Biology and Behaviour
Chapter 3 - Pg. 83-101
• Francis Galton (1869), a cousin of Charles Darwin, concluded that talent runs in families.
• According to Mill, Galton’s subjects rose to eminence more because of environmental
factors than hereditary ones.
• Our understanding of how characteristics are transmitted from parent to offspring
originated with in- sights achieved by Gregor Mendel.
• James Watson and Francis Crick’s 1953 identification of the structure of DNA, the basic
component of hereditary transmission.
• The number of genes that humans have: the current estimate of around 26,000 genes
(Pennisi,2003). Most of those genes are also possessed by all living things.
• We humans share a large proportion of our genes with bears, barnacles, beans, and
• We differ from one another by only about 1 to 1.5% of our genes.
Genetic and environmental forces
• Genotype—the genetic material an individual inherits
• Phenotype—the observable expression of the genotype, including both body
characteristics and behavior
• Environment—every aspect of the individual and his or her surroundings other than the
1. Parent’s Genotype–Child’s Genotype • Relation 1 involves the transmission of genetic material- DNA genes
chromosomes-from parent to offspring.
• DNA carries all the biochemical instructions involved in the formation and functioning
of an organism.
• Genes affect development and behavior only through the manufacture of
proteins-“DNA’s information translated into flesh and blood” (Levine & Suzuki, 1993,
• Humans normally have a total of 46 chromosomes in the nucleus of every cell. Eggs
and sperm each contain only 23 chromosomes. These 46 chromosomes are actually
• One member of each chromosome pair was inherited from each parent. Thus, every
individual has two copies of each gene. Your children will each receive half of your
genes, and your grandchildren will have one-quarter.
• Females have two identical, largish sex chromosomes, called X chromosomes.
• Males have one X chromosome and one much smaller Y chromosome.
• Because a female has only X chromosomes, her eggs having an X.
• Because a male is XY, half his sperm contain an X chromosome and half contain a
• For this reason, it is always the father who determines the sex of offspring: if an X-
bearing sperm fertilizes an egg, a female (XX) zygote results; if the egg is fertilized
by a Y-bearing sperm, the zygote is male (XY).
Diversity and individuality
• Genes guarantee that we will be similar in certain ways to other people both at the
species level, and at the individual level. Genes also guarantee differences at both
• Mutation contribute to genetic diversity among people. Most are harmful. Those that
occur in germ cells(testis, eggs) can be passed on to offspring.
• Sometimes a mutation that occurs in a germ cell or early in prenatal development
makes individuals more viable, that is, more likely to survive, perhaps by increasing
their resistance to some disease or by increasing their ability to adapt to some crucial
aspect of their environment. Such mutations provide the basis for evolution.
• Random assortment of chromosomes in the formation of egg and sperm is the
second mechanism that promotes variability among individual. During germ-cell
division, the 23 pairs of chromosomes are shuffled randomly. • This means that, for each germ cell, there are 223, or 8.4 million, possible
combinations of chromosomes. Thus, when a sperm and an egg unite, the odds are
essentially zero that any two individuals—even members of the same family—would
have the same genotype (except, of course, identical twins).
• The two members of a pair of chromosomes sometimes swap sections of
DNA(crossing over) some of the chromosomes that parents pass on to their
2. Child’s Genotype–Child’s Phenotype
• Although every cell in your body contains copies of all the genes you received from
your parents, only some of those genes are expressed. At any given time in any cell
in the body, some genes are active (turned on), while others are not.
Gene expression: Developmental changes
• Particular genes are active at any given point in development. A given gene
influences development and behavior only when it is turned on, and human
development proceeds normally, only if genes get switched on and off in the right
place, at the right time, and for the right length of time.
• Some genes are involved in the basic functioning of almost all cells almost all the
time. The switching on and off of genes is controlled primarily by regulator genes.
• When one gene is switched on, it causes another gene to turn on or off, which has
an impact on the status of yet other genes.
• The continuous switching on and off of genes underlies development throughout life.
• External factors can also affect the switching on and off of genes.
Gene expression: Dominance patterns
• Many of an individual’s genes are never expressed. About a third of human genes
have two or more different forms, known as alleles. The alleles of a given gene
influence the same trait or characteristic (e.g., eye color), but they contribute to
different developmental outcomes (e.g., brown, blue, hazel, gray eyes).
• Pattern of gene expression discovered by Mendel and referred to as the dominant-
recessive pattern. Some genes have only two alleles, one of which is dominant and
the other recessive. In this pattern, a person can inherit either two of the same allele
—two dominant or two recessive—and thus be homozygous for the trait in question;
or the person can inherit two different alleles—one dominant and the other recessive
—and thus be heterozygous for the trait. • When an individual is homozygous, with either two dominant or two recessive alleles,
the corresponding trait will be expressed.
• When an individual is heterozygous for a trait, the instructions of the dominant allele
will be expressed (see Figure 3.3).
• Example: To illustrate, let us consider two traits of no importance to human survival:
the ability to roll one’s tongue lengthwise and curliness of hair. If you can roll your
tongue lengthwise into the shape of a tube, then at least one, but not necessarily
both, of your parents must also possess this remarkable but useless talent. From this
statement (and Figure 3.3), you should be able to figure out that tongue rolling is
governed by a dominant allele. In contrast, if you have straight hair, then both of your
parents must carry an allele for this trait, although it is possible that neither of them
actually has straight hair. This is because straight hair is governed by a recessive
gene, and curly hair is governed by a dominant gene.
