326 ❯ Glossary
Klinefelter syndrome (XXY) Syndrome in which
individuals have male sex organs but are sterile and
display several feminine body characteristics.
Krebs cycle Energy-producing reaction that occurs
in the matrix of the mitochondria, in which pyru-
vate is broken down completely to H 2 O and CO 2
to produce 3 NADH, 1 FADH 2 , and 1 ATP.
K-selected populationsPopulations of a roughly
constant size whose members have low reproduc-
tive rates. The offspring produced by K-selected
organisms require extensive postnatal care.
lac operon Operon that aids in control of transcrip-
tion of lactose metabolizing genes.
lactic acid fermentationOccurs in human muscle
cells when oxygen is unavailable. Pyruvate is
directly reduced to lactate by NADH to regenerate
the NAD+needed for the resumption of glycolysis.
lagging strand The discontinuous strand produced
during DNA replication.
larynx Passageway from the pharynx to the trachea.
Commonly called the “voicebox.”
lateral meristems Cells that extend all the way
through the plant from roots to shoots and provide
the secondary growth that increases the girth of a
plant.
lateral rootsRoots that serve to hold a plant in place
in the soil.
law of dominance When two opposite pure-breeding
varieties (homozygous dominant vs. homozygous
recessive) of an organism are crossed, all the off-
spring resemble one parent. This is referred to as
the “dominant” trait. The variety that is hidden is
referred to as the “recessive” trait.
law of independent assortment Members of each
pair of factors are distributed independently when
the gametes are formed. In other words, inheri-
tance of one particular trait or characteristic does
not interfere with inheritance of another trait (in
unlinked genes). For example, if an individual is
BbRr for two genes, gametes formed during meio-
sis could contain BR, Br, bR, or br. The B and b
alleles assort independentlyof the R and r alleles.
law of multiplication Law that states that to deter-
mine the probability that two random events will
occur in succession, you simply multiply the prob-
ability of the first event by the probability of the
second event.
law of segregation Every organism carries pairs of
factors, called alleles,for each trait, and the members
of the pair segregate out (separate) during the for-
mation of gametes. For example, if an individual is
Bb for eye color, during gamete formation one
gamete would receive a B and the other made from
that cell would receive a b.
leading strand The continuous strand produced
during DNA replication.
LH Seeluteinizing hormone.
LH surge Giant release of LH that triggers
ovulation—the release of a secondary oocyte
from the ovary.
lichen A symbiotic collection of organisms (fungus
and algae) living as one.
life cycleSequence of events that make up the repro-
ductive cycle of an organism.
limiting factorsEnvironmental factors that keep
population sizes in check (predators, diseases, food
supplies, and waste).
linkage mapA genetic map put together using
crossover frequencies.
linked genesGenes along the same chromosome
that tend to be inherited together because the
chromosome is passed along as a unit.
lipaseThe major fat-digesting enzyme of the human
body.
lipidsHydrophobic organic compounds used by
cells as energy stores or building blocks. Three
important lipids are fats, steroids, and phospho-
lipids.
logistic growth A population grows at a rate that
creates an S-shaped curve.
long day plantsPlants, such as spinach, which flower
if exposed to a night that is shorter than a critical
period.
luteinizing hormone (LH)A gonadotropin that stimu-
lates ovulation and formation of a corpus luteum, as
well as the synthesis of estrogen and progesterone.
lymphatic systemImportant part of the circulatory
system that functions as the route by which pro-
teins and fluids that have leaked out of the blood-
stream can return to circulation. The lymphatic
system also functions as a protector for the body
because of the presence of lymph nodes.
lymph nodes Structures found in the lymphatic
system that are full of white blood cells, which live to
fight infection. These nodes often swell up during
infection as a sign of the body’s fight against the
infectious agent.