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?

Streptomycin is an antibiotic that inhibits protein synthesis in bacteria and some eukaryotic organisms, like Chlamydomonas, a single-celled green alga. Streptomycin resistance can emerge due to random mutations, which alter the target protein structure so that the antibiotic can no longer bind effectively. In Chlamydomonas, such resistance could occur either in the chloroplast or the nuclear genome.

When studying the inheritance of streptomycin resistance, scientists might cross resistant strains with sensitive ones. A Chlamydomonas strain that is resistant in both genetic areas (RR) will pass on the resistant trait to its progeny even if crossed with a sensitive strain (SS). Hence, understanding the genetics behind streptomycin resistance in Chlamydomonas not only provides insight into basic genetic principles but also has implications for studying antibiotic resistance in more complex organisms.

A Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is a simple way to calculate the chances of offspring having a particular genotype.

Consider tossing two coins: the Punnett square would help us understand the probability of getting heads or tails. Similarly, in genetics, the Punnett square gives us the possible genotypes of offspring from two parents by considering their alleles. Genotypes, which are the genetic makeup of organisms, are represented within the cells of the Punnett square, while phenotypes, which are observable characteristics, can often be inferred from them.

In the Chlamydomonas exercise, the Punnett square reveals the possible genotypes of the offspring when a resistant strain (RR) is crossed with a sensitive strain (SS), predicting that all offspring will carry the resistant allele (R).

Alleles are different versions of a gene that determine distinct traits that can be passed from parents to offspring. When it comes to genetic inheritance, alleles can be either dominant or recessive. A dominant allele is always expressed in the phenotype, even if only one copy is present, while a recessive allele is masked when a dominant allele is present.

For example, in the Chlamydomonas resistance study, the resistant allele (R) is dominant, meaning it will dictate the organism's resistance to streptomycin when present. On the other hand, the sensitive allele (S) is recessive and is only expressed when two copies are present (SS). When an organism has one copy of each (Rs), it will be resistant due to the dominance of the R allele.

This fundamental concept of dominant and recessive alleles is crucial in understanding inheritance patterns, predicting offspring traits, and exploring more complex genetic principles, such as codominance and incomplete dominance.