Kathleen Stassen Berger
Part I
Chapter Three
Heredity and Environment
The Genetic Code
From One Cell to Many
From Genotype to Phenotype
Chromosomal and Genetic
Abnormalities
Prepared by Madeleine Lacefield
Tattoon, M.A.
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The Genetic Code
• “Genes play a leading role in the
drama of human development, yet
they rarely take center stage. Genes
are pervasive and powerful, but they
are also hidden and elusive.”
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What Genes Are
• DNA (deoxyribonucleic acid)
– Molecule that contains the chemical instructions for cells
to manufacture various proteins.
– Chromosome
• a molecule of DNA that contains the instructions to make
proteins
• Humans have 46 chromosomes (23 pairs), and about 25,000
genes.
– Genome
• the code for making a human being
• Every person has a slightly different code, but the human
genome is 99.5% the same for any 2 people.
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What Genes Are
• Genes are as section of
chromosomes and the basic unit for
the transmission of heredity,
consisting of a string of chemicals
that code for the manufacture of
certain proteins.
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The Beginnings of Life
“…development begins at conception…each
human reproductive cell or gamete,
contains 23 chromosomes, half of that
person’s 46…”
gamete
A reproductive cell; that is, a sperm or
ovum that can produce a new individual if it
combines with a gamete from the other sex
to make a zygote
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The Beginnings of Life
• Matching Genes
– conception occurs in the usual way
• zygote
– the single cell formed from the fusing of two
gametes, a sperm and an ovum
• genotype
– An organism’s entire genetic inheritance, or
genetic potential
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The Beginnings of Life
• Male or Female?
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From One Cell to Many
– phenotype
• the observable characteristic of a
person, including appearance,
personality, intelligence, and all other
traits
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From One Cell to Many
• New Cells, New Functions
– Gene-Gene Interactions
• occurs through cell differentiation, gene-gene
(polygenic), and gene-environment interaction
– Multifactorial
• refers to a trait that is affected by many factors, both
genetic and environmental
– The Human Genome Project is an international effort to
map the entire human genome
• researchers have found that humans have only about
25,000 genes, 99% of which are present in the genomes
of other creatures as well
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From One Cell to Many
• Additive Heredity
– an allele is a slight, normal variation of a
particular gene
• some alleles are…
– additive genes combine to make a
phenotype
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From One Cell to Many
• Dominant-Recessive Heredity
– the interaction of a pair of alleles in such a
way that the phenotype reveals the influence
of one allele (the dominant gene) more than
that of the other (the recessive gene)
– a special case of the dominant-recessive
pattern occurs with genes that are x-linked,
located on the x chromosome
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From One Cell to Many
• More Complications
– A small alteration in the sequence of
base pairs or several extra repetitions in
one triplet ma be inconsequential or
may cascade to create a major problem
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From One Cell to Many
• Twins, Clones, Assisted Reproduction (ART)
– dizygotic (fraternal) twins
• result from two sperm penetrating two ova, and share 50% of their
genes
– monozygotic (identical) twins
• originate from one zygote, and share 100% genes
– a clone
• originates from a live organism
– ART
• general term for the technique designed to help infertile
couples conceive and then sustain a pregnancy
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From One Cell to Many
• Assisted Reproduction (ART)
– general term for the technique designed to
help infertile couples conceive and then
sustain a pregnancy
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From Genotype to Phenotype
• Scientist in many nations have studied
thousands of twins, both monozygotic and
dizygotic, raised together in the same home
and raised separately in different homes
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From Genotype to Phenotype
• Genes affect every aspect of human
behavior, including social and cognitive
behavior
• Most environmental influences on
children raised in the same home are
not shared
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From Genotype to Phenotype
• Each child’s genes elicit other people’s
responses, and these responses shape
development. In other words, a child’s
environment is partly the result of his or
her genes.
• Children, adolescents, and especially
adults choose environments that are
compatible with their genes (called nichepicking), and thus genetic influences in
adulthood
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From Genotype to Phenotype
• Carrier
– a person whose genotype includes a gene that
is not expressed in the phenotype…such an
unexpressed gene occurs in half of the
carrier’s gametes and thus is passed on to half
of the carrier’s children, who will most likely be
carriers, too…
– Generally, only when the gene is inherited
from both parents does the characteristic
appear in the phenotype.
