930
UNIT SIX
f
rst studies to detect it were perFormed on inmates at a high-
security mental Facility. However, 96% oF men with XYY
syndrome share only great height, acne, and speech and
reading problems. Perhaps teachers, employers, parents,
and others may expect more oF these physically large boys
and men than oF their peers, and a small percentage oF them
cope with this stress by becoming aggressive.
A Fertilized ovum that has one Y chromosome and no X
chromosome has never been observed. Apparently, when a
zygote lacks an X chromosome, so much genetic material is
missing that only a Few, iF any, cell divisions are possible.
Monosomic
Monosomic
Trisomic
Trisomic
Euploid
Euploid
Monosomic
Trisomic
Zygotes
Fertilization of
euploid egg
Sperm
Meiosis II
AA
aa
AA
aa
AA
aa
Meiosis I
First division
nondisjunction
Primary spermatocyte
(a)
(b)
Secondary
spermatocyte
Second
division
nondisjunction
AA
aa
A
a
A
a
a
a
AA
FIGURE 24.12
Extra or missing chromosomes constitute aneuploidy. Unequal division of chromosome pairs into sperm and egg cells can
occur at either the F
rst or the second meiotic division. (
a
) A single pair of chromosomes is unevenly partitioned into the two cells arising from the
F
rst division of meiosis in a male. The result: two sperm cells that have two copies of the chromosome and two sperm cells that have no copies
of that chromosome. When a sperm cell with two copies of the chromosome fertilizes a normal egg cell, the zygote produced is trisomic for that
chromosome; when a sperm cell lacking the chromosome fertilizes a normal egg cell, the zygote is monosomic for that chromosome. Symptoms
depend upon which chromosome is involved. (
b
) This nondisjunction occurs at the second meiotic division. The two products of the F
rst division are
una±
ected, so two of the mature sperm are normal, and two are aneuploid. Egg cells can undergo nondisjunction as well, leading to zygotes with
extra or missing chromosomes when they are fertilized by normal sperm cells.
TABLE
24.1
|
Comparing and Contrasting
Trisomies 13, 18, and 21
Type of Trisomy
Incidence at Birth
Percent of Conceptions That
Survive 1 Year After Birth
13 (Patau)
1/12,500–1/21,700
<5%
18 (Edward)
1/6,000–1/10,000
<5%
21 (Down)
1/800–1/826
85%
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