses helps clarify the inheritance of traits linked to chloroplast and nuclear genes.