Essentials of Anatomy and Physiology

Mitosis

Stem cell

Oogonium New stem cell

Figure 20–2. Oogenesis. The processes of mitosis and meiosis are shown. For each

primary oocyte that undergoes meiosis, only one functional ovum is formed.

QUESTION:When does the secondary oocyte begin the second meiotic division?

BOX20–1 TRISOMY AND DOWN SYNDROME

every 5000 live births. Both of these trisomies are

characterized by severe mental and physical retar-

dation, heart defects, deafness, and bone abnor-

malities. Affected infants usually die within their first

year.

Down syndrome (Trisomy 21) is the most

common trisomy, with a frequency of about one

per 750 live births. Children with Down syndrome

are mentally retarded, but there is a great range of

mental ability in this group. Physical characteristics

include a skin fold above each eye, short stature,

poor muscle tone, and heart defects. Again, the

degree of severity is highly variable.

Women over the age of 35 are believed to be at

greater risk of having a child with Down syndrome.

The reason may be that as egg cells age the process

of meiosis is more likely to proceed incorrectly.

Trisomymeans the presence of three (rather than

the normal two) of a particular chromosome in the

cells of an individual. This may occur because of

non-disjunction (nonseparation) of a chromosome

pair during the second meiotic division, usually in

an egg cell. The egg cell has two of a particular

chromosome, and if fertilized by a sperm, will then

contain three of that chromosome, and a total of

47 chromosomes.

Most trisomies are probably lethal; that is, the

affected embryo will quickly die, even before the

woman realizes she is pregnant. When an embryo-

fetus survives and a child is born with a trisomy,

developmental defects are always present.

The severity of trisomies may be seen in two of

the more rarely occurring ones: Trisomy 13 and

Trisomy 18, each of which occurs about once for

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