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From Genotype to Phenotype
• Addiction
– …inherited biochemistry making people
vulnerable to various addition…
– …any one can abuse drugs or
alcohol…but genes create an addictive
pull that can be overpowering, extremely
weak, or somewhere in between…
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From Genotype to Phenotype
• Visual Acuity
– New borns cannot focus more than 2 feet
away
– Children see better each year until about
age 8
– Many adolescents become nearsighted
when eyeball shape changes
– Vision is more likely to improve than to
worsen until age 40
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From Genotype to Phenotype
• Visual Acuity
– In middle age, the elasticity of the lens
decrease and the eyeball shape change
again, so that many people become
farsighted and need reading glasses
– Among the old, eye diseases, including
cataracts, are common
– About 10 percent of people over age 90 are
blind
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From Genotype to Phenotype
• Nearsightedness and Genes
– If children have a vision problem it is most
often myopia (nearsightedness)
– Nearsightedness is a symptom in more than
150 genetic syndromes
– Caused by physical trauma or illness, such as
the rubella virus, or poor nutrition (such as
vitamin A deficiency
– These factors cause “high” nearsightedness,
so severe that it can lead to blindness
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From Genotype to Phenotype
• Culture and Cohort
– genes are not the major cause of poor
vision
• historical and multicultural research finds
that environment also influences
nearsightedness
– if diet is deficient of vitamin A
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From Genotype to Phenotype
• Practical Application
– developmental application of naturenurture interaction
• family history of genetic problems
• someone inherited a problem
– alcoholism in the genes
– lack of outdoor play
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From Genotype to Phenotype
• Practical Application
– type 2 diabetes (adult-onset diabetes)
• a chronic disease which the body does
not produce enough insulin to
adequately metabolize carbohydrate
(glucose)… it typically developed in
people aged 50 - 60…today it often
appears in younger people
– begins when a person is vulnerable and
has more body fat than is ideal
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From Genotype to Phenotype
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Chromosomal and Genetic Abnormalities
• abnormalities caused by identifiable problems…those
with an extra chromosome or a single gene
– study of these problems is relevant to the study of
development…
• providing insight into the complexities of nature and
nurture
• knowing their origins helps limit these effects
• information combats the prejudice that surrounds
such problems
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Chromosomal and Genetic Abnormalities
• Not Exactly 46 Chromosomes
– a variable that most often correlates with
chromosomal abnormalities is the age of the
mother
– occur not only in the formation of gametes but
also in their early duplication
– mosaic is having a condition (mosaicism) that
involves having a mixture of cells, some
normal and some with an odd number of
chromosomes or a sense of missing genes
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Chromosomal and Genetic Abnormalities
• Down Syndrome
– a condition in which a person has 47
chromosomes instead of the usual 46, with
three rather than two chromosomes at the
21st position
– people with Down Syndrome typically
have distinctive characteristics, including
unusual facial features, heart abnormities,
and language difficulties
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Chromosomal and Genetic Abnormalities
• Abnormalities of the 23rd Pair
– humans have at least 44 autosomes and
one X chromosome
• an embryo cannot develop without an X
chromosome
• an odd number of X chromosomes impairs
cognition and psychosocial development
and sexual maturation
• if a child has three sex chromosomes
instead of two he/she may seem normal
until puberty
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Chromosomal and Genetic Abnormalities
• Dominant-Gene Disorders
– everyone carries genes or alleles that
could produce serous diseases or
handicaps in the next generation
– 7,000 single-gene disorders
• their dominant effects are apparent in the
phenotype
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Chromosomal and Genetic Abnormalities
• Fragile X Syndrome
– a genetic disorder in which part of the X
chromosome seems to be attached to the
rest of it by a very thin string of molecules
– the actual cause is too many repetitions of
a particular part of a gene’s code
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Chromosomal and Genetic Abnormalities
• Recessive-Gene Disorder
– most recessive disorders are not X-linked
– double recessive patterns are lethal…one
recessive gene is protective
– sometimes a person who carried a lethal
gene has many descendants who marry
each other… the genetic disease then
becomes common in that group
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Chromosomal and Genetic Abnormalities
• Genetic Counseling and Testing
– consultation and testing by trained experts
that enable individuals to learn about their
genetic heritage, including harmful
conditions that they might pass along to
any children they may conceive
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Chromosomal and Genetic Abnormalities
• Who Should Get Counseling, and When?
– genetic counseling
• consultation and testing by trained experts
that enable individuals to learn about their
genetic heritage, including harmful conditions
that they might pass along to any children
they may conceive
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Chromosomal and Genetic Abnormalities
• Is knowledge Always Power?
– Genetic counselors, scientist, and the
general public usually favor testing
• having some information is better than
having none
– high risk individuals (who might hear bad
news) do not always want to know
• the truth might jeopardize their marriage,
their insurance coverage, or their chance of
parenthood
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Chromosomal and Genetic Abnormalities
• Coping with Uncertainty
– much is uncertain in genetic testing and
counseling
– those who learn that they have a harmful
dominant gene have new information, as
well as new uncertainties
– interaction of genes and the environment
makes development overt the life span
unpredictable, even if the genes are known
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