One tool for studying human genetics is population sampling,
in which research select a small number of individual that represent the
whole population. Geneticists also study identical twins to distinguish
between genetic and environmental influences on specific traits. Geneticists
may also trace a pedigree, a family record that shows how a trait is inherited
over several generations. A pedigree often reveals a carrier, someone who
is heterozygous for a trait. Traits controlled by a single allele of a
gene are called dominant-allele traits. Sickle-cell disease, often called
sickle-cell anemia, is a single-allele disorder that operates in a codominant
system. Huntington disease, or HD, is a single-allele trait that is caused
by a dominant allele. The first symptoms of HD - mild forgetfulness and
irritability - appear when the victim are in their thirties or forties.
Recently, however, geneticists discover a genetic marker - a short section
of DNA that indicates the presence of an allele that codes for a trait
- for the HD allele. In humans, characteristics are sometimes controlled
by a signal gene, but frequently a characteristic may be controlled by
several genes. A trait that is controlled by two or more genes is called
a polygenic trait. Some human traits are controlled by multiple alleles,
three or more alleles of the same gene that code for a single trait. An
antigen is a substance that causes the body to produce an antibody. Certain
human traits are sex linked; that is, the alleles for these traits appear
only on the X chromosome. 
A recessive sex-linked trait that is found primarily
in males is hemophilia. The presence of male or female sex hormones influences
the expression of certain human traits, called sex-influenced traits. Some
genetic disorders result from nondisjunction. A zygote with 45 chromosomes
has only one of a particular chromosome, a condition called monosomy. A
zygote with 47 chromosomes has three of a particular chromosome, a condition
called trisomy. An extra chromosome 21, for example, results in Down syndrome,
a disorder characterized by mental retardation, a fold of skin above the
eyes, and weak muscles. Genetic screening is an examination of a person's
genetic makeup. It may involve making a karotype, which is a picture of
an individual's chromosomes. Scientists can then spot chromosomal abnormalities.
Couples at genetic risk may wish to undergo genetic counseling, a type
of counseling that informs them about problems that could affect their
offspring. In amniocentesis the physician uses a needle and syringe to
remove a small amount of amniotic fluid from the amniotic, the sac that
surrounds the fetus. In fetoscopy the physician inserts a tiny camera through
ha small incision in the uterus. In chorion villi sampling the physician
analyzes a sample of the chorion villi, which grow between the mother's
uterus and the placenta. Physicains may also analyze fetal development
by using ultrasound, a technique in which high-frequency sound waves bounce
off the fetus, forming an image.
