Biological Bases of Behavior 75researchers 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
formation of sex cells, fertilization, embryonic development, and protein synthesis.
Each DNA segment of a chromosomethat determines a trait is a gene. Chromosomes
carry information stored in genes to new cells during reproduction. Normal human body
cells have 46 chromosomes, 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 chromosomes. 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 chromosomes results in a zygote,
and subsequently an individual, with chromosomal abnormalities. Turner syndrome
females 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.
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 expressed gene is called the dominant gene; the hidden gene is the reces-
sive gene. Numerous recessive genes are responsible for syndromes in the homozygous con-
dition.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 abnor-
mal 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.