Biology Now, 2e

Dog Days of Science ■ 129

After their success with dog size, Lark and

Ostrander identified genes responsible for other

traits—fur color, leg length, skull shape, and

more. They also identified genes related to cancer

and other complex traits that might tell us some-

thing about human disease (see “Most Chronic

Diseases Are Complex Traits,” page 127). In

border collies, Ostrander’s team identified a gene

Colder extremities

(<37°C)

Colder extremities

(<37°C)

Body temperature

(37°C)

Figure 7.10

The environment can alter the effects

of genes

Coat color in Siamese cats is controlled by a

temperature-sensitive allele.

Q1: The gene that brings about the pale

Siamese body fur is also responsible

in part for the typical blue eyes of the

species. What is the term for this type of

inheritance?

Q2: Siamese kittens that weigh more tend

to have darker fur on their bodies. Why

might this be?

Q3: The Siamese cat pictured is called a

“seal point” because it has seal-colored

(dark brown) extremities. Some Siamese

cats show the same color pattern, but the

dark areas are of a lighter color or even a

different shade—for example, lilac point,

red point, blue point. What results would

you predict if the experiments described

in the text (shaving the cat and then

increasing or decreasing temperature)

were conducted on cats with these color

patterns?

Only 50 years and 35 generations separate this tame silver

fox from its wild relative

T

he silver fox is the same species as the more familiar red fox.

Because of its soft, silver coat, it has been bred in captivity

for over 100 years to provide fur coats, stoles, and hats for the

wealthy.

In 1959, a Russian geneticist, Dmitr y Belyaev, began to conduct

breeding experiments on silver foxes he had purchased from a fur

breeder, pairing only the tamest individuals of each generation. He

determined how tame a fox was by obser ving its response when

approached and of fered food. As the foxes became tamer in each

generation, they did not show a “fear response” until they were

older—9 weeks instead of 6 weeks. (Domestic dogs develop a fear

response at about 8–12 weeks.) In addition, in the tame foxes,

the hormones

associated with

a fear response

did not increase

until later. These

traits were all

clearly influenced

by the same

gene or genes—

an example of

pleiotropy.

Another

surprising result

was that the

foxes’ appearance

began to change

along with the

changes in

behavior. They

began to develop

shorter tails, wider

faces, and floppier

ears. All of these

made the adult

foxes look more puppy-like, and they are similar to the differences

that can be observed when comparing the domestic dog to its

ancestor, the wolf. Scientists conjecture that in both cases,

tameness and associated changes in development, physiology, and

anatomy were brought about by breeding for juvenile features.

The New Family Pet?

Georgie was Gordon Lark’s first Portuguese water dog,

a fiercely loyal and playful friend.

GEORGIE