Mules result from a cross between a horse (2 n = 64) and a donkey
(2 n = 62), have 63 chromosomes, and are almost always sterile.
However, in the summer of 1985, a female mule named Krause who was
pastured with a male donkey gave birth to a male foal (O. A. Ryder et al.
1985. Journal of Heredity 76:379–381). Blood tests established that the
foal, appropriately named Blue Moon, was the offspring of Krause and
that Krause was indeed a mule. Both Blue Moon and Krause were
fathered by the same donkey (see the accompanying pedigree). The foal,
like his mother, had 63 chromosomes—half of them horse chromosomes
and the other half donkey chromosomes. Analyses of genetic markers
showed that, remarkably, Blue Moon seemed to have inherited a
complete set of horse chromosomes from his mother, instead of the
random mixture of horse and donkey chromosomes that would be
expected with normal meiosis. Thus, Blue Moon and Krause were not
only mother and son, but also brother and sister.
a. With the use of a diagram, show how, if Blue Moon inherited only
horse chromosomes from his mother, Krause and Blue Moon are mother
and son as well as sister and brother.
b. Although rare, additional cases of fertile mules giving birth to offspring
have been reported. In these cases, when a female mule mates with a male
horse, the offspring is horselike in appearance, but when a female mule
mates with a male donkey, the offspring is mulelike in appearance. Is this
observation consistent with the idea that the offspring of fertile female
mules inherit only a set of horse chromosomes from their mule mothers?
Explain your reasoning.
c. Can you suggest a possible mechanism for how fertile female mules
might pass on a complete set of horse chromosomes to their offspring?
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