Transcript Heredity and Prenatal Development

CHAPTER 2
Heredity and Conception
Learning Outcomes
LO1 Explain the influences of heredity on
development, referring to chromosomes and
genes, mitosis and meiosis, twins, and dominant
and recessive traits.
LO2 Describe the features and causes of various
chromosomal abnormalities.
LO3 Describe the features and causes of various
genetic abnormalities.
LO4 Discuss methods of detecting genetic
abnormalities.
LO5 Describe methods of determining our genotypes
and our phenotypes.
LO6 Describe the process of conception.
LO7 Discuss the causes of infertility and alternate
ways of becoming parents.
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TRUTH OR FICTION?
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Your father determined whether you
are female or male.
You can carry the genes for a deadly
illness and not become sick yourself.
Approximately 120 to 150 boys are
conceived for every 100 girls.
Sperm travel about at random inside
the woman’s reproductive tract.
“Test-tube” babies are grown in a
laboratory dish throughout their 9month gestation period.
You can select the sex of your child.
© iStockphoto.com
LO1 The Influence of
Heredity on Development
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The Influence of Heredity on
Development
• Heredity defines our nature.
– It is based on the biological transmission of
physical and psychological traits from
generation to generation.
• The field of biology that studies heredity
is called GENETICS.
– Traits are transmitted by:
• Chromosomes
• Genes
Chromosomes and Genes
• CHROMOSOMES
– Rod-shaped structures found in cells
– Typical human cells have 46 (23 pairs)
– Each contains thousands of segments called genes.
• GENES
– Biochemical materials that regulate how traits develop
– Some traits are transmitted by a single pair of genes.
– Other traits are POLYGENIC (using several pairs).
– We (humans) have approximately 20-25,000 genes.
– Genes are segments of strands of deoxyribonucleic acid
(DNA).
– DNA forms a double spiral (HELIX) that looks like a twisting
ladder.
– Each “rung” on the ladder consist of 4 basic chemicals that
are placed in pairs of either:
• Adenine and Thymine
• Cytosine and Guanine
– The sequence of the rungs are the individual genetic code
that cause the developing organism to grow arms, wings, skin,
or scales.
Figure 2.1 – The Double Helix of DNA
Beginnings of Life – Mitosis &
Meiosis
• Life begins as a single cell or zygote that
divides repeatedly.
• There are TWO types of cell division.
• Mitosis and Meiosis
Mitosis
• Mitosis: genetic code carried into new cells in
our bodies
• DNA breaks apart (unzips); and the double
helix duplicates.
• DNA forms two camps on either side of cell;
cell divides. Each incomplete rung combines
with its partner to form a new ladder;
resulting identical copies of the DNA strand
separate when cell divides; each is newly
formed cell; genetic code is identical in new
cells unless mutations occur through
environmental influences such as radiation;
mutations occur by chance
Figure 2.2 – The Double Helix of DNA
Meiosis
• Sperm and ova are produced through
meiosis or reduction division.
• 46 chromosomes within the cell nucleus first
line up into 23 pairs.
• DNA ladders unzip, leaving unpaired halves
of chromosome; when cell divides each
member of each pair goes to each newly
formed cell.
• Each new cell nucleus contains only 23
chromosomes, not 46.
• 23 chromosomes come from the mother and
23 chromosomes come from the father; 22
pairs are autosomes and 23rd pair are sex
chromosomes.
Identical and Fraternal Twins
• Sometimes a zygote divides into two cells that separate
and becomes two individuals or twins. There are two types
of twins:
– Monozygotic (MZ): zygote divides into two cells that
separate so each develops into individual with same
genetic makeup - IDENTICAL
– Dizygotic (DZ): two ova are produced in the same
month and they are each fertilized by a different sperm
cell - FRATERNAL
• DZ runs in families through the maternal side; if
mother or grandmother was a twin, or mother has
had twins already the chances increase of her
bearing twins again
• Ovulation: as a woman nears end of child-bearing years,
ovulation becomes less regular causing multiple ovum to
be released some months, thus increasing likelihood of
twins
• Fertility Drugs also increase the chances of multiple births
by causing more than one ovum to be released.
Dominant and Recessive Traits
• Traits are determined by pairs of genes; each member
of a pair of genes is termed an allele.
