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Chapter 3
Forming A New
Life:
Conception,
Heredity, and
Environment
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prior written consent of McGraw-Hill Education.
How Fertilization Takes Place
 Fertilization
 Union of sperm and ovum to produce a zygote.
 Also called conception
 Zygote:
 One-celled organism resulting from fertilization.
 Duplicates itself by cell division to create a baby
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How Fertilization Takes Place
 Women
 At birth, have 2 million ova in their ovaries, each
contained in a follicle.
 During ovulation, when sexual maturity is attained, a
mature follicle is ruptured and the ovum is expelled
from the ovary.
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How Fertilization Takes Place
 Men
 Several hundred million sperm are produced in the
testicles each day.
 Sperm enter the vagina through ejaculation and attempt
to reach the cervix.
 Few will arrive in the fallopian tubes where fertilization
takes place.
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Infertility
 Inability to conceive a baby after 12 months of trying.
 Women’s fertility begins to decline in the late 20s.
 Men’s fertility begins to decline in the late 30s.
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Table 3.1 - Common Causes of
Infertility in Men and Women
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Table 3.1 - Common Causes of
Infertility in Men and Women
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Treatments for Infertility
Hormone
Treatment
Drug Therapy
Surgery
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Alternative Ways to
Parenthood
 In vitro fertilization (IVF) - Increases the likelihood of
multiple, usually premature, births.
 In vitro maturation (IVM) - Diminishes the likelihood of
multiple births.
 Performed earlier in the monthly cycle
 Makes hormone injections unnecessary
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Alternative Ways to
Parenthood
 Male infertility
 Intracytoplasmic sperm injection (ICSI)
 Artificial insemination
 Artificial insemination by a donor (AID)
 Gamete intrafallopian transfer (GIFT)
 Zygote intrafallopian transfer (ZIFT)
 Surrogate motherhood
 Surrogate - Fertile woman impregnated by a prospective
father by artificial insemination.
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Mechanisms of Heredity
 Heredity
 Genetic transmission of heritable characteristics from
parents to offspring.
 Genetic code
 Sequence of bases within the DNA molecule.
 Set of rules that govern the formation of proteins that
determine the structure and functions of living cells.
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Figure 3.1 - DNA: The Genetic
Code
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Genetic Code
 Chromosomes: Coils of DNA that consist of
genes.
 Genes: Small segments of DNA located in
definite positions on particular
chromosomes.
 Functional units of heredity
 Located in a definite position on chromosome
 Human genome: Complete sequence of
genes in the human body.
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Cell Division
 Meiosis
 Type of cell division which the sex cells undergo when they
are developing.
 Crossing Over
 Each sex cell ends up with only 23 chromosomes.
 Mitosis
 Process by which the nonsex cells divide in half over and over
again.
 DNA replicates itself, so that each newly formed cell has the
same DNA structure as all the others.
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Genotypes and Phenotypes
 Phenotype: Observable characteristics of a person.
 Genotype: Genetic makeup of a person, containing
both expressed and unexpressed characteristics.
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Patterns of Genetic Transmission
 Dominant and recessive inheritance
 Every offspring gets a pair of alleles for each
characteristic, one from each parent.
 Alleles: Two or more alternative forms of a gene that can
occupy the same position on paired chromosomes and
affect the same trait.
 Homozygous: Possessing two identical alleles for a trait.
 Heterozygous: Possessing differing alleles for a trait.
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Patterns of Genetic
Transmission
 Polygenic inheritance
 Pattern of inheritance in which multiple genes at
different sites on chromosomes affect a complex trait.
 Traits may also be affected by mutations.
 Mutations: Permanent alterations in genes or
chromosomes.
 Multifactorial transmission
 Combination of genetic and environmental factors to
produce certain complex traits.
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Epigenesis
 Mechanism that turns genes on or off and determines
functions of body cells.
 Chemical molecules that are attached to a gene.
 Epigenetic changes can respond to environmental
factors such as nutrition, sleep habits, stress, and
physical affection.
 Epigenetic modifications may be heritable.
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Table 3.2 - Some Birth
Defects
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Table 3.2 - Some Birth
Defects
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Table 3.2 - Some Birth
Defects
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Dominant or Recessive
Inheritance of Defects
 Dominant traits
 Achondroplasia - Type of dwarfism
 Huntington’s disease
 Recessive traits
 Tay-Sachs
 Sickle-cell anemia - Can be incomplete dominance
 Incomplete dominance: Pattern of inheritance in which
a child receives two different alleles, resulting in partial
expression of a trait.
