88 ❯ STEP 4. Review the Knowledge You Need to Score High
their biological parents, but not their adoptive families, with respect to a given trait,
researchers infer a genetic component for that trait. Such constellations of behaviors as
alcoholism, schizophrenia, and general intelligence have shown both genetic and environ-
mental components.Transmission of hereditary Characteristics
Transmission of hereditary characteristics is achieved by biological processes, including for-
mation of sex cells, fertilization, embryonic development, and protein synthesis. Each DNA
segment of a chromosome that determines a trait is a gene. Chromosomes carry information
stored in genes to new cells during reproduction. Normal human body cells have 46 chromo-
somes, except for eggs and sperms that have 23 chromosomes. Males have 44 chromosomes,
plus X and Y. Females have 44 chromosomes, plus X and X. At fertilization, 23 chromosomes
from the sperm unite with 23 chromosomes from the egg to form a zygote with 46 chromo-
somes. If the male contributes an X chromosome, the baby is female; if the male contributes
a Y chromosome, the baby is male. The presence of a Y chromosome makes the baby a male.
All of the cells of the embryo/baby have the same 23 pairs of chromosomes, which carry genes
for the same traits. Fertilization that includes a sperm or egg with the wrong number of chro-
mosomes results in a zygote, and subsequently an individual, with chromosomal abnormalities.
Females with Turner syndrome have only one X sex chromosome (XO). Girls with Turner
syndrome are typically short with a webbed neck, lack ovaries, and fail to develop secondary
sex characteristics at puberty. Although usually of normal intelligence, they typically evidence
specific cognitive deficits in arithmetic, spatial organization, and visual form perception. Males
with Klinefelter’s syndrome arise from an XXY zygote. The syndrome becomes evident at
puberty when male secondary sex characteristics fail to develop, but breast tissue does. Males
with Klinefelter’s tend to be passive. The presence of three copies of chromosome-21 results
in the expression of Down syndrome. Individuals with Down syndrome typically have intel-
lectual disability and have a round head, a flat nasal bridge, a protruding tongue, small round
ears, a fold in the eyelid, and poor muscle tone and coordination.
The genetic makeup for a trait of an individual is called its genotype. The expres-
sion of the genes is called its phenotype. For traits determined by one pair of genes, if
they are the same (homozygous), the individual expresses that phenotypic characteristic.
If the genes are different, the expressed gene is called the dominant gene; the hidden
gene is the recessive gene. Numerous recessive genes are responsible for syndromes in the
homozygous condition. Tay-Sachs syndrome produces progressive loss of nervous func-
tion and death in a baby. Albinism arises from a failure to synthesize or store pigment
and also involves abnormal nerve pathways to the brain, resulting in quivering eyes and
the inability to perceive depth or three-dimensionality with both eyes. Phenylketonuria
(PKU) results in severe, irreversible brain damage unless the baby is fed a special diet
low in phenylalanine within 30 days of birth; the infant lacks an enzyme to process this
amino acid which can build up and poison cells of the nervous system. Thus, heredity
and environment interact to determine a trait. Huntington’s disease is an example of
a dominant gene defect that involves degeneration of the nervous system. Progressive
symptoms involve forgetfulness, tremors, jerky motions, loss of the ability to talk, per-
sonality changes such as temper tantrums or inappropriate accusations, blindness, and
death. Recessive genes for color blindness are located on the X chromosome with no
corresponding gene on the Y chromosome. As a result, males show sex-linked traits like
color blindness much more frequently than females. Behaviors and diseases may have
variations only some of which are genetically based. A form of familial Alzheimer’s dis-
ease has been attributed to a gene on chromosome 21, but not all cases of Alzheimer’s
disease are associated with that gene.