Question

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?

Short answer

Question: Explain the role of maternal genotype in determining the direction of shell coiling in snails. Answer: In snails, the direction of shell coiling, either dextral (right-handed) or sinistral (left-handed), is determined by the maternal genotype, not the offspring's genotype. When crosses were performed between dextral and sinistral snails, the resulting offspring's shell coiling direction was found to depend entirely on the genotype of the mother, regardless of the father's genotype. This phenomenon is due to maternal-effect inheritance, where certain gene products are deposited in the egg cytoplasm by the mother during oogenesis, affecting the offspring's phenotype.

Step-by-step solution

(a) Inheritance of phenotypes due to chloroplasts

One way to establish that particular phenotypes are inherited as a result of genetic information in the chloroplast is by performing reciprocal crosses. If the nuclear genes were responsible, the offspring of reciprocal crosses would show indistinguishable phenotypes. However, if the phenotype depends on genes in the chloroplast, the offspring will have different phenotypes in the reciprocal crosses. For example, in the experiments with Chlamydomonas, a green alga, crosses showed that the offspring's chlorophyll content depended on the maternal parent, indicating that the genetic info controlling this trait was in the chloroplasts instead of the nucleus.

(b) Discovery of petite mutations in yeast and extranuclear inheritance

The discovery of three categories of petite mutations in yeast led researchers to postulate extranuclear inheritance of colony size because they observed differences in the inheritance patterns of these mutations. During the experiments, it was found that the segregation ratios of the petite mutants were not consistent with Mendelian nuclear inheritance but correlated with the proportion of defective mitochondrial DNA. These findings suggested that the colony size was a result of extranuclear inheritance involving the genes in the mitochondrial genome.

(c) Observations supporting the endosymbiotic theory

The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotic cells, which were engulfed by a larger eukaryotic cell. Several observations support the endosymbiotic theory, such as: 1. Mitochondria and chloroplasts have their own DNA and ribosomes, like prokaryotes. 2. The genetic sequences in mitochondrial and chloroplast DNA are more similar to sequences found in bacterial DNA than those in nuclear DNA. 3. Both organelles are surrounded by a double membrane, consistent with the engulfment of a prokaryotic cell by a eukaryotic cell. 4. Mitochondria and chloroplasts reproduce by binary fission, a process similar to bacterial reproduction.

(d) Observations on dextrally and sinistrally coiled snails

Experiments on snails with dextral (right-handed) and sinistral (left-handed) coiling showed that the phenotype depended not on the snails' own genotype but on the maternal genotype. When crosses were performed, the direction of the coiling of the offspring's shells depended entirely on the genotype of the mother, regardless of the father's genotype. This suggests that the offspring's shell coiling direction is a result of maternal-effect inheritance.

(e) Findings demonstrating maternal effect on inheritance

Maternal effect on inheritance is demonstrated by the fact that offspring's phenotype depends on the mother's genotype, and not their own genotype, for certain traits. This occurs because certain gene products, such as RNAs and proteins, are deposited in the egg cytoplasm by the mother during oogenesis. As embryos develop using these maternal gene products before zygotic gene activation, the early development of the offspring may be determined by the mother's genotype. Examples of maternal-effect inheritance include the shell coiling direction in snails and certain developmental defects in Drosophila.