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Sex-Linked Inheritance of
Defects
 Sex-linked inheritance: Pattern of inheritance in which
certain characteristics carried on the X chromosome
inherited from the mother are transmitted differently to
her male and female offspring.
 Certain recessive disorders are linked to genes on the sex
chromosomes.
 Male and female children affected differently.
 Carriers - Heterozygote females who carry one bad copy
of a recessive gene and one good one.
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Figure 3.3 - Genetic and
Chromosomal Abnormalities
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Chromosomal Abnormalities
 Errors in cell division
 Extra or missing chromosome
 Triple X syndrome
 Klinefelter syndrome XXY
 Turner syndrome XO
 Down syndrome: Chromosomal disorder characterized by
moderate-to-severe mental retardation and by such
physical signs as a downward-sloping skin fold at the inner
corners of the eyes.
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Table 3.3 - Genetic and
Chromosomal Abnormalities
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Genetic Counseling
 Clinical service that advises prospective parents of
their probable risk of having children with hereditary
defects.
 Karyotype - Chart that can show chromosomal
abnormalities.
 Indicates whether a person who appears normal might
carry a genetic defect that could be transmitted to a child.
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Relative Influences of Heredity
and Environment
 Behavioral genetics: Quantitative study of relative
hereditary and environmental influences on behavior.
 Heritability: Statistical estimate of contribution of
heredity to individual differences in a specific trait
within a given population at a particular time.
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Relative Influences of Heredity
and Environment
 Measuring heritability - Estimating how much of a trait
is due to genetics and how much is the result of
environmental influences.
 Types of heritability studies
 Family studies - Measure the degree to which biological
relatives share certain traits.
 Whether the closeness of the familial relationship is associated
with the degree of similarity
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Relative Influences of Heredity
and Environment
 Adoption studies - Look at similarities between adopted
children and their adoptive parents.
 Studies on twins - Compare pairs of monozygotic twins
with same-sex dizygotic twin.
 Concordant - Describing the tendency of twins to share the
same trait or disorder.
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How Heredity and Environment
Work Together
 Developmental system - From conception on, a
combination of constitutional, social, economic,
and cultural factors help shape development.
 More advantageous circumstances and experiences lead
to increased likelihood of optimum development.
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Reaction Range and
Canalization
 Reaction Range: Potential variability, depending on
environmental conditions, in the expression of a
hereditary trait.
 Canalization: Limitation on variance of expression
of certain inherited characteristics.
 Highly canalized traits - Eye color, for example, strongly
programmed by genes with little opportunity for
variance in their expression.
 Cognition and personality are not highly canalized.
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Genotype-Environment
Reaction
 Genotype-environment interaction: Effect of the
interaction between genes and the environment on
phenotypic variation.
 Genotype-environment correlation: Tendency of
certain genetic and environmental influences to
reinforce each other; may be passive, reactive
(evocative), or active.
 Also called genotype-environment covariance
 Genetically similar children often develop differently
depending on their home environments.
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Genotype-Environment
Reaction
Passive
• Parents, who provide the genes that
predispose a child toward a trait, also
tend to provide an environment that
encourages the development of that trait.
Reactive or
evocative
• Children with differing genetic makeups
evoke different responses from adults.
Active
• As children grow older, they select
experiences consistent with their genetic
tendencies.
• Niche picking - Tendency to seek out
environments compatible with one’s
genotype.
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Nonshared Environment
 Nonshared environmental effects: Unique
environment in which each child grows up,
consisting of distinctive influences or influences
that affect one child differently from another.
 Effects of experience on development
 Interactions of parenting
 Nonfamilial influences
 Broader context in which families live
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Characteristics Influenced by
Heredity and Environment
 Physical and physiological traits - Monozygotic twins
more concordant than dizygotic twins in risk for
medical disorders.
 Obesity: Extremely overweight in relation to age, sex,
height, and body type.
 Intelligence - Heredity exerts a strong influence on
general intelligence and lesser extent on specific
abilities such as memory, verbal, and spatial ability.
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Characteristics Influenced by
Heredity and Environment
 Personality - Genes directly linked with specific aspects
of personality.
 Temperament: Characteristic disposition, or style of
approaching and reacting to situations.
 Psychopathology - Evidence for a strong hereditary
influence on such mental disorders as schizophrenia,
autism, alcoholism, and depression.
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Characteristics Influenced by
Heredity and Environment
 Schizophrenia: Neurological disorder marked by:
 Loss of contact with reality
 Hallucinations and delusions
 Loss of coherent, logical thought
 Inappropriate emotionality
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