• Homozygous: both of the alleles for a trait are the
same
• Heterozygous: alleles for a trait differ
• Incomplete dominance/codominance: effects of
both alleles are shown - a sort of “averaging”
• Dominant trait: trait whose influence will be shown
each time the gene is present
• Recessive trait: trait whose influence will be shown
only when it is paired with a second recessive gene
• Carriers: people who bear one dominant gene and
one recessive trait
Figure 2.4 – Transmission of Dominant and
Recessive Traits
Figure 2.1 – Examples of Dominant and
Recessive Traits
LO2 Chromosomal
Abnormalities
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Chromosomal Abnormalities
• Chromosomal or genetic abnormalities can
cause health problems.
• Some disorders are caused by abnormalities in
the 22 pairs of autosomes.
• Others are found in the 23rd pair (sex
chromosomes).
• Some genetic abnormalities are caused by
combinations of genes called:
– Multifactorial problems
– These reflect both a predisposition AND
environmental contributors.
Down Syndrome
• Caused by an extra chromosome of the 21st
pair, resulting in 47 chromosomes.
• Probability of having a Down Syndrome child
increases with advancing age of parents.
• Characteristic features:
– rounded face, protruding tongue, broad, flat nose,
sloping fold of skin over the inner corners of the
eyes
• Show deficits in cognitive and motor
development
• Typically die from cardiovascular problems by
middle age, although modern medicine has
extended life expectancy.
The development and
adjustment of children
with Down syndrome
are related to their
acceptance by their
families. Children with
Down syndrome who
are reared at home
develop more rapidly
and achieve higher
levels of functioning
than those who are
reared in institutions.
© U.P. Images/iStockphoto.com / © Tomasz Markowski/iStockphoto.com
Figure 2.5 – Down Syndrome
Sex-Linked Chromosomal Abnormalities
• Sex-linked chromosomal abnormalities:
disorders stemming from abnormal
number of sex chromosomes
• Most individuals with disorder are
infertile.
• Approx. 1 male in 700/1000 has extra Y
chromosome resulting in heightened
male secondary sex characteristics
Sex-Linked Chromosomal Abnormalities
• XYY’s somewhat taller and develop
heavier beards.
• Once termed “supermales” due to the
characteristics.
• But often have more problems than XY’s
• Such as mild delays in language
development
Klinefelter Syndrome (XXY)
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1 male in 500 has syndrome
Caused by extra X sex chromosome
Produces less testosterone than normal males
Therefore, the testes, deepening of voice,
musculature, and male pattern of body hair does not
develop properly.
• Usually have enlarged breasts (gynecomastia)
• Typically mildly retarded, particularly language
• Treated with testosterone replacement therapy, they
see improvement in sex characteristics and mood
elevation but remain infertile.
Turner Syndrome (X)
• Approx. 1 girl in 2,500 has syndrome
• Female has single X chromosome.
• External genitals are normal, ovaries poorly
developed, producing little estrogen
• Shorter than average and infertile
• Cognitive deficits with low estrogen:
problems with visual-spatial skills,
mathematics, and nonverbal memory
Triple X Syndrome (XXX)
• Approx. 1 girl in 1,000 has syndrome
• Normal in appearance
• Tend to show lower-than-average language
skills
• Poorer memory for recent events
• Development of external sex organs appear
normal
• Increased incidence of infertility
LO3 Genetic Abnormalities
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Genetic Abnormalities
• A number of disorders attributed to genes:
– Phenylketonuria (PKU)
– Huntington’s Disease
– Sickle-Cell Anemia
– Tay-Sachs Disease
– Cystic Fibrosis
– Sex-Linked Genetic Abnormalities
Phenylketonuria (PKU)
• Enzyme disorder transmitted by a recessive
gene affecting 1 child in 8,000.
