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Y chromosome The nuclei of human cells contain 22
autosomes (any chromosome that is not a sex chromo-
some) and two sex chromosomes. In females, the sex
chromosomes are the two X chromosomes, and in males
there is one X chromosome and one Y chromosome.
The presence of the Y chromosome leads to a baby boy.

yeast Considered to be the oldest “plant” cultivated
by humans. Yeast is a unicellular fungus that belongs to
the family Saccharomycetaceae. It lives in the soil, on
plants, and is airborne, and it has been used in the pro-
duction of bread, beer, and wine because it drives the
process of fermentation. It produces carbon dioxide
and alcohol when in the presence of sugar.

yellow fever A tropical mosquito-borne viral hep-
atitis caused by an arbovirus (yellow fever virus) from
the Flaviviridae family. It is transmitted by the

mosquitoes Aedes aegypti and Haemagogus capri-

corni,among others.

yolk sac An extraembryonic tissue (membranous

sac) that is attached to an embryo and contains the

nutritive food yolk. In fish larvae, the alevin-stage

embryos swim with the yolk sac attached until it is

absorbed. In placental mammals, it is vestigial (con-

tains no nutrients). The primitive yolk sac divides into

two parts: one portion becomes the fetal gastrointesti-

nal tract, while the second portion connects to the fetal

body via the vitelline duct and is located in the fluid

space outside the amnion, the extraembryonic coelom.

It produces the embryo’s first blood cells and germ cells

that develop into gonads; is the source of the mucous

membrane of almost the entire intestinal tract; and

deals with the transfer of nutritive fluid to the embryo

from the trophoblast, primary mesenchyme, and

extraembryonic coelom.

Y