genotype. The links between genes and particular
normal and abnormal phenotypes are being identi-
fied using results of the Human Genome Project.
Identification of these links is improving child health
by providing genetic counselors with detailed infor-
mation on the causes, phenotypic variability, and
environmental factors that lead to birth defects and
other diseases. Nongenetic factors influencing a phe-
notype that results from a particular genotype can be
used by genetic counselors to suggest environmental
or lifestyle changes that can lessen disease severity.
See also: GENETIC COUNSELING; GENOTYPE; HUMAN
GENOME PROJECT
Bibliography
Oak Ridge National Laboratory. ‘‘The Science behind the Human
Genome Project.’’ Available from http://www.ornl.gov/hgmis/
project/info.html; INTERNET.
David W. Threadgill
Robert E. Boykin
PHENYLKETONURIA
Phenylketonuria (PKU) is due to a deficiency of phe-
nylalanine hydroxylase, the enzyme that converts
phenylalanine to tyrosine. High levels of phenylala-
nine accumulate in the body resulting in severe men-
tal retardation, seizures, and a musty odor. PKU is an
autosomal recessive genetic disorder. The incidence
is 1 in 10,000 to 1 in 12,000 live births. PKU is detect-
ed on newborn screens, which are most sensitive when
performed between forty-eight and seventy-two hours
of life, and after the infant has been fed a diet contain-
ing protein. Phenylalanine levels are easily detected
by a few drops of blood, and early treatment prevents
complications making it a model disease for screen-
ing. Treatment includes a diet low in phenylalanine
and frequent monitoring of blood levels. Dietary
therapy should begin immediately in the newborn pe-
riod. The duration of therapy is controversial. It is
recommended that children maintain a very restric-
tive diet through the age of six, and they are encour-
aged to maintain some restriction throughout life.
See also: BIRTH DEFECTS; NUTRITION
Bibliography
Berry, Gerard. ‘‘Inborn Errors of Carbohydrate Ammonia, Amino
Acid, and Organic Acid Metabolism.’’ In H. William Taeusch
and Roberta Ballard eds., Avery’s Diseases of the Newborn. Phila-
delphia: W. B. Saunders, 1998.
Goodman, Stephen, and Carol Greene. ‘‘Metabolic Disorders of
the Newborn.’’ Pediatrics in Review 15 (1994):359–365.
Rezvani, Iraj. ‘‘Defects in Metabolism of Amino Acids.’’ In Richard
E. Behrman, Robert M. Kliegman, and Ann M. Arvin eds.,
Nelson Textbook of Pediatrics. Philadelphia: W. B. Saunders
Company, 1996.
Meica M. Efird
PHYSICAL GROWTH
Physical growth usually refers to changes in size or
mass; so it is correct to say that a child grows in stature
(height) or body weight. Even though most people
usually think of growth at the level of the whole child,
the cells and internal structures that make up the
child also grow, primarily by increasing in number or
size. Consequently, auxologists (those who study child
growth) may be interested in the growth of bones to
help understand fractures and osteoporosis; the
growth of the heart walls to help understand hyper-
tension (high blood pressure) and heart disease; or
the growth of adipose tissue (body fat) to help under-
stand obesity.
The measurement of body dimensions such as
those used in growth studies is called anthropometry.
Past growth is usually measured as the size attained
at a chronological age, for example the weight of a
child at eight years old. Assessment of the rate of
growth requires that a body dimension, such as
weight, be measured twice over a period, and then the
change is expressed in terms of the increment or ve-
locity of growth, for example in pounds or kilograms
per year.
Some physical changes in childhood are more
complicated than just size or mass. These changes in-
clude alterations in body structures and functions and
can be termed development. Physical developmental
changes are as diverse as the closing of the fontanels
(soft spots) in a baby’s skull, the erupting of teeth,
learning to walk, or the deepening of the voice of boys
during adolescence.
Some developmental changes are considered
maturational, or indicators of physical maturity. Mat-
uration is the progression of developmental changes
toward the characteristics of adults. Physical matura-
tion occurs from the time of conception, but some of
the most commonly recognized indicators of matura-
tion become apparent during adolescence. Changes
in body shape, breast development in girls, pubic hair
development in both genders, and development of fa-
cial hair in boys are visible indicators of maturation
toward adult appearance of the body, and they signal
adult reproductive functioning. The cessation of the
growth of long bones, associated with the final attain-
ment of adult stature is also a maturational event.
Although growth and maturation are certainly re-
lated, distinguishing between them is important be-
cause some physiological and hormonal processes
affect growth and maturation differentially, as do
some diseases. It is easy to observe that children of the
same size can differ in maturational status and that
fully mature individuals (adults) can be of different
sizes.306 PHENYLKETONURIA