• Cannot metabolize an amino acid called
phenylalanine; builds up in body and impairs
functioning of the central nervous system
(CNS)
• Results are mental retardation, psychological
disorders, physical problems
• No cure, but PKU can be detected in new
born children through blood or urine analysis;
if identified placed on diets low in
phenylalanine within three weeks of birth and
develop normally
Huntington’s Disease
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Affects approx. 1 in 18,000 Americans
Fatal, progressive degenerative disorder
Dominant trait
Physical symptoms include uncontrollable
muscle movements
Psychological symptoms include loss of
intellectual functioning and personality change
Onset during middle adulthood
Half of their offspring will have disorder
Affects 1 in 18,000 Americans
No cure, but helpful medicines
Sickle-Cell Anemia
• Caused by a recessive gene
• Most common among African Americans
– 1 in 10 African Americans
– 1 in 20 Latino/a Americans
• Red blood cells take on the shape of a sickle
and clump together, obstructing blood and
oxygen supply.
• Decreased oxygen can impair cognitive and
academic functions.
• Physical problems include: painful swollen
joints, jaundice, and potentially fatal
conditions such as pneumonia, stroke, and
heart and kidney failure
Tay-Sachs Disease
• Caused by recessive gene
• Causes CNS to degenerate resulting in death
• Commonly found among children in Jewish
families of Eastern European background
• 1 in 30 Jewish Americans carry recessive
gene
• Children with disorder progressively lose
control of muscles experiencing visual and
auditory sensory losses, develop mental
retardation, become paralyzed, and die by
end of early childhood (age 5).
Cystic Fibrosis
• Caused by recessive gene
• Most common fatal hereditary disease
among European Americans
• Approx. 30,000 Americans have disorder, 10
million more are carriers (1 in 31 people)
• Children suffer from excessive production of
thick mucus that clogs the pancreas and
lungs.
• Most victims die of respiratory infections in
their 20s.
Sex-Linked Genetic Abnormalities
• Genetic defects only carried on the X sex
chromosome
• Hemophilia: inability of the blood to clot
© Photo 12/The Image Works
– Queen Victoria was a carrier
of hemophilia
Sex-Linked Genetic Abnormalities
• Duchenne Muscular Dystrophy: weakening of
muscles, inability to walk, general wasting
away, and sometimes death
• Involve recessive genes
• Females with two X sex chromosomes are
less likely to show sex-linked disorder.
• Sons of female carriers are more likely to be
afflicted.
LO4 Genetic Counseling
and Prenatal Testing
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Genetic Counseling and Prenatal Testing
• Genetic counselors compile information
about a couple’s genetic heritage to explore
if their children will have a genetic
abnormality.
• Couples with likelihood of passing on genetic
abnormality can make informed decision as
to alternative family plans, such as adoption.
• Prenatal testing can also indicate if the
embryo or fetus is carrying genetic
abnormalities.
• Amniocentesis, Chorionic Villus Sampling,
Ultrasound, and Blood Tests are examples of
prenatal testing.
Amniocentesis
• Performed on mother usually around 14 – 16 weeks
after conception, although sometimes earlier
• Amniotic Fluid containing cells sloughed off by fetus is
withdrawn from the amniotic sac with a syringe.
• Cells are separated, grown in a culture, and examined
for chromosomal abnormalities.
• Routine test for women over 35 to detect for Down’s
Syndrome as chances for syndrome dramatically
increase as women approach age 40.
• Test also can determine the sex of child.
• Amniocentesis does carry some risk of miscarriage.
– Approx. 1 in 100
Figure 2.6 – Amniocentesis
Chorionic Villus Sampling (CVS)
• Similar to Amniocentesis
• Performed between 9 - 12 weeks
• Syringe inserted through vagina into uterus
and sucks out threadlike projections (villi)
from the outer membrane that covers the
amniotic sac and fetus.
• Results available in days
• CVS slightly higher risk than amniocentesis
of spontaneous abortion; both increase the
risk of miscarriage
Ultrasound
• Sound waves that are too high in frequency
to be heard by human ear are used to obtain
information about the fetus.
• Ultrasound waves are reflected by the fetus,
and the computer uses the information to
generate a picture of the fetus.
• Picture is termed a sonogram.
• Used to guide the syringe in amniocentesis
and CVS and to locate fetal structures when
intrauterine transfusions are needed for
survival of fetus as with Rh disease
• Used to track growth of fetus, detect multiple
pregnancies, detect structural abnormalities
• Procedure is able to detect sex of fetus
Blood Tests
• Used to identify sickle-cell anemia, Tay-Sachs
disease, and cystic fibrosis
• Alpha-fetoprotein (AFP) assay used to
detect neural tube defects such as spina bifida
and chromosomal abnormalities.
