Sperm cells have either an X or a Y chromosome.
But an egg cell always has an X
chromosome. Therefore, when a sperm
unites with an egg, the new cell has
either one X and one Y chromosome or
two X chromosomes. A cell with one X
and one Y chromosome produces a boy.
A cell with two X chromosomes produces
a girl.
If a sperm with an X chromosome unites
with an egg cell, the result is a baby girl. If
a sperm carrying a Y chromosome unites
with an egg cell, the result is a baby boy.
BRITANNICA STUDENT ENCYCLOPEDIA Genetics 93
How Traits Are Inherited
A baby’s chromosomes carry a set of
genes from the father and another set
from the mother. These genes work
together to produce the baby’s traits.
Each gene has a special task—for
example, giving color to the baby’s eyes.
Some genes are dominant, or stronger
than others. A child may get a browneye
gene from the father and a blue-eye
gene from the mother. Because the
brown-eye gene is dominant, it takes
over the job of coloring the child’s eyes.
A child has blue eyes only if he or she
receives two blue-eye genes, one from
each parent.
Children of the same parents can end up
with different traits. This is because each
parent carries two genes for every task
but gives each child only one of these.
The father, for example, may have one
gene for brown eyes and one gene for
blue eyes. His first child may get the
brown-eye gene. His second child may
get the blue-eye gene.
There are two kinds of twins: identical
twins and fraternal twins. Identical twins
have exactly the same traits because they
have exactly the same genes. Identical
twins form from one egg that has united
with a sperm. This egg splits in half, and
each half grows into a separate baby.
Fraternal twins have different sets of
genes. This is because they form from
two separate eggs that united with two
different sperm.
Genetic Problems
Sometimes a gene does not work as it
should, or it works in a harmful way.
The result may be a defect in a part of
the body or a disease. A condition called
albinism is an example. It occurs when
the gene that controls the body’s production
of coloring substances does not
work properly. People with this condition
are born without color in their eyes,
hair, or skin. Muscular dystrophy is a
disease caused by a defective gene. The
disease causes the muscles to weaken
over time.
Every human has two genes to control traits
such as eye color. They get one gene from
each parent. This is how traits get passed
from one generation to the next.
Identical twins can be hard to tell apart.
94 Genetics BRITANNICA STUDENT ENCYCLOPEDIA
History of Genetics
In the 1850s and 1860s an Austrian
monk named Gregor Mendel studied
pea plants in his garden. He found that
there were rules for how traits passed
from one generation of pea plants to the
next. The rules are the same for every
plant and animal. During his lifetime no
one understood how important these
findings were.
In 1900 people rediscovered Mendel’s
work. From then on, the new science of
genetics grew rapidly. In 1953 James
Watson of the United States and Francis
Crick of England discovered the structure
of DNA. Their studies helped scientists
understand how genes work and
how they make copies of themselves.
By the mid-1970s, scientists had learned
how to locate, remove, and insert specific
genes in DNA. This work is called
genetic engineering. By the 1990s scientists
could clone animals, or produce
animals that have exactly the same DNA
as another animal. In 1996 researchers
in Scotland produced the first clone of
an adult mammal—a sheep. Some scientists
worked toward cloning human
beings. But others saw this work as dangerous
and wrong.
In 2003 a team of researchers finished a
project to identify and locate all the
genes in all human DNA. The results
will help scientists in the study of
human biology and medicine.
#More to explore
Chromosome • DNA • Evolution
Genghis Khan
Genghis Khan was a conqueror who
brought together the wandering peoples
of central Asia more than 800 years ago.
He made their lands into the state of
Mongolia. Then he took his armies
beyond Mongolia. They built up a huge
empire. They also killed many people
and destroyed many cities.
Genghis Khan was born in about 1162
in northern Mongolia. His name at first
was Temujin. At that time Mongolia was
home to a number of related, but separate,
peoples called Mongols. Temujin’s
father was Yesugei, the chief of one
Mongol group. When Temujin was 9
years old, his father was killed by
enemies from another Mongol group,
the Tatars. Temujin then became chief.
Though he was young, Temujin won
many followers.
Genghis Khan
BRITANNICA STUDENT ENCYCLOPEDIA Genghis Khan 95
Temujin gradually built up a large army.
He used it to conquer neighboring
groups, including theTatars.He unified
those groups into a singleMongol nation.
In 1206 the people named him Genghis
Khan, which means “universal ruler.”
Genghis Khan then set out to conquer
other lands. First he invaded northern
China. In less than 10 years he and his
generals took over most of China. Genghis
Khan next conquered the land that
now makes up Turkmenistan and
