WHAT WILL YOU KNOW?
What is the relationship between genes and
chromosomes?
Is twinning genetic or can a person choose to have
twins?
How could a person have the gene for something that is
never apparent?
If both parents are alcoholics, will their children be
alcoholics too?
Why are some children born with Down syndrome?
What Genes Are
Protein
Contains sequence of chemicals consisting of long
string of amino acids
Consists of instructions for stringing together the
right amino acids in the right order via pairs of four
chemicals called bases
What Genes Are
All living things are composed of cells that
promote growth and sustain life according
to instructions in their molecules of DNA
DNA (deoxyribonucleic acid)
• Molecule that contains the chemical instructions for
cells to manufacture various proteins
• Promotes growth and sustains life
Chromosomes
• Molecules of DNA
• Consists of 46 chromosomes arranged in 23 pairs
What Genes Are
Allele
Variation of a gene or any of the possible forms in
which a gene for a particular trait can occur
Effects of variations vary greatly from causing lifethreatening conditions to having no detectable effect at
all
Genetic diversity
Distinguishes each person
Allows the human species to adapt to pressures of the
environment
What Genes Are
Methylation
Processes additional DNA and RNA that
enhances, transcribes, connects, and alters genes
Genome
Involves the full set of genes that are the
instructions to make an individual member of a
certain species
How Proteins Are Made
Variations
Differences begin with genes
Single nucleotide polymorphism (SNP) gene
• Two, three, or more versions
• May be inconsequential or destructive
Two Parents,
Millions of Gametes
Zygote
Two gametes (sperm and
ovum) combine and produce a
new individual with 23
chromosomes from each
parent.
Gametes
Reproductive cells (sperm
and ova)
Each gamete consists of 23
chromosomes.
This is a computer illustration of a
small segment of one gene. Even a
small difference in one gene can
cause major changes in a person’s
phenotype.
The Beginnings of
Life
Matching genes
Genotype
• Organism’s entire
genetic inheritance, or
genetic potential.
Homozygous
• Two genes of one pair
that are exactly the
same in every letter of
their code
Heterozygous
• Two genes of one pair
that differ in some way
The Moment of Conception
This ovum is about to become a
zygote.
Uncertain Sex
Every now and then, a baby is born with “ambiguous genitals,” meaning that the child's
sex is not abundantly clear. When this happens, a quick analysis of the chromosomes is
needed, to make sure there are exactly 46 and to see whether the 23rd pair is XY or XX.
The karyotypes shown here indicate a normal baby boy (left) and girl (right).
Male or Female?
Humans usually possess 46
chromosomes.
44 autosomes and 2 sex chromosomes
Females: XX
Males: XY
Sex of offspring depends on whether the father's Y
sperm or X sperm fertilizes the ovum.
Determining a Zygote’s Sex
Too Many Boys?
Is sex selection the parents’
right or a social wrong?
Preference for boys in many
areas of world
My Strength, My Daughter
That’s the slogan these girls in New
Delhi are shouting at a
demonstration against abortion of
female fetuses in India.
Ways to prevent female
birth
• Inactivating X sperm before
conception
• In vitro fertilization (IVF)
• Aborting XX fetuses
New Cells, New Functions
Duplication and division begins
within hours after conception
23 pairs of chromosomes
carrying all the genes duplicate,
forming two complete sets of
the genome
Two cells duplicate and divide,
becoming four, which
themselves duplicate and
divide, becoming eight, and so
on
Two sets move toward opposite
sides of the zygote, and the
single cell splits neatly down the
middle into two cells, each
containing the original genetic
code
First Stages of the Germinal Period
ANATOMICAL TRAVELOGUE/SCIENCE SOURCE
(a)
(b)
(c)
The original zygote as it divides into (a) two cells, (b) four cells, and (c) eight cells.
Occasionally at this early stage, the cells separate completely, forming the beginning of
monozygotic twins, quadruplets, or octuplets.
New Cells, New Functions
At birth
Zygote becomes 10 trillion cells
Each cell carries complete copy of genetic instructions
Stem cells
Result from early duplication and division
Are able to produce any other cell
Placenta
Allows early genetic testing
Siblings and Twins
Monozygotic
(identical) twins
Dizygotic (fraternal)
twins
Originate from one zygote that
splits apart very early in
development
Result from fertilization of two
separate ova by two separate
sperm.
Incomplete split results in
conjoined twins.
Dizygotic twins have half their
genes in common and occur twice
as often as monozygotic twins.
Same genotype but slight
variations in phenotype are
possible due to environmental
influences.
Incidence is genetic and varies by
ethnicity and age.
Assisted Reproduction
Assisted reproductive technology (ART)
General term for the techniques designed to help
infertile couples conceive and sustain a pregnancy
in vitro fertilization (IVF)
Involves fertilization that takes place outside a
woman’s body
Involves mixing sperm with surgically removed ova
from the woman’s ovary and implanting zygote into
a woman’s uterus
Male/Female Differences
One-third of all fertility problems originate in
woman; another third from man; final third
from unknown causes.
• Fertility decreases with age; faster decline for
women
Infertility: Failure to conceive a child after a
year of trying.
Subfertile: Less fertile than ideal; not sterile
From Genotype to Phenotype
Phenotype
Observable characteristics of an organism, including
appearance, personality, intelligence, and all other traits
Polygenic
Trait influenced by many genes
Multifactorial
Trait affected by many factors, both genetic and
environmental, that enhance, halt, shape, or alter the
expression of genes, resulting in a phenotype that may
differ markedly from the genotype.
From Genotype to Phenotype
Epigenetic
Environmental factors that affect genes and
genetic expression—enhancing, halting,
shaping, or altering the expression of genes
and resulting in a phenotype that may differ
markedly from the genotype
Gene-Gene Interactions
Human Genome Project
Involves international effort to map the complete
human genetic code
Was essentially completed in 2001, though analysis is
ongoing
Some interesting findings
Humans have far fewer genes than previously
believed
All living creatures share many genes
Gene-Gene Interactions
Additive genes
Genes that add something to some aspect of the
phenotype
Partly dependent on all inherited genes
Example
Height is affected by the contributions of about 180
genes
Gene-Gene Interactions
Dominant-recessive heredity
Dominant gene is far more influential than the
recessive gene (non-additive).
Dominant gene can completely control the
phenotype with no noticeable effect of recessive
gene.
Effect of recessive genes can sometimes be
noticed.
A Changeling?
No. Many brown-eyed people carry a recessive gene for blue eyes. The only way to
know for sure is if they have a blue-eyed parent (who gives one gene for blue eyes to
every child) or if they have a blue-eyed child. Other recessive genes include red hair,
Rh negative blood, and many genetic diseases.
More Complications
Copy number variations
Occur in abundance
Involve genes with various repeats or detections of base pairs
May be inconsequential, lethal, or something in between
Parental imprinting
Occurs when genes from either mother or father are affected
May differ for XX versus XY embryos
Syndrome examples: Prader-Willi syndrome and Angelman
syndromes
Genotype and Phenotype
Current consensus
• Genes affect every aspect of behavior
• Most environment influences on children raised in
the same home are not shared
• Genes elicit responses that shape development
• Lifelong, people choose friends and environments
that encourage their genetic predispositions
Genotype and
Phenotype
Alcoholism
Genes create addictive
pull
Alcoholism is polygenic
and culture is pivotal
Is He Drunk?
Risk
Biological sex
Gender
Contexts
Ethnicity
Nature and Nurture Connected: Alcoholism
Genes can cause an overpowering addictive pull in some
people.
Inherited psychological traits affect alcoholism.
Biological sex (XX or XY) and gender (cultural) also
affect the risk of alcoholism.
Environmental conditions can modify the genetic effects.
Nature and nurture must combine to create an alcoholic.
Genotype and Phenotype
Nearsightedness
Age: Limited focus at birth; change in eyeball shape at
puberty; myopia at middle age
Genes: Alleles of Pax6 contribute to nearsighted
Heritability: Heritability of 90%
Other causes
Environment
Homework and lack of play
Chromosomal and Genetic Problems
Why study conditions caused by extra
chromosome or single destructive gene?
They provide insight into the complexities of nature
and nurture.
Knowing their origins helps limit their effects.
Information combats prejudice: Difference is not
always deficit.
Chromosomal and
Genetic Problems
Not exactly 46
Some gametes have more or
fewer than 23 chromosomes
Some times only part of 23rd
chromosome missing
Down syndrome
Daniel has trisomy-21, and this
photograph was taken at the only
school in Chile where normal and
special-needs children share
classrooms.
Called trisomy-21 because the
person has three copies of
chromosome 21
Involves around 300 distinct
characteristics from third
chromosome; unique
individuals
Chromosomal and Genetic Problems
Dominant disorders
7,000 known single-gene disorders are dominant
Evident in phenotype
Rare because people rarely live long enough to reproduce
Exceptions
Huntington disease: Fatal CNS disorder caused by genetic
miscode of more than 35 repetitions of particular triplet
Rare type of early onset (before age 60) Alzheimer disease
Chromosomal and Genetic Problems
Recessive disorders
• Millions of different types,
lethal conditions are rare
• Several types are sexlinked
Fragile X syndrome
• Caused by more than 200
repetitions of one triplet on
one gene
• Most common form of
inherited form of inherited
mental retardation
Sickle-cell trait
• Offers some protection
against malaria
• African carriers are more
likely than non-carriers to
survive
Cystic fibrosis
• More common among
people with northern
European ancestors
• Carriers may have been
protected against cholera
Genetic Testing for Psychological Disorders
Genes are part of every psychological disorder
Few scientists advocate genetic testing for schizophrenia
In several U.S. states selling genetic tests is illegal
Some testing is legally required
Genetic Testing for Psychological Disorders
Genetic counseling
Involves consultation and testing by trained
experts
Enable individuals to learn about their genetic
heritage
Helps couples clarify values and determine genetic
risks
Ethical rules
Tests are confidential
Decisions are made by the clients, not the counselors