host the larger and more independent member in a
symbiotic relationship. (14.3)
Human Genome Project (HGP) joint effort of
thousands of researchers from laboratories
worldwide that determined the sequence of the
three billion base pairs that make up the human
genome. A complete draft of the human genome
was first published in February 2001. Among the
HGP’s immediate findings was that the DNA of
all humans is more than 99.9 percent identical.
(9.2)
human growth hormone (HGH) small protein
molecule that spurs body growth by increasing
intestinal absorption of calcium, increasing cell
division and development, and stimulating
protein synthesis and lipid metabolism. The
protein’s action prompts the body to use lipids
for energy instead of proteins and carbohydrates.
(6.1)
hydrogen bonds weak attractions between polar
molecules that contain hydrogen atoms bonded
to the more electronegative atoms oxygen,
nitrogen, or fluorine. (1.1)
hydrolysis type of reaction in a molecule is split,
or lysed, by the addition of water. (1.3)
hydrolytic enzymes enzymes that catalyze the
addition of water in reactions. (2.2)
hydrophilic “water-loving”; describes molecules
that interact with water. (1.1)
hydrophobic “water-hating”; describes molecules
that do not interact with water. (1.1)
hydrothermal vent fissures in Earth’s crust that
release hot water and gases. (2.2)
hyperopia far-sightedness or difficulty seeing things
that are nearby. The condition is caused by a
too-short eyeball or weak ciliary muscles. (5.3)
hyperthermia heat exhaustion and unusually high
body temperature. The condition is caused by the
body’s inability to release enough excess heat by
vasodilation; can be fatal. (4.1)
hyperthyroidism (Grave’s disease) an autoimmune
disorder caused by an excess of thyroxine.
Antibodies attach to TSH receptors on thyroid
cells, causing the thyroid gland to produce too
much thyroxine. Typically, the production of
excess thyroxine results in increased metabolism,
excessive heat production, dilation of blood
vessels, and weight loss despite increased
appetite. (6.1)
hypothalamus the part of the brain that acts as the
main control centre for the autonomic nervous
system, re-establishes homeostasis, and controls
the endocrine hormone system. (5.1)
hypothermia condition in which the body no longer
has the energy to shiver, extremities freeze, and
blood flow to the brain slows, resulting inimpaired judgment, sleepiness, eventual loss of
consciousness, and even death. (4.1)
hypothyroidism deficiency in thyroxine (caused by
iodine deficiency or an autoimmune disease)
causes continual production of TSH and cell
division in thyroid tissue. Typically, this
deficiency results in reduced metabolism,
reduced tolerance of cold, decreased heart rate,
and weight gain despite decreased appetite. The
condition is also known as myxedema. (6.1)immigration movement of individuals into a region
from elsewhere; a factor that increases a
population. (14.2)
immunity the ability to resist a disease after having
been exposed to it in the past. (4.4)
inbreeding mating between closely related partners.
This type of mating results in increased numbers
of homozygous members of the population. (11.3)
incomplete dominance blending of the traits of two
different alleles at one locus that occurs when
neither allele is dominant. (11.1)
induced in mutations, describes a permanent
change in the genetic material of a cell caused by
a mutagen outside the cell. (9.1)
induced fit change in shape of the active site of an
enzyme to accommodate the substrate and to
facilitate a chemical reaction. (2.2)
inducer substance, such as a lactose molecule, that
stops the action of a repressor. RNA polymerase
can then bind to the promoter sequence and
begin transcription. (8.4)
Industrial Revolution period of transition in
lifestyle during the eighteenth century during
which people shifted from working in traditional
agricultural jobs and making goods by hand to
working in factories and making mass-produced
goods. (15.1)
inheritance of acquired characteristics Lamarck’s
theory that characteristics acquired during an
organism’s lifetime could be passed to its
offspring. (10.2)
inhibin hormone that acts on the hypothalamus to
stimulate the production of the releasing factors
that trigger release of FSH. The interaction of
FSH and inhibin controls the rate of formation of
sperm. (6.3)
inhibitors chemicals that bind to specific enzymes.
(2.2)
inhibitory response process during which an
inhibitory neurotransmitter is sent through the
synapse from the presynaptic neuron to the
postsynaptic neuron. This transmission raises the
threshold of the all-or-none response for that
postsynaptic neuron, making it less likely thatI
578 MHR • Glossary