They can cause unusual features, diseases,
and other problems.
Structure
DNA has a complex structure. It is
made of chemical substances that are
linked together like a chain. Each piece
of DNA has two long strands, or chains.
The two strands are joined together.
They form a shape like a ladder that has
been twisted into a spiral.
Chemicals called phosphates and sugars
make up the sides of the ladder. DNA
also has chemicals called bases. Each
base on one strand is joined to a base on
the other strand. The linked bases form
the rungs of the ladder.
Genetic Code
There are four different bases in DNA:
adenine, thymine, guanine, and
cytosine. These four chemicals are
repeated in different orders over and
over again in each strand of DNA.
Human DNA contains about 3 billion
pairs of these bases.
The order in which the bases are
arranged is very important. It forms a
code that tells cells to make certain
kinds of proteins. The differences in
these proteins is what makes different
living things—such as a cactus, a gerbil,
and two different people—different.
#More to explore
Cell • Chromosome • Genetics • Living
Thing • Mutation
Dog
The dog is one of the most popular animals
in the world. It was one of the first
animals to be domesticated, or trained
for use by humans. The dog’s scientific
name is Canis familiaris. It is related to
the coyote, wolves, foxes, and jackals.
Where Dogs Live
People around the world keep dogs as
pets, guards, or work animals. Some
dogs, called feral dogs, do not live with
people. These homeless dogs often roam
around in groups, called packs. One
type of dog, called the dingo, lives in the
wild in Australia.
Physical Features and Breeds
The dog is a mammal with sharp teeth,
an excellent sense of smell, and a fine
sense of hearing. Each of its four legs
ends in a foot, or paw, with five toes.
Each toe has a soft pad and a claw. A
coat of hair keeps the dog warm. It cools
off by panting and hanging its tongue
out of its mouth.
Some dogs are purebred. This means that
both parents were of the same breed. A
dog with parents of different breeds is
called a mutt or mongrel.
BRITANNICA STUDENT ENCYCLOPEDIA Dog 61
Some Breeds of Dog
Breed Origin Height of Males (inches)
Sporting dogs
English cocker spaniel England 16–17
Golden retriever Scotland 23–24
Irish setter Ireland 27
Weimaraner Germany 25–27
Golden retriever
Hounds
Afghan hound Afghanistan 27
Beagle England 13 or 15
Dachshund Germany 7–10
Greyhound Egypt 27–30
Beagle
Working dogs
Great Dane Germany minimum 30
Mastiff England minimum 30
Saint Bernard Switzerland minimum 27.5
Siberian husky northeastern Asia 21–23.5
Siberian husky
Terriers
Bull terrier England 21–22
Miniature schnauzer Germany 12–14
Scottish terrier Scotland 10
Welsh terrier Wales 15–15.5
Scottish terrier
Toys
Chihuahua Mexico 5
Maltese Malta 5
Pomeranian Iceland, Lapland maximum 11
Pug China 10–11
Pug
Non-sporting dogs
Bulldog England 12–14
Dalmatian Croatia 19–23
Lhasa apso Tibet 10–11
Poodle France minimum 15
Dalmatian
Herding dogs
Collie Scotland 24–26
German shepherd Germany 24–26
Old English sheepdog England minimum 22
Pembroke Welsh corgi Wales 10–12
Collie
Dog breeds are placed in seven different groups, according to the original function of the
breed. For example, sporting dogs and hounds hunt animals. Working dogs do such jobs
as pulling sleds or guarding homes. The table shows some dog breeds in each group.
62 Dog BRITANNICA STUDENT ENCYCLOPEDIA
Apart from these common features, dogs
come in many different sizes, shapes,
and colors. Dogs that have similar sizes,
looks, and behaviors make up groups
called breeds.
Behavior
Dogs are carnivores, which means that
they eat meat. Pet dogs eat dog food
made with meat products.Wild dogs
hunt animals.
Dogs claim territory and mark it as their
own. They do this by urinating and rubbing
their scent on things. But dogs also
like to be around people and other dogs.
People can even train dogs to obey
simple commands.
Dogs make many sounds, including
barks, howls, and growls. They also use
movements to communicate. For
example, when a dog wags its tail, it is
happy. If it shows its teeth and growls, it
is making a threat.
Life Cycle
A female dog gives birth to a litter of
puppies about two months after mating.
A litter contains two to 12 puppies.
Newborn puppies depend on their
mother for her milk and protection.
Puppies become more independent
when they are three or four months old.
Dogs live for 10 to 15 years.
Dogs and Humans
Dogs have been with humans since prehistoric
times. The ancient Egyptians
thought dogs were holy. The ancient
Romans kept watchdogs. In the palaces
of ancient China, people kept small dogs
in the sleeves of their robes.
Over the years, people developed different
breeds of dog for different purposes.
Toy dogs, including the poodle and the
pug, were meant to be pets. Other dogs
were meant to do certain jobs. For
example, the golden retriever and other
sporting dogs helped people to hunt
birds. Some of these dogs are now
mainly pets. Others still do their traditional
jobs.
Working dogs do many other jobs today.
Some sniff bags in search of explosives or