• Neural tube defects cause elevation in the
AFP level in the mother’s blood.
• High AFP levels related to increased risk of
fetal death.
LO5 Heredity and the
Environment
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Heredity and the Environment
• Inheritance, nutrition, learning, exercise, accident, and
illness all influence development of traits.
• Genotypes
– Set of traits we inherit from our parents (our blueprint)
• Phenotypes
– Actual set of traits; develop because of both genetic and
environmental influences (what actually transpires)
• Canalization
– Environmental conditions may prevent an individual from
reaching their full potential, but if environmental influences
improve, there is a tendency to “snap back” to the genetically
determined “canal.”
– Personality and intelligence are apparently less canalized
(environment playing stronger role than with physical
development).
Genetic - Environmental Correlation
• One problem in sorting out the influences of heredity (nature) and
environment (nurture) is that genes partly determine the
environments to which people are exposed.
• Psychologist Sandra Scarr describes 3 types of correlations
between genetic and environmental influences as they relate to
age of individuals:
– Passive Correlation
• Parents intentionally and unintentionally place children in certain
environments; it is called passive because child has no choice
– Evocative Correlation
• A child’s genotype is connected with behaviors that elicit or
evoke certain types of social responses from others, and these
responses in turn become part of the environment of the child
– Active Correlation
• As we mature, we become more proactive in creating our
environment.
• Choosing environments that allow us to develop inherited
preferences is termed niche-picking.
The Epigentic Framework
• The relationship between genetic and environmental
influences is not a one-way street; it is
BIDIRECTIONAL.
• Genes affect the development of
traits and behaviors, but likewise
traits and behaviors lead us to
certain environments.
• According to Epigenesis,
development is a continuum of
bidirectional exchanges between
nature and nurture.
© Brand X Images/Jupiterimages
Strategies for Understanding
• Research strategies used to aid in sorting out
effects of heredity and environment
– Kinship Studies
• Study distribution of traits among relatives with differing
degrees of relatedness
– If genes are dominant in manifesting a trait, people who are
most closely related should be more likely to share it.
– Twin Studies
• Study both MZ twins sharing 100% genes and DZ twins
sharing 50% genes
– Study of MZ twins raised apart from infancy show similarities
much the same as those raised together, leading us to
believe there is a strong genetic component to traits.
– Adoption Studies
• Study children separated from biological parents at an
early age
– When children raised by adoptive parents demonstrate more
similarities of traits with natural parents, again a strong
argument for genetic predominance seems to prevail.
LO6 Conception: Against
All Odds
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CONCEPTION: The Players (Ova & Sperm)
• Conception: union of an ovum and a sperm
cell that occurs when the chromosomes of
each combine to form 23 new pairs
– Ova
• Women are born with approx. 400,000 ova - all they will
ever have but in an immature form
• During woman’s reproductive years, approx. 400 (1 in
1,000) will ripen and be released.
• Ova are much larger than sperm cells but barely visible to
the human eye.
– Sperm
• Develop in stages, starting with 46 chromosomes, and
after meiosis ends with 23 - half X’s and half Y’s
• Each is about 1/500th of an inch - one of the smallest
types of cells in the body.
CONCEPTION: The Journey & Meeting
Against All Odds
• Journey of the OVUM
– At signal from female hormones (estrogen and progesterone),
some ova begin to mature (puberty).
– Approx. once a month one, midway through menstrual cycle,
one (maybe more) is released from follicle into nearby fallopian
tube
– Takes 3 to 4 days to reach the uterus
– Not self-propelled; need assistance from cilia (small hair-like
structures) and contractions of tube wall
– If not fertilized, it is discharged through
uterus and vagina along with endometrium
formed to support an embryo in the menstrual
flow.
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Figure 2.8 – Female Reproductive Organs
CONCEPTION: The Journey & Meeting
Against All Odds
• Journey of the SPERM
– Y chromosomes have greater motility (faster
swimmers).
– 120 to 150 boys are conceived for every 100 girls.
• Male fetuses experience higher rate of
spontaneous abortion during first month of
pregnancy.
