110 ❯ STEP 4. Review the Knowledge You Need to Score HighDown syndromeis a classic aneuploid example, affecting one out of every 700 chil-
dren born in this country. It most often involves a trisomy of chromosome 21, and leads
to mental retardation, heart defects, short stature, and characteristic facial features. Most
people with trisomy 21 are sterile.
Trisomy 21 is not the only form of nondisjunction caused by error in the chromosome
separation process. Trisomy 13, also known as Patau syndrome,causes serious brain and
circulatory defects. Trisomy 18, also known as Edwards syndrome,can affect all organs.
It is rare for a baby to survive for more than a year with either of these two conditions.
There are also syndromes involving aneuploidy of the sex chromosomes. Males can receive
an extra Y chromosome (XYY). Although this nondisjunction does not seem to produce a
major syndrome, XYY males tend to be taller than average, and some geneticists believe
they display a higher degree of aggressive behavior. A male can receive an extra X chromo-
some, as in Klinefelter syndrome(XXY). These infertile individuals have male sex organs
but show several feminine body characteristics. Nondisjunction occurs in females as well.
Females who are XXX have no real syndrome. Females who are missing an X chromosome
(XO) have a condition called Turner syndrome.XO individuals are sterile females who
possess sex organs that fail to mature at puberty.
Trisomies are not the only kind of chromosomal abnormalities that lead to inherited
diseases. A deletionoccurs when a piece of the chromosome is lost in the developmental
process. Deletions, such as cri-du-chat syndrome,can lead to problems. This syndrome
occurs with a deletion in chromosome 5 that leads to mental retardation, abnormal facial
features, and a small head. Most affected individuals die very young.
Chromosomal translocations,in which a piece of one chromosome is attached to
another, nonhomologous chromosome, can cause major problems. Chronic myelogenous
leukemiais a cancer affecting white blood cell precursor cells. In this disease, a portion of
chromosome 22 has been swapped with a piece of chromosome 9.
Achromosome inversionoccurs when a portion of a chromosome separates and reattaches
in the opposite direction. This can have no effect at all, or it can render a gene nonfunctional if
it occurs in the middle of a sequence. A chromosome duplicationresults in the repetition of a
genetic segment. A chromosome duplicationresults in the repetition of a genetic segment...
whoops... sorry.... Duplications often have serious effects on an organism.
These are the major concepts of heredity with which the AP Biology exam writers
would like you to be familiar. Try the practice problems that follow and be sure you are
able to construct, read, and analyze both Punnett squares and pedigrees, keeping in mind
the laws of probability.❯ Review Questions
- The following crossover frequencies were noted
via experimentation for a set of five genes on a
single chromosome:
A and B →35%
B and C →15%
A and C →20%
A and D →10%
D and B →25%
A and E →5%
B and E →40%
BIG IDEA 3.A.3
Certain genetic dis-
orders can provide
understanding of
how genes are
passed from parent
to offspring.