The Molecular Biology of Eukaryotes Practice Problems (page 2)

By — McGraw-Hill Professional
Updated on Aug 23, 2011

Solution 2

(a)  Although we know the genotypes of the parents, we have no information concerning the genotype of the immediate maternal ancestor that was responsible for the organization of the egg cytoplasm from which our parental individuals developed. Therefore, we are unable to determine what phenotypes these individuals exhibit. Let us assume for the purpose of diagramming part (b) that the maternal parent is dextral and the paternal parent is sinistral.

(b)  Let D = dextrally organized cytoplasm; S = sinistrally organized cytoplasm.

Solved Problems

Practice 3

Slow-growing yeast cells called neutral petites lack normal activity of the respiratory enzyme cytochrome oxidase associated with the mitochrondria. Petites can be maintained indefinitely in vegetative cultures through budding, but can sporulate only if crossed to wild type. When a haploid neutral petite cell fuses with a haploid wild-type cell of opposite mating type, a fertile wild-type diploid cell is produced. Under appropriate conditions, the diploid cell reproduces sexually (sporulates). The four ascospores of the ascus (Fig. 6-4) germinate into cells with a 1 : 1 mating type ratio (as expected for nuclear genes), but they are all wild type. The petite trait never appears again, even after repeated backcrossings of both mating types to petite. The mitochondrial factors for petite are able to perpetuate themselves vegetatively, but are "swamped," lost, or permanently altered in the presence of wild-type factors. Neutral petite behaves the same in reciprocal crosses regardless of mating type. Assume that a neutral petite yeast has the chromosomal genes for normally functioning mitochondria, but has structurally defective mitochondria. Another kind of yeast is known, called segregational petite, which has structurally normal mitochondria that cannot function because of inhibition due to a recessive mutant chromosomal gene. What results would be expected among the sexual progeny when the neutral petite crosses with the segregational petite?

Solution 3

The diploid zygote receives from the segregational petite parent structurally normal mitochondria that should be able to function normally in the presence of the dominant nuclear gene from the neutral petite parent. Sporulation would probably distribute at least some structurally normal mitochondria to each ascospore. The nuclear genes would segregate 1 normal : 1 segregational petite. Let shaded cytoplasm contain defective mitochondria.

Solved Problems

Practice 4

A condition called "poky" in Neurospora is characterized by slow growth due to an abnormal respiratory enzyme system similar to that of petite yeast. The poky trait is transmitted through the maternal (protoperithecial) parent. A chromosomal gene F interacts with poky cytoplasm to produce a faster growing culture called "fast-poky," even though the enzyme system is still abnormal. Poky cytoplasm is not permanently modified by transient contact with an F genotype in the zygote. It returns to the poky state when the genotype bears the alternative allele F'. Gene F has no phenotypic expression in the presence of a normal cytoplasm. If the maternal parent is fast-poky and the paternal (conidial) parent is normal, predict the genotypes and phenotypes of the resulting ascospores.

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