2' Genetic Bases of Child Development

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2. Genetic Bases of Child Development

• 2.1 Mechanisms of Heredity
• Each egg and sperm contain 23 chromosomes, tiny
  structures in the nucleus that contain genetic
  material. When they combine, there are 23 pairs
  of chromosomes.

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New Techniques

• To couples who cannot conceive, there are new
  techniques such as in vitro fertilization.
• Sperm and egg are put together in a lab dish and
  then the fertilized eggs are placed in the
  mothers uterus. Within 24 hours they will
  hopefully implant in the uterus wall. Only 20
  of these attempts are successful.

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Chromosomes

• 23 pairs define a childs heredity. The first 22
  pairs are called autosomes (each pair is about
  the same size). The 23rd pair, determines the
  childs sex and the X is larger than the Y
  chromosome. An X chromosome is always contained
  in the egg. The sperm contains an X or a Y. A
  pairing of two X chromosomes results in a girl
  and the pairing of an X and a Y results in a boy.

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DNA and Genes

• Each chromosome consists of a molecule of
  deoxyribonucleic acid (DNA) and carries the
  genetic code consisting of pairs of nucleotide
  bases. Each group of nucleotide bases that
  provides a set of biochemical instructions is a
  gene. Genes regulate the development of all
  human abilities and characteristics.

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Genes

• A complete set of genes makes up a persons
  heredity and is known as the individuals
  genotype. Genetic instructions along with the
  environment produces a phenotype, an individuals
  physical, behavioral, and psychological
  characteristics.

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Single-Gene Inheritance

• Genes come in different forms known as alleles.
  When alleles are the same, they are homozygous
  and when they are different, they are
  heterozygous.

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Gene Inheritance

• How does a genotype become a phenotype?
• If the individual is homozygous, the alleles are
  the same, the instructions are the same and that
  phenotype results. If an individual is
  heterozygous, the results are more complex.
  Usually one allele is dominant (instructions
  followed) and the other recessive (instructions
  ignored). However,

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Gene Inheritance

• Sometimes one allele does not dominate
  completely, which is known as incomplete
  dominance. With incomplete dominance, the
  phenotype that results often falls between the
  phenotypes associated with either allele. How
  does this process work in the case of sickle cell
  disease?

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Single Gene Inheritance
Figure 2-2
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Behavioral Genetics

• Behavioral genetics deals with the inheritance of
  behavioral and psychological traits which are
  complex and represent a range of outcomes.
  Phenotypes that reflect the activity pattern of
  many different genes is known as Polygenic
  Inheritance.

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Polygenic Inheritance

• When so many genes are involved in polygenic
  inheritance, it is almost impossible to trace the
  effects of each gene. The example shows a
  behavioral phenotype that spans a continuum.

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Polygenic Inheritance
Figure 2-3
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Methods of Behavioral Genetics

• Twin studies heredity is implicated when
  identical twins (monozygotic, from a single
  fertilized egg) are more alike than fraternal
  twins (dizygotic, from two eggs fertilized by two
  sperm). In studies, the two types of twins are
  compared to measure the influence of heredity.

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Studies

• Adoption studies heredity is implicated when
  children are more like their biological parents
  than their adoptive parents.
• There are problems with both types of studies.
  Identical twins may be treated more alike than
  fraternal twins, creating more similar
  environment.

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Studies

• Often, adoption agencies place children in homes
  with adoptive parents that are similar to the
  homes of their biological parents. Therefore,
  the environments are alike.

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DNA Marker

• Individuals are identified that differ in the
  trait of interest and their DNA is analyzed.
• DNA markers for the groups are compared to
  isolate specific alleles that may contribute to a
  particular behavior or psychological trait.

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DNA Marker

• Techniques like the one mentioned have the
  potential to identify the many different genes
  that contribute to complex behavioral and
  psychological traits.

