there are two X chromosomes. In males, there is one Y
and one X. It is the presence of the Y chromosome that
leads to the development into a male.
sex-linked gene Agene located on one of the sex
chromosomes. Some genetic diseases are located on a
sex gene. For example, hemophilia A, which is a blood-
clotting disorder caused by a mutant gene, is located on
the X chromosome. Genes located on the X chromo-
some are called X-linked, while genes located on the Y
chromosome are called Y-linked.
sexual dimorphism The existence of noticeable
physical differences between males and females.
sexual reproduction The creation of a new individ-
ual following the union of two gametes (e.g., egg and
sperm). In humans and most other eukaryotes, the two
gametes differ in structure (anisogamy) and are con-
tributed by different parents.
sexual selection Sexual selection based on sec-
ondary sex characteristics and on the success of
certain individuals over others in relation to the propa-
gation of the species and the enhancement of sexual
dimorphism.
Sherrington, Charles Scott (1857–1952) English
Neurophysiologist Charles Scott Sherrington was
born on November 27, 1857, in Islington, London, to
James Norton Sherrington and Anne Brookes. He
attended Ipswich Grammar School from 1870 to 1875
and in 1876 began medical studies at St. Thomas’s
Hospital. In 1879 he went to Cambridge, where he
studied physiology under the “father of British physiol-
ogy,” Sir Michael Foster (1836–1907). He was profes-
sor of physiology at the Universities of Liverpool and
London and at Oxford.
From 1891 to 1897 he conducted research on the
nervous system, the efferent nerve (part of the peripher-
al nervous system, not in the brain or spinal cord) sup-
ply of muscles, and discovered that about one-third of
the nerve fibers in a nerve supplying a muscle are effer-
ent, the remainder being motor. He explained proprio-
ception, the function of the nerve synapse, a word he
created. He received the 1932 Nobel Prize in physiolo-
gyor medicine with E. D. ADRIANfor their discoveries
regarding the function of the neuron.
In 1906 he published his well-known book The
Integrative Action of the Nervous System.His other
books include Mammalian Physiology: A Course of
Practical Exercises,1919; The Brain and Its Mecha-
nism,1933; and Man on His Nature,1940.
In 1913 he became the Waynfleet professor of
physiology at Oxford, a post he held until his retire-
ment in 1936. He died suddenly of heart failure at
Eastbourne in 1952.shock A life-threatening condition where blood
pressure is too low to sustain life. Occurs when a low
blood volume (due to severe bleeding, excessive fluid
loss, or inadequate fluid uptake), inadequate pumping
action of the heart, or excessive dilation of the blood
vessel walls (vasodilation) causes low blood pressure.
This in turn results in inadequate blood supply to
body cells, which can quickly die or be irreversibly
damaged.
Anaphylactic shock is the severest formof allergy,
which is a medical emergency; a Type I reaction accord-
ing to the Gell and Coombs classification. An often
severeand sometimes fatal systemic reaction in a sus-
ceptible individual upon exposure to a specific antigen
(such as wasp venom or penicillin) following previous
sensitization. Characterized especially by respiratory
symptoms, fainting, itching, urticaria, swelling of the
throat or other mucous membranes, and a sudden
decline in blood pressure.shoot system The aboveground system of stems,
leaves, and flowers.short-day plant A plant that flowers when the days
areshort; requires less than 12 hours of daylight to
bloom or long periods of darkness before it can bloom,
e.g., poinsettia.
See alsoPHOTOPERIODISM.306 sex-linked gene