Understanding Psychology (10th Ed)

 GENETIC INFLUENCES ON THE FETUS 
The process of fetal growth that we have just described refl ects normal development, 
which occurs in 95–98% of all pregnancies. Some individuals are less fortunate; in 
the remaining 2–5% of cases, children are born with serious birth defects. A major 
Study Alert
Critical (or sensitive) periods, 
which can occur before or 
after birth, are important 
because they indicate the 
time that organisms are 
particularly susceptible to 
damage that may aff ect them for 
the rest of their lives.
These remarkable photos of live fetuses display the degree of physical development at 
prenatal ages 4 and 15 weeks.
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390 Chapter 
12 
Development
cause of such defects is faulty genes or chromosomes. Here are some of the more 
common genetic and chromosomal diffi culties.
• Phenylketonuria (PKU) . A child born with the inherited disease phenylketonuria 
cannot produce an enzyme that is required for normal development. This 
defi ciency results in an accumulation of poisons that eventually cause 
profound mental retardation. The disease is treatable, however, if it is caught 
early. Most infants today are routinely tested for PKU, and children with the 
disorder can be placed on a special diet that allows them to develop normally 
(Ievers-Landis et al., 2005; Christ, Steiner, & Grange, 2006; Widaman, 2009).
• Sickle-cell anemia . About 10% of the African-American population has the 
possibility of passing on sickle-cell anemia, a disease that gets its name from 
the abnormally shaped red blood cells it causes. Children with the disease 
may have episodes of pain, yellowish eyes, stunted growth, and vision 
problems (Taras & Potts-Datema, 2005; Selove, 2007).
• Tay-Sachs disease . Children born with Tay-Sachs disease, a disorder most often 
found in Jews of Eastern European ancestry, usually die by age 3 or 4 because 
of the body’s inability to break down fat. If both parents carry the genetic 
defect that produces the fatal illness, their child has a 1 in 4 chance of being 
born with the disease (Leib et al., 2005; Weinstein, 2007).
• Down syndrome . Down syndrome, one of the causes of mental retardation, 
occurs when the zygote receives an extra chromosome at the moment of 
conception. Down syndrome is often related to the mother’s age; mothers over 
35 and younger than 18 stand a higher risk than other women of having a 
child with the syndrome (Roizen & Patterson, 2003; Sherman et al., 2007).