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Psychological Characteristics Most Affected by
Heredity

• Intelligence
• Psychological Disorders
• Personality

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Studies

• Research has consistently shown that the I.Q.
  test scores of identical twins is more alike than
  that of fraternal twins.
• Identical twin studies show that heredity is an
  important component for the personality disorders
  of depression and schizophrenia.
• Look at the Hereditary Environmental Roots of
  Infants Social Behavior.

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2.2 Genetic Disorders

• Inherited Disorders sickle-cell disease, PKU
• Abnormal Chromosomes Down syndrome, Turners
  syndrome What else could be added to the list?

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Inherited Disorders

• Inherited through 2 recessive Alleles
• Sickle cell disease (trouble with oxygenation of
  body and blockage of white blood cells), Albinism
  (skin lacks melanin), Cystic Fibrosis (excess
  mucus clogs respiratory tracts), Phenylketonuria
  (PKU) (Phenylalanine accumulates in the body and
  damages the nervous system), Tay-sachs disease
  (nervous system degeneration in infancy).

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Inherited Disorders

• Serious disorders caused by dominant alleles are
  rare. One exception is Huntingtons disease ( a
  disease characterized by degeneration of the
  nervous system in middle adulthood).

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Genetic Counseling

• Genetic counseling is used to construct a family
  history of possible genetic disorders.
• Samples of blood from the prospective parents can
  be tested in advance of a planned pregnancy to
  determine any genetic problems.
• Tests can also be conducted during pregnancy to
  look for genetic disorders.

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Abnormal Chromosomes

• Disorders caused by the wrong number of
  chromosomes is more common inherited disorders.
• Down Syndrome is the best example of one too many
  chromosomes on the 21st chromosome, usually
  contributed by the egg. Older mothers have a
  higher risk of having a Down Syndrome child.

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Abnormal Chromosomes

• Abnormal sex chromosomes can disrupt development.
  Examples are Klinefelters Syndrome (male with
  XXY) XYY is the complement ( both cases have
  below normal intelligence), Turners Syndrome
  (female with X) limited secondary sex
  characteristics, XXX has delayed language and
  motor development.

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Abnormal Chromosomes

• Notice that there are no disorders with only a Y
  chromosome, it seems the presence of the X
  chromosome is necessary for life. What can you
  conclude from this?

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2.3 Heredity is Not Destiny

• Paths from Genes to Behavior
• Reaction Range
• Changing Relations Between Nature and Nurture
• The Nature of Nurture

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Paths from Genes to Behavior

• Genes affect behavior indirectly, genes are fixed
  but phenotypes are not.
• The impact of genes on behavior depends on the
  environment in which the genetic instructions are
  carried out.

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Reaction Range

• Heredity and environment determine the direction
  of development. Reaction range refers to the
  fact that a genotype is manifested in reaction to
  the environment where development takes place.
  The example will illustrate how phenotypic
  intelligence might vary, depending on the
  environment. Lets look at the variation.

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Reaction Range
Figure 2-5
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Changing Relations between Nature and Nurture

• passive gene-environment relation parents
  provide genes and much of the early environments.
  This relationship is common with infants and
  young children.

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Changing Nature/Nuture Relationships

• evocative gene-environment relation genotypes
  evoke different responses from the environment. A
  young childs genotype evokes or prompts people
  to respond differently to the child.
• active gene-environment relation children
  actively seek environments that match their
  genetic makeup, called niche-picking.

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The Nature of Nurture

• Siblings are not much alike. Why?
• Family environments affect each child differently
  (nonshared environmental influences are the
  forces that make children different from one
  another).

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Non-shared Environments

• Studies show that siblings are not alike in their
  cognitive and social development.
• Contrasting parental influences tend to make
  siblings different, parents provide a unique set
  of genes and a unique environment.

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Figure 2-6
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Summary

• The 46 chromosomes are a roadmap guiding
  development. Behavioral development results from
  polygenic inheritance, or combined effects of
  many pairs of genes and the environment.
  Heredity can disrupt development with inherited
  diseases or the wrong number of chromosomes.

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Summary

• A single genotype can lead to many phenotypes and
  genes can influence experience. Development is
  always a joint process influenced by both
  heredity and environment.