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. Identify the genetic basis of streptomycin resistance in Chlamydomonas, which can be due to mutations in either chloroplast genes or nuclear genes.
2. Understand that Chlamydomonas has a uniparental inheritance pattern for chloroplast genes, typically inherited from the mt⁺ parent.
3. In the cross between an mt⁺ strain resistant in both genes and an mt⁻ strain sensitive to the antibiotic, consider that the chloroplast gene conferring resistance will be inherited from the mt⁺ parent, leading to resistant offspring.
4. For the nuclear gene, since it follows Mendelian inheritance, the offspring will be heterozygous for the nuclear gene, potentially showing resistance if the resistance allele is dominant.
5. In the reciprocal cross, where the mt⁻ strain is resistant and the mt⁺ strain is sensitive, the offspring will inherit the sensitive chloroplast gene from the mt⁺ parent, and the nuclear gene will follow Mendelian inheritance, potentially leading to sensitive offspring if the resistance allele is recessive.

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 confer resistance to antibiotics like streptomycin, affecting the organism's phenotype. 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 parental genotypes to observe the effects of gene inheritance from both parents. In this case, crossing an mt⁺ strain resistant in both genes with a sensitive strain, and then performing the reverse cross, allows for the analysis of maternal versus paternal contributions to the offspring's phenotype. This concept helps clarify the role of maternal inheritance in traits linked to chloroplast genes.