Chapter 3 The Genetic Bases of Human Development

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Chapter 3The Genetic Bases of Human Development

• Human Development

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Mechanisms of Heredity

• The Biology of Heredity
• On average, an ejaculation of semen into the
  vagina contains 200 to 500 million sperm.
• Only a few hundred sperm complete the journey to
  an egg.
• Each egg and sperm cell contains 23 chromosomes,
  which are tiny structures in the nucleus that
  contain genetic material.

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Mechanisms of Heredity

• When a sperm penetrates an egg, their chromosomes
  combine to produce 23 pairs of chromosomes.
• Thus, the development of a new human being is
  underway.

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Mechanisms of Heredity

• The first 22 pairs of chromosomes are called
  autosomes and are about the same size.
• In the 23rd pair, however, the chromosome labeled
  X is much larger than the chromosome labeled Y.
• It is the 23rd pair which determines the sex of
  the child--- called the sex chromosomes.

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The Mechanisms of Heredity

• An egg always contains an X 23 rd chromosome, but
  a sperm contains either an X or Y.
• When an X-carrying sperm fertilizes an egg, the
  23rd pair is XX which results in a girl.
• When a Y carrying sperm fertilizes the egg, the
  23rd pair is XY which results in a boy.

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The Mechanism of Heredity

• Each chromosome actually consists of one molecule
  of deoxyribonucleic acid, also known as DNA.
• Each group of nucleotide bases that provides a
  specific set of biochemical instructions is a
  gene.

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The Mechanisms of Heredity

• The complete set of genes that makes up a
  persons heredity is known as a persons
  genotype.
• Genetic instructions, in conjunction with
  environmental influences, produce a phenotype,
  which is an individuals physical, behavioral,
  and psychological features.

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The Mechanisms Of Heredity

• In June of 2000, Former President Clinton
  gathered with a group of leading scientists to
  announce a historic milestone.
• The announcement was that a working draft of the
  human genome was complete.
• The project came to be known as the Human Genome
  Project, a multinational effort among governments
  and scientists to map the order of every
  nucleotide base and locate the position of every
  gene in the human genome.

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The Mechanisms of Heredity

• A genome is the set of genes that provides the
  instruction for an entire organism.
• The Human Genome Project was started in 1990 and
  has been the most ambitious and technically
  challenging project ever undertaken by scientists.

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The Mