Human Growth
and
Development
Chapter Three
Heredity and Environment
PowerPoints prepared by Cathie Robertson, Grossmont College
Revised by Jenni Fauchier, Metropolitan Community College
The Genetic Code
• Development that is dynamic,
ongoing, interactional, and unique;
just four chemicals are the basic
building blocks of the genetic
code
What Genes Are
• Genes are made up of DNA—the
complex protein code of genetic
information
• DNA directs the form and
function of each body cell as it
develops
• Each molecule of DNA is called a
chromosome
• Chromosomes contain instructions to
make all the proteins a living being
needs
• The packet of instructions is called a
genome
• Each person has 23 sets of
chromosomes, or 46 chromosomes
• The human genome contains 30,000
genes
The Beginnings of Human Life
• Gamete—reproductive cell that
directs process by which genetic
information combined and
transmitted
• Father gametes—sperm
• Mother gametes—ovum
Zygote and Genotype
• Male and female gametes fuse and
become a zygote
• Zygote begins process of duplication
and division
– two reproductive cells
• Genotype—the genetic information
from the 46 chromosomes
– set at human conception and endures
through life
Sex Determination and
Sex Ratio
• Of 22 out of 23 pairs of human chromosome,
the matching chromosomes are very closely
matched
– but not identical
• some genes come in slight, normal
variations called alleles
• The 23rd pair is different
– in females, it is designated XX
– in males, it is designated XY
Sex Determination and Sex Ratio, cont.
• Females always contribute one X
• Males will have 1/2 of the sperm
contributing an X and the other half
contributing a Y
• Critical factor in determining the
sex of a zygote is which sperm
reaches the ovum first
Sex Determination and Sex Ratio, cont.
• Other factors include
– rarely, male sperm may only carry either X or Y
– sometimes a woman’s uterus either unusually
alkaline or acid, giving either an X or Y sperm an
advantage
– in a stressful pregnancy XY embryos are more
likely to be expelled than are XX embryos in a
spontaneous abortion, or miscarriage
– current sex ratio in United States is 52 males to
48 females
Multiple Zygotes
• Monozygotic twins—identical
twins (or quadruplets) originate
from one zygote
– share identical instructions
– possibility of cloning
– 1/3 of twins monozygotic
Multiple Zygotes, cont.
• Dizygotic twins—from two
separate zygotes
– Dizygotic births occur once in every 60
births, and occur as frequently as 1 in
6 pregnancies, but usually only 1 twin
develops past embryo stage
Multiple Zygotes, cont.
• Dizygotic twins
– women in late 30’s are three times more
likely to have dizygotic twins
• as menopause approaches, ovulation
becomes irregular with some cycles
producing no ovas and others producing
multiple ovas
– share no more genes than other offspring
(about 50 percent)
• 50 percent of the time one twin is male
Duplication, Division, and
Differentiation
• The zygote contains a complete
set of instructions to create a
person
• Complex instructions on
duplication, cell division, and
differentiation
• Zygote begins duplication and division
within hours after conception
– the 23 pairs of chromosomes duplicate, forming
two complete sets of the genetic code for that
person (zygote)
– these two pair sets move toward the opposite
sides of the zygote and the single cell in the
zygote splits down the middle
– the zygote’s outer membrane surrounds two cells,
each containing a complete set of the original
genetic code
• these two cells then duplicate and
divide to become four, then eight,
and so on
• by birth, your original zygote has
duplicated and divided into 10
trillion cells . . . by adulthood, it’s
100 trillion cells
• Every cell carries an exact copy
of the complete genetic
instructions inherited by the onecelled zygote
Differentiation
• Not just any cell found in the
zygote can become a person
• At the 8-cell stage a third
process, differentiation, occurs
• Cells begin to specialize
– they take different forms
– they reproduce at different rates,
depending on where in the growing mass
they are located
Differentiation, cont.
• Certain genes affect
differentiation by switching other
genes on and others off so that the
other genes produce the right
proteins at the right times—on-off
switching mechanisms
• Genotype—inheritance that can be
observed or is expressed
Gene - Gene Interactions
• Multifactoral traits—inherited
traits produced by interaction of
genes and environment
• Polygenetic traits—inherited traits
produced by gene interaction
• These are affected by on-off
switching mechanisms, additive
genes, and dominant-recessive
genes
Additive Genes
• Additive genes—one of a number of
genes affecting a specific trait
– each additive gene contributes to the trait
• skin color and height are determined by
them
• every additive gene has some impact on a
person’s phenotype
– when genes interact this way, all the involved
genes contribute fairly equally
Dominant and Recessive
Genes
• Nonadditive genes—phenotype
shows one gene more influential
than other genes
• This is also referred to as the
dominant-recessive pattern
– gene showing the most influence is referred
to as dominant
– gene showing the least influence is referred
to as recessive
• X-linked genes—located on X
chromosome
– if recessive gene is X-linked, that it is on the
X chromosome is critical
– female has one X chromosome and one Y;
males have only 2 X
• females’ recessive genes can be
counterbalanced by Y chromosome, but
males’ recessive genes can not because they
have only the X chromosome
– if X-linked gene recessive, may cause color
blindness, many allergies, several diseases,
and learning disabilities
More Complications
• Genes direct the creation of 20
amino acids that produce thousands
of proteins forming the body’s
structure and directing biochemical
functions
– proteins of each body cell are continually
affected by other proteins, nutrients, and
toxins that influence the cell functioning
More Complications, cont.
• genetic imprinting—tendency of
certain genes to be expressed
differently when inherited from
mother than from father
(tagging)
– some of the genes which influence
height, insulin production, and several
forms of mental retardation affect a
child differently depending on which
parent they came from
Mechanisms of Genetic
Diversity
• Since each gamete contains only
23 chromosomes, why is every
conception genetically unique?
– 8 million chromosomally different ova x
8 million of the same = 64 trillion
different possibilities of children from
each couple
Health Benefits of Genetic
Diversity
• Genetic diversity safeguards
human health
• Minute differences can affect
the ability to stave off certain
diseases
• Genetic diversity maintains the
species
From Genotype to Phenotype
• Every psychological
characteristic is genetically
influenced
• Every psychological
characteristic and personal
trait is affected by the
environment
From Genotype to Phenotype, cont.
• Genotype—genetic potential
• Phenotype—combination of
genetic potential and
expression
– we are all carriers of the
unexpressed genes
• we can pass them along through
the sperm or ova
Behavior Genetics
• Behavior genetics—study of
effects of genes on behavior
– personality patterns, psychological
disorders, and intellectual abilities
Senility Caused by
Alzheimer’s Disease
• Most common and feared type of
senility is Alzheimer’s disease
– amyloid B protein accumulates in the
brain, leading to dysfunction and
destruction of brain cells and
disruption of the mind
• Can be genetic—but only when
“early-onset”
Senility Caused by Alzheimer’s
Disease, cont.
• If “late-onset,” may be a
combination of genes and
environment
– other predictors may include
hypertension, diabetes, high cholesterol,
diet, exercise, not smoking, weight
control, mental alertness, and physical
health
Alcoholism
• Inherited biochemistry makes
some people highly susceptible to
alcohol addiction
– addictive pull can be overpowering, or
weak, or something in the middle
– may explain ethnic variations
Alcoholism, cont.
• Not simply a biochemical reaction—it
is psychological and physical, and
biological; thus alcoholism is
polygenetic, with alcoholics
inheriting a combination of
biochemistry-affecting and
temperament-affecting genes
• Culture counts too(whether alcohol
is present in environment)
Chromosomal and Genetic
Abnormalities
• We now give attention to these
because we can recognize
– disruptions of normal development
– origins of genetic and chromosomal
abnormalities
– misinformation and prejudice add to
problems of people with these
abnormalities
Chromosomal Abnormalities
• A gamete with more than or less
than 23 chromosomes creates a
zygote with chromosomal
abnormalities
– most likely variable that creates
chromosomal abnormalities is mother’s
age (over 35)
– father’s age (over 40) also a variable
Chromosomal Abnormalities, cont.
• Most zygotes with chromosomal
abnormalities never come to term
– spontaneous abortion occurs in about
one-half of all fetus with chromosomal
abnormalities
Down Syndrome
• Three chromosomes at gene
#21 (trisomy-21)
• Syndrome—a cluster of distinct
characteristics that occur
together in a given disorder
Abnormalities of the
23rd Pair
• Location of sex chromosome
• Kleinfelters syndrome—XXY
– seemingly normal child has delayed
puberty
• Fragile X syndrome
– hanging on by a thread (mutated gene)
– intensifies from generation to
generation
Genetic Testing and Genetic
Counseling
• Individuals with a parent, sibling, or child
with a serious genetic condition known to
be dominant or recessive
• Couples with history of early spontaneous
abortions, stillbirths, or infertility
• Couples from the same ethnic group or
subgroup—especially if closely related
• Women over 35 and men over 40
The Process of Genetic
Counseling
• Counselor constructs couples’
family history
– charts patterns of health and illness
over generations
• Some tests provide information
before conception
The Process of Genetic Counseling, cont.
• Other tests are prenatal
–
–
–
–
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alpha-fetoprotein assay
ultrasound (AKA sonogram)
amniocentesis
chorionic villi sampling
pre-implantation testing (used in in vitro
fertilization)
– gamete selection; ova/and or sperm are
screened to select ones free of
particular problems
A Basis for Decision
• Many want to know ahead of time
• Some do not
• There is a more knowledge of
what is to come—or not
Alternatives
• If both partners are carriers of a
serious condition or are at high risk
because of age or family
characteristics, they may turn to
–
–
–
–
–
in-vitro fertilization (IVF)
gamete intrafallopian transfer (GIFT)
zygote intrafallopian transfer (ZIF)
artificial insemination donor (AID)
postponement of pregnancy until promising
treatments are further developed