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Chapter 9: Problem 3

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

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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

01

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.
02

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.
03

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.

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Most popular questions from this chapter

In this chapter, we focused on extranuclear inheritance and how traits can be determined by genetic information contained in mitochondria and chloroplasts, and we discussed how expression of maternal genotypes can affect the phenotype of an organism. At the same time, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How was it established that particular phenotypes are inherited as a result of genetic information present in the chloroplast rather than in the nucleus? (b) How did the discovery of three categories of petite mutations in yeast lead researchers to postulate extranuclear inheritance of colony size? (c) What observations support the endosymbiotic theory? (d) What key observations in crosses between dextrally and sinistrally coiled snails support the explanation that this phenotype is the result of maternal- effect inheritance? (e) What findings demonstrate a maternal effect as the basis of a mode of inheritance?

Tools are now available to sequence mtDNAs from an individual's genome sequence data to accurately measure heteroplasmy and to easily recognize the most functionally important mitochondrial variants. What is the significance of these tools for the diagnosis of human mtDNA diseases? If your sibling is diagnosed with a mitochondrial degenerative neurological disorder, would you want your mtDNA to be sequenced for mutations and heteroplasmy?

The maternal-effect mutation bicoid ( \(b c d\) ) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles \(\left(b c d^{+} / b c d^{-}\right)\) and a male homozygous for the mutation \(\left(b c d^{-} / b c d^{-}\right)\) (a) How is it possible for a male homozygous for the mutation to exist?. (b) Predict the outcome (normal vs. failed embryogenesis) in the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) generations of the cross described.

In Drosophila subobscura, the presence of a recessive gene called grandchildless (gs) causes the offspring of homozygous females, but not those of homozygous males, to be sterile. Can you offer an explanation as to why females and not males are affected by the mutant gene?

Why is the rate of mutation in mitochondrial DNA higher than that in nuclear DNA but the incidence of genetic diseases caused by mutations in mitochondrial DNA relatively low?
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