Bma Illustrated Medical Dictionary

and is absent in every female body cell.

Each sperm carries either a single X or a

single Y chromosome.

Unlike the X chromosome, the Y chro-

mosome carries little genetic material.

Its major function is to stimulate the

development of the testesin the embryo.

There are no significant diseases related

to abnormalities of the Y chromosome,

but hairy ears is a trait thought to be

determined by a Y-linked gene.

yeastsTypes of fungiin which the body

of the fungus comprises individual cells

that occur either singly, in pairs, or in

longer chains. Certain yeasts can cause

infections of the skin or mucous mem-

branes; the most important of these

disease-causing yeasts is CANDIDA ALBI-

CANS, which causes candidiasis.

yellow feverAn infectious disease of

short duration and variable severity that

is caused by a virus transmitted by mos-

quitoes. In severe cases, the skin yellows

due to jaundice, from which the name

yellow fever derives. The infection may

be spread from monkeys to humans in

forest areas through various species of

mosquito; and in urban areas it can be

transmitted between humans by AEDES

AEGYPTImosquitoes.

Today, yellow fever is contracted only in

Central America, parts of South America,

and a large area of Africa. Eradication of

the causative mosquito from populated

areas has greatly reduced its incidence.

Yellow fever is characterised by a sud-

den onset of fever and headache, often

with nausea and nosebleeds and, despite

the high fever, a very low heart-rate. In

YEASTS

yawningAn involuntary act, or reflex

action, usually associated with drowsi-

ness or boredom. The mouth is opened

wide and a slow, deep breath is taken

through it in order to draw air into the

lungs. The air is then slowly released.

Yawning is accompanied by a momen-

tary increase in the heart-rate, and, in

many cases, watering of the eyes.

The purpose of yawning is unknown,

but one theory suggests it is triggered

by raised levels of carbon dioxide in the

blood; thus, its purpose could be to

reduce the level of carbon dioxide and

increase that of oxygen in the blood.

yawsAn infectious disease that tends

to be found throughout poorer subtrop-

ical and tropical areas of the world.

Yaws is caused by a spirochaete (a spi-

ral-shaped bacterium), and it spreads

principally in conditions of poor hygiene.

The infection is almost always acquired

in childhood, and it mainly affects the

skin and bones.

The bacteria enter the body through

abrasions in the skin. Three or 4 weeks

after infection, an itchy, raspberry-like

growth appears at the site of infection,

sometimes preceded by fever and pains.

Scratching spreads the infection and

causes more growths to develop else-

where on the skin. Without treatment,

the growths heal slowly over the course

of about 6 months, but recurrence is

common. In about 10 per cent of un-

treated cases, widespread tissue loss

eventually occurs. This may eventually

lead to gross destruction of the skin,

bones, and joints of the legs, nose,

palate and upper jaw.

Yaws can be cured by a single large

dose of a penicillin druggiven as an

injection into muscle.

Y chromosomeA sex chromosome that

is present in every normal male body

cell. It is paired with an X chromosome

YAWNING YELLOW FEVER

Y

Y

Cytoplasm

Nucleus

Mitochondrion

Cell wall

YEAST CELL