316 CHAPTER 8
Sometimes the chromosome itself is the problem. Although each egg and each sperm
are only supposed to have 23 chromosomes, in the creation of these cells a chromosome
can end up in the wrong cell, leaving one cell with only 22 and the other with 24. If either
of these cells survives to “mate,” the missing or extra chromosome can cause mild to
severe problems in development (American Academy of Pediatrics, 1995; Barnes & Carey,
2002; Centers for Disease Control and Prevention, 2009c; Gardner & Sutherland, 1996).
Examples of chromosome disorders include Down syndrome, a disorder in which
there is an extra chromosome in what would normally be the 21st pair. Symptoms com-
monly include the physical characteristics of almond-shaped, wide-set eyes, intellec-
tual disability, and the increased risk of organ failure later in life (Barnes & Carey, 2002;
Hernandez & Fisher, 1996: Patel et al., 2015). Other chromosome disorders occur when
there is an extra sex chromosome in the 23rd pair, such as Klinefelter’s syndrome, in which
the 23rd set of sex chromosomes is XXY, with the extra X producing a male with reduced
masculine characteristics, enlarged breasts, obesity, and excessive height (Bock, 1993;
Frühmesser & Kotzot, 2011); and Tu r n e r ’s s y n d r o m e, in which the 23rd pair is actually
missing an X, so that the result is a lone X chromosome (Ranke & Saenger, 2001). These
females tend to be very short, infertile, and sexually underdeveloped (American Acad-
emy of Pediatrics, 1995; Cramer et al., 2014; Hong et al., 2009; Rovet, 1993).
Down syndrome is a form of
intellectual disability caused by an
extra chromosome 21.
Figure 8.2 Dominant and Recessive Genes and PKU
This figure shows the variation of one or two parents carrying recessive genes and the result of this in their
offspring. (a) If only one parent carries the PKU gene, their children might be carriers but will not have PKU. (b)
Only if both parents are carriers of PKU, will a child have the 1-in-4 possibility of having PKU.
Carries recessive
PKU gene
Does not carry
recessive
PKU gene Normal Normal Carrier Carrier
Mother Father
Mother
contributes
either P or p
Father
contributes
either P or P
Result: No child will be afflicted with PKU,
although 2 in 4 will carry the recessive
a. gene
PP
PP
PP pP pP
Carries recessive
PKU gene
Carries recessive
PKU gene
Normal Carrier Carrier Afflicted
with PKU
Mother Father
Mother
contributes
either P or p
Father
contributes
either P or p
Result: 1-in-4 children will inherit two
dominant genes and will not have PKU;
2 in 4 will inherit one recessive gene and,
will not be afflicted with PKU but will carry
the recessive gene; and 1 in 4 will have
b. PKU
PP Pp
Pp
Pp
Pp
pP pp
Interactive
