T
he probability of an event is the chance that the event will occur. For
example, there is a probability of 0.5 that a coin will turn up “heads”
when it is tossed. A probability of 0.5 is the same thing as a 50 percent
chance, or^1 / 2 odds, or a ratio of one heads to one tails (1:1). If you
toss the coin only a few times, the observed percentage of heads may
differ greatly from 50 percent. But if you toss it many, many times, that
observed percentage will be very close to 50 percent. Each toss of a
coin is an independent
event, in the sense
that the outcome of
one toss does not
affect the outcome
of the next toss. The
probability of getting
two heads in a row is a
product of the separate
probabilities of each
individual toss: 0.5 ×
0.5, which is 0.25. In our
cross, the probability of
getting a brown puppy
is^1 / 4 , or 0.25. To go
back to a Punnett square to predict the ratio of puppies from a genetic
cross of two heterozygous (Bb) black-coated dogs, the probability of
getting a black puppy is^3 / 4 , or 0.75.
We cannot know with certainty what the actual phenotype or
genotype of a particular offspring is going to be, except when true-
breeding individuals are crossed. For example, two brown dogs, both
of whom have a bb genotype, will have only bb-genotype, brown-
phenotype offspring. Moreover, the probability that a particular
offspring will display a specific phenotype is completely unaffected by
how many offspring there are. The likelihood that we will see the 3:1
black-to-brown outcome, however, increases when we analyze a larger
number of offspring, just as Mendel analyzed thousands of pea plants.What Are the Odds?
124 ■ CHAPTER 07 Patterns of InheritanceGENETICSGENETICS
Going to the Dogs
Like Mendel, Elaine Ostrander planned to study
plants to unravel the secrets of genetics and in -
heritance. But when she arrived at UC Berkeley
to open her lab, the space was not yet available.
So she wandered down the hall and into the
office of Jasper Rine, a geneticist who normally
studied yeast but was looking for someone to
start a mammalian genome research project.
Ostrander volunteered.
But which mammal to study? “I was allergic
to cats, and I didn’t know enough about cows or
pigs or horses,” she recalls, so she picked dogs.
Not only was Ostrander a dog lover, but the
American Kennel Club had just begun offering
funding to researchers trying to identify genes
associated with dog diseases.
In 1993, Ostrander began identifying all the
genes unique to dogs—that is, making a map
of the dog genome. Some colleagues said she
was nuts, that no one would give her money to
support the research. But Ostrander is nothing
if not persistent, and she knew the potential
value of the research: dogs have more than 350
inherited diseases, and up to 300 of those are
similar to conditions in people, including cancer,
epilepsy, heart disease, and Addison’s disease,
the illness that killed Lark’s dog Georgie. The
genetics of bladder cancer is difficult to study
in humans, for example, but the disease is quite
common in Scottish terriers and would be easier
to study in a dog species. By cracking the genetic
code of dogs, Ostrander hoped to uncover causes
and potential treatments for human diseases.
In 2005, she published the first full dog
genome sequence, for a female boxer named
Tasha. The achievement gained her scientific
fame and raised awareness among scientists
of the importance of dog genetics to human
health. “Of the more than 5,500 mammals living
today, dogs are arguably the most remarkable,”
Ostrander’s coauthor, Eric Lander, a professor of
biology at the Massachusetts Institute of Tech-
nology, said when the first dog genome sequence
was published. “The incredible physical and
behavioral diversity of dogs—from Chihuahuas
to Great Danes—is encoded in their genomes.
It can uniquely help us understand embryonic
development, neurobiology, human disease, and
the basis of evolution” (Figure 7.7).Mendel’s laws by identifying chromosomes
as the paired factors (where each homolo-
gous chromosome in a pair has one allele for
a gene) that are shuffled and recombined, and
then separated randomly into sperm and egg
cells during meiosis (see Figures 6.7 and 6.8 in
Chapter 6). Then, during fertilization, a one-in-
a-million sperm fuses with a one-in-a-million
egg to create a unique individual. That is how
offspring can have genotypes and phenotypes
that were not present in either parent, such as
a brown puppy born to two black dogs.