• The Y chromosome, being smaller than the X chromosome, has only about a third as
many genes on it.
• Example: Suppose, then, that a woman inherits a recessive allele on the X
chromosome from her mother. Chances are, she will have a dominant allele on the
chromosome from her father to suppress it. Now suppose that a man inherits the
same recessive allele on the X chromosome from his mother. Chances are, because
of the much smaller size of the Y chromosome he inherits from his father, he will not
have a dominant allele to override it, so he will develop the trait.
• This difference in sex-linked inheritance is one reason for the greater vulnerability of
males: they are more likely to suffer a variety of inherited disorders caused by
recessive alleles on their X chromosome.
3. Child’s Environment–Child’s Phenotype
• Because of the continuous interaction of genotype and environment, a given
genotype will develop differently in different environments. This idea is expressed by
the concept of the norm of reaction (Dobzhansky, 1955), which refers to all the
phenotypes that could theoretically result from a given genotype in relation to all the
environments in which it could survive and develop.
Examples of genotype–environment interaction
• Scientists randomly assign animals with known genotypes to be raised in a wide
variety of environmental conditions. If genetically identical animals develop differently
in different environments, researchers can infer that environmental factors must be
responsible. • The researchers wanted to determine why some children with a particular genotype
who experience severe maltreatment become violent and antisocial as adults,
whereas others who are exposed to the same parenting abuse do not.
• Figure 3.5, shows- suffering abusive treatment as a child and possessing a particular
variant of MAOA, an X-linked gene known to inhibit brain chemicals associated with
• 85% of the maltreated group with the relatively inactive gene developed some form
of antisocial be- havior, and they were almost 10 times more likely to be convicted of
a violent crime.
• The important point here is that neither factor by itself (inactive MAOA gene or being
abused) predisposed boys to become highly aggressive. The higher incidence of
antisocial behavior was observed only for the group with both factors.
Parental contributions to the child’s environment
• A highly salient and important part of a child’s environment is the parents’ relationship
with the child—the manner in which they interact with him or her, the general home
environment they provide, the experiences they arrange for the child, the
encouragement they offer for particular behaviors, attitudes, and activities, and so on.
• The environment that parents provide for their children is due in part to their own genetic
makeup. Thus, the child of a parent with a high level of musical ability is likely to hear
more music while growing up than is a child of less musically talented parents.
4. Child’s Phenotype–Child’s Environment
• The child as a source of his or her own development. • First, by virtue of their nature and behavior, they actively evoke certain kinds of
responses from others (Scarr, 1992; Scarr & McCartney, 1983). Babies who enjoy
being cuddled are more likely to receive cuddling than are squirmy babies. Impulsive
children hear “No,” “Don’t,” “Stop,” and “Be careful” more often than inhibited children
• Indeed, the degree to which parent–child relationships are mutually responsive is
largely a function of the child’s genetically influenced behavioral characteristics
(Deater-Deckard & O’Connor, 2000).
• Children actively selecting surroundings and experiences that match their interests,
talents, and personality characteristics (Scarr, 1992). As soon as infants become
capable of self-locomotion, for example, they start selecting certain objects in the
environment for exploration.
Genetic Transmission of Diseases and Disorders
• More than 5000 human diseases and disorders are presently known to have genetic
• Many conditions show straight forward Mendelian (dominant–recessive) patterns of
inheritance, occurring only when an individual has two recessive alleles for the
• Disorders that are caused by a dominant gene include Huntington disease and
• Severe speech and language difficulties that are common in a particular family in
England have been traced to a mutation of a single gene (referred to as FOXP2) that
acts in a dominant fashion (Marcus & Fisher, 2003).
• In some cases, a single gene can have both harmful and beneficial effects. One such
case is sickle-cell disease, a debilitating and sometimes fatal blood disorder that
affects about 1 out of every 500 African Americans. It is a recessive-gene disorder,
so a child who inherits two sickle-cell genes (one from both parents) will suffer from
the disease. People with one normal and one sickle-cell gene have some
abnormality in their blood cells, but usually experience no negative effects. In fact, if
they live in regions of the world— like West Africa—where malaria is common, they
benefit, because the sickle cells in their blood confer resistance against this deadly
• Many common human diseases and disorders are believed to result from multiple
inherited genes, often in conjunction with environmental factors. Among the many
diseases in this category are some forms of cancer and heart disease, as well as
asthma. • Psychiatric disorders, such as schizophrenia, and behavior disorders, such as attention-
deficit hyperactivity disorder (ADHD), probably also involve multiple genes.
• Some single- gene conditions are carried on the X chromosome and are much more
common in males.
• Sex-linked disorders range from relatively minor problems, like male-pattern baldness
and red-green color blindness, to very serious disorders, including hemophilia,
Duchenne muscular dystrophy and fragile-X syndrome.
• Some genetic disorders originate with errors in germ-cell division that result in a zygote
that has either more or less than the normal complement of chromosomes. Most such
zygotes cannot survive, but some do.
• Down syndrome most commonly originates when the mother’s egg cells do not divide
properly, and an egg that is fertilized contains an extra copy of chromosome 21. The
probability of such errors in cell division increases with age, with the incidence of giving
birth to a child with Down syndrome being markedly higher for women over 35.
• Down syndrome, occurs in about 1 of every 1000 births in the United States. The