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Unformatted text preview: exact mirror images, for any disadvantage that a fertilization, the gene produces a protein, found in the sinistral twist in itself could confer on its possessor is cytoplasm of the egg, that controls the pattern of cell almost inconceivable. But left- and right-handed snails division and thus handedness. In experiments, an are not actually true mirror images of one another. Their injection of cytoplasm from dextral eggs changes the shapes are noticeably different. Sinistral rarity might, then, pattern of sinistral eggs, but an injection from sinistral be a consequence of possible disadvantages conferred by eggs does not influence dextral eggs. One explanation for these other concomitant structural features. In addition, the differing effects is that all Lymnaea peregra eggs perhaps left- and right-handed snails cannot mate with begin left-handed but most switch to being right-handed. each other, having incompatible twist directions. Thus, the path to a solution to the puzzle of handedness in Presumably an individual of the rarer form would have all snails appears to be as twisted as the helix itself. relative difficulty in finding a mate of the same hand, thus (472 words) keeping the rare form rare or creating geographically
separated right- and left-handed populations.
But this evolutionary mechanism combining
dissymmetry, anatomy, and chance does not provide an
adequate explanation of why right-handedness should
have become predominant. It does not explain, for
example, why the infrequent unions between snails of
opposing hands produce fewer offspring of the rarer than
the commoner form in species where each parent
contributes equally to handedness. Nor does it explain
why, in a species where one parent determines
handedness,a brood is not exclusively right- or
left-handed when the offspring would have the same 160 4. The second paragraph of the passage is primarily 6. According to the passage, which of the following is concerned with offering possible reasons why true of Lymnaea peregra? (A) it is unlikely that evolutionary mechanisms (A) Handedness within the species was at one time could discriminate against sinistral snails equally distributed between left and right. (B) sinistrality is relatively uncommon among snail (B) Under laboratory conditions, dextral eggs from species Lymnaea peregra can be artificially induced (C) dextral and sinistral populations of a snail to develop into sinistral snails. species tend to intermingle (C) Broods of Lymnaea peregra are, without varia- (D) a theory based on a developmental mechanism tion, exclusively sinistral or dextral. inadequately accounts for the predominance (D) Handedness in Lymnaea peregra offspring is of dextrality across snail species determined by two parents together. (E) dextral snails breed more readily than sinistral (E) fewer sinistral offspring in dextral broods than snails, even within predominantly sinistral dextral offspring in sinistral broods populations
7. Which of the followin...
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- Fall '13