Once in a Blue Moon
One of the best-known facts of genetics is that a cross
between a horse and a donkey produces a mule. Actually,
it’s a cross between a female horse and a male donkey that
produces the mule; the reciprocal cross, between a male
horse and a female donkey, produces a hinny, which has
smaller ears and a bushy tail, like a horse ( FIGURE 9.1).
Both mules and hinnies are sterile because horses and donkeys
are different species with different numbers of chromosomes:
a horse has 64 chromosomes, whereas a donkey
has only 62. There are also considerable differences in the
sizes and shapes of the chromosomes that horses and donkeys
have in common. A mule inherits 32 chromosomes
from its horse mother and 31 chromosomes from its donkey
father, giving the mule a chromosome number of 63.
The maternal and paternal chromosomes of a mule are not
homologous, and so they do not pair and separate properly
in meiosis; consequently, a mule’s gametes are abnormal
and the animal is sterile.
?
In spite of the conventional wisdom that mules are
sterile, reports of female mules with foals have surfaced over
the years, although many of them can be attributed to mistaken
identification. In several instances, a chromosome
check of the alleged fertile mule has demonstrated that she
is actually a donkey. In other instances, analyses of genetic
markers in both mule and foal demonstrated that the foal
was not the offspring of the mule; female mules are capable
of lactation and sometimes they adopt the foal of a nearby
horse or donkey.
In the summer of 1985, a female mule named Krause,
who was pastured with a male donkey, was observed with a
newborn foal. There were no other female horses or donkeys
in the pasture; so it seemed unlikely that the mule had
adopted the foal. Blood samples were collected from
Krause, her horse and donkey parents, and her male foal,
which was appropriately named Blue Moon. A team of geneticists
led by Oliver Ryder of the San Diego Zoo examined
their chromosomal makeup and analyzed 17 genetic
markers from the blood samples.