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I 9.2 Blue Moon resulted from a cross between a fertile mule and a donkey. The probable pedigree of Blue Moon, the foal of a fertile mule, is shown.

her son is not known; possibly all Krause's donkey chromosomes passed into the polar body during the first division of meiosis (see Figure 2.22), leaving the oocyte with only horse chromosomes.

Krause later gave birth to another male foal named White Lightning. Like his brother, White Lightning possessed mule chromosomes and appeared to have inherited only horse chromosomes from his mother. Additional reports of fertile female mules support the idea that their offspring inherit only horse chromosomes from their mother. When the father of a mule's offspring is a horse, the offspring is horselike in appearance, because it apparently inherits horse chromosomes from both of its parents. When the father of a mule's offspring is a donkey, however, the offspring is mulelike in appearance, because it inherits horse chromosomes from its mule mother and donkey chromosomes from its father.

Most species have a characteristic number of chromosomes, each with a distinct size and structure, and all the tissues of an organism (except for gametes) generally have the same set of chromosomes. Nevertheless, variations in chromosome number and structure do periodically arise. Individual chromosomes may lose or gain parts; the sequence of genes within a chromosome may become altered; whole chromosomes can even be lost or gained. These variations in the number and structure of chromosomes are termed chromosome mutations, and they frequently play an important role in evolution.

We begin this chapter by briefly reviewing some basic concepts of chromosome structure, which we learned in Chapter 2. We then consider the different types of chromosome mutations, their definitions, their features, and their phenotypic effects. Finally, we examine the role of chromosome mutations in cancer.

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