Question

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 \(m t^{+}\) strain resistant in both genes and a member of a strain sensitive to the antibiotic? What results would occur in the reciprocal cross?

Short answer

Additionally, what are the results of the reciprocal cross? Answer: The offspring of the initial cross, where the maternal parent is from the strain with streptomycin resistance in both the chloroplast and nuclear genes, will be phenotypically resistant to streptomycin. On the other hand, the offspring of the reciprocal cross, where the sensitive strain is the maternal parent, will be phenotypically sensitive to streptomycin.

Step-by-step solution

Understand the inheritance pattern and phenotype of streptomycin resistance in Chlamydomonas

Chlamydomonas is a unicellular green algae. The resistance to streptomycin in Chlamydomonas can be due to a mutation in either the chloroplast gene or the nuclear gene. The inheritance pattern of chloroplast genes is maternal, which means that they are inherited only from the maternal parent. On the other hand, nuclear genes are inherited from both parents in a Mendelian fashion. \(m t^{+}\) strain has a mutation in both the chloroplast and nuclear genes that causes resistance to streptomycin.

Determine the phenotypic results of the initial cross

In the initial cross, we have a member of the \(m t^{+}\) strain (resistant in both chloroplast and nuclear genes) as the maternal parent and a member of the sensitive strain as the paternal parent. The offspring will inherit the resistant chloroplast gene from their maternal parent (because chloroplast inheritance is maternal) and will also inherit one nuclear gene from each parent. As a result, the offspring will be heterozygous for nuclear gene (Nucleus R/n) and will have resistance in the chloroplast gene (Chloroplast R). Therefore, these offspring will be phenotypically resistant to streptomycin.

Determine the phenotypic results of the reciprocal cross

In the reciprocal cross, we have a member of the sensitive strain as the maternal parent and a member of the \(m t^{+}\) strain (resistant in both chloroplast and nuclear genes) as the paternal parent. The offspring will inherit the sensitive chloroplast gene from their maternal parent. The sensitive strain phenotype dominates over the resistant phenotype, and the offspring will be heterozygous for nuclear gene inheritance (Nucleus R/n). However, due to the dominant phenotype of the sensitive chloroplast gene, overall, the offspring will be phenotypically sensitive to streptomycin. In conclusion, the offspring of the initial cross, where the maternal parent is from the \(m t^{+}\) strain, will be phenotypically resistant to streptomycin. On the other hand, the offspring of the reciprocal cross, where the sensitive strain is the maternal parent, will be phenotypically sensitive to streptomycin.