• At birth, boys outnumber girls by ratio of
106:100.
• Boys also have higher incidence of infant
mortality, further equalizing sex ratio.
CONCEPTION: The Journey & Meeting
Against All Odds
• Journey of the SPERM
– Approx. 150 million sperm ejaculated but only 1 in
1,000 will approach an ovum
– Sperm surviving initial obstacles may reach
fallopian tubes 60 to 90 minutes after ejaculation.
• About 2,000 may finally enter correct tube.
• Still fewer manage to maneuver the final 2
inches against current
• Journey is “blind” but not random.
• Sperm are “egged on”  by a change in calcium
ions that occurs when an ovum is released by a
follicle.
CONCEPTION: The Journey & Meeting
Against All Odds
• Destination: Conception
– Only ONE sperm that reach an egg enters.
– Ova are surrounded by a gelatinous layer that must
be penetrated for fertilization to occur.
– Many sperm secrete an enzyme that can briefly thin
the layer.
• But it only allows one sperm to penetrate.
– Once a sperm has entered, CONCEPTION has
taken place and the layer thickens, locking other
sperm out.
– Chromosomes from the sperm cell line up across
from corresponding chromosomes within the egg
cell.
– Form 23 new pairs with unique set of genetic
instructions
LO7 Infertility
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INFERTILITY
• Approximately 1 in 6 American couples
experiences problems with conception.
• The term infertility is not usually applied
until conceiving on their own has been
unsuccessful for 1 year.
• Once viewed as primarily a female
problem
• It is a male problem 40% of the time.
INFERTILITY PROBLEMS IN MEN
• Low sperm count
– Most common problem in men
• Deformed sperm
• Genetic factors
• Environmental poisons
• Low motility (poor swimmers)
• Infectious diseases (such as STI’s)
• Chronic diseases (such as diabetes)
• Injury of the testes
– Overheating, as seen in long distance runners or from
using narrow bicycle seats
• An “autoimmune” response
– Man’s body attacks his own sperm as a foreign agents
INFERTILITY PROBLEMS IN WOMEN
• Irregular ovulation or failure to ovulate
– Most common problem in women
• Declining hormone levels
– So-called “fertility” drugs (e.g., clomiphene & pergonal) may cause
multiple births by stimulating more than one ovum to ripen during a
month
• Endometriosis
– Inflammation of the tissue that is sloughed off during menstruation
• Can be treated with hormones or surgery
• More common among women who delay childbearing
• Obstructions/malfunctions of reproductive tract
– Infections may scar the fallopian tubes and other organs impeding
passage of sperm or ova
– Pelvic inflammatory disease (PID)
• From bacterial or vial infections, including STIs gonorrhea &
chlamydia
• Antibiotics may help infections but infertility may be irreversible.
Methods Used to Help Infertile
Couples Bear Children
• Artificial Insemination
– Sperm collected and quick-frozen
– Injected into woman’s uterus at ovulation
• In Vitro Fertilization (IVF) “test-tube” babies
– Ova and sperm are placed in a laboratory dish for
fertilization.
– Injected for implantation into mother’s uterus for
gestation
– May take several attempts because only a minority
of tries meet with success; several embryos may be
injected at once
– Performed rather routinely; expensive; no
guarantees
Methods Used to Help Infertile Couples
Bear Children
• Surrogate Mothers
– Women who carry babies to term for other women
– Usually paid and sign a contract to surrender
parental rights to child
• Can sometimes be a woman and man’s ova and
sperm fertilized IVF and implanted in surrogate
• Or can be surrogates ova and artificial
insemination by man if woman’s ovum are not
viable
Methods Used to Help Infertile
Couples Bear Children
• Adoption
© Alex Wong/Getty Images
– May be occasional conflicts between adoptive
parents and biological parents who change their
minds
– Can be time intensive; many Americans find it
easier to adopt foreign infants or children with
special needs
Selecting the Sex of Your Child
• Preimplantation Genetic Diagnosis (PGD)
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Developed to detect genetic disorders
Also revels sex of embryo
Ova are fertilized in vitro
After a few days of cell division, chromosomal
structure examined microscopically to reveal sex
– Embryos of the desired sex are implanted where
one or more may grow to term
– Again...implantation cannot be guaranteed.