Predict the results of a cross between ascospores from a segregational petite strain and a neutral petite strain. Indicate the phenotype of the zygote and the ascospores it may subsequently produce.

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?

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 mt⁺ strain resistant in both genes and a member of a strain sensitive to the antibiotic? What results would occur in the reciprocal cross?

A male mouse from a true-breeding strain of hyperactive animals is crossed with a female mouse from a true-breeding strain of lethargic animals. (These are both hypothetical strains.) All the progeny are lethargic. In the F₂ generation, all offspring are lethargic. What is the best genetic explanation for these observations? Propose a cross to test your explanation.

In diploid yeast strains, sporulation and subsequent meiosis can produce haploid ascospores, which may fuse to reestablish diploid cells. When ascospores from a segregational petite strain fuse with those of a normal wild-type strain, the diploid zygotes are all normal. Following meiosis, ascospores are petite and normal. Is the segregational petite phenotype inherited as a dominant or a recessive trait?

A plant may have green, white, or green-and-white (variegated) leaves on its branches, owing to a mutation in the chloroplast that prevents color from developing. Predict the results of the following crosses:

  Ovule Source        Pollen Source

(a) Green branch    x  White branch

(b) White branch     x  Green branch

(c) Variegated branch x  Green branch

(d) Green branch    x  Variegated branch

In a cross of Lymnaea, the snail contributing the eggs was dextral but of unknown genotype. Both the genotype and the phenotype of the other snail are unknown. All F₁ offspring exhibited dextral coiling. Ten of the F₁ snails were allowed to undergo self-fertilization. One-half produced only dextrally coiled offspring, whereas the other half produced only sinistrally coiled offspring. What were the genotypes of the original parents?