Create an account to track your scores
and create your own practice tests:
Test: MCAT Biology
Passage:
This has seemed a fatal objection to the chromosome view, but it may not be so, as Spillman has argued, so long as it has not yet been shown that all of the dominant characters may be present at the same time. But even admitting this possible way of eluding the objection, the other point raised above concerning the absence of groupings of characters in Mendelian inheritance seems a fatal objection to the chromosome theory, so long as that theory attempts to locate each character in a special chromosome. We shall have occasion to return to this point later.
In recent years most workers in Mendelian inheritance have adopted a new method of formulating their theory. Characters that Mendelize are no longer allelomorphic to each other, but each character has for its pair the absence of that character. This is the presence and absence theory. We can apply this hypothesis to the chromosome theory. For examples, let us assume a new variety or race arises by the loss of a character from that chromosome that has heretofore carried it. The chromosome still remains in existence, since it may carry many other characters besides the one that was lost, and it becomes in the hybrid the mate of the one still retaining that character. If now separation occurs, two classes of germ-cells result, one with and the other without the character; and the observed numerical proportions follow. There is nothing in this assumption that meets with any greater difficulty on the chromosome separation hypothesis than on the earlier view of paired allelomorphs, but it meets with the same difficulties, and as an assumption is neither more nor less in accord with the postulated mechanism.
Excerpt from Morgan, T. H. 1910. Chromosomes and heredity. The American Naturalist, 44:449–496.
1. | Asssume that in a particular studied species of rodents, coat color is an autosomally-inherited trait, with black fur being the dominant phenotype and white fur being the recessive phenotype. In an experiment, a mouse with black fur mates with a mouse with a mouse with white fur. The resultant 26 offspring mice all have grey fur color. Which of the following genetic principles is best demonstrated by this result? |
Nondisjunction
X-linkage
Codominance
Incomplete Dominance
Spontaneous mutation
Certified Tutor