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Biological Bases of Behavior ❮ 75specific cognitive deficits in arithmetic, spatial organization, and visual form perception.
Klinefelter’s syndrome males arise from an XXY zygote. The syndrome becomes
evident at puberty when male secondary sex characteristics fail to develop, but breast
tissue does. Klinefelter’s males tend to be passive. The presence of three copies of
chromosome-21 results in the expression of Down syndrome. Down syndrome
individuals are typically mentally retarded 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 expression
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 one that is expressed 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 syndromeproduces progressive loss of nervous function and death
in a baby. Albinismarises 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 diseaseis 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, personality 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 chromo-
some. As a result, males show sex-linked traitslikecolor blindnessmuch more frequently
than females. Behaviors and diseases may have variations only some of which are genetically
based. A form of familial Alzheimer’s diseasehas been attributed to a gene on chromosome
21, but not all cases of Alzheimer’s disease are associated with that gene.❯ Review Questions
- A neuron without terminal buttons would be
unable to
(A) receive information from neighboring neurons
(B) generate an action potential
(C) direct the synthesis of neurotransmitters
(D) secrete neurotransmitters to postsynaptic
neurons
(E) transport ions across the cell membrane - Paul Broca found that the loss of the ability to
speak intelligibly is associated with damage to a
region of the brain in the
(A) thalamus
(B) right parietal lobe
(C) right occipital lobe
(D) left temporal lobe
(E) left frontal lobe
3. Scientists are able to see changes in the brain as it
processes information by means of
(A) lesioning
(B) autopsy
(C) CT
(D) MRI
(E) PET
4. The simplest behaviors we carry on
(A) are learned when we are infants
(B) do not involve the central nervous system
(C) are called instincts
(D) include sneezing and blinking
(E) must be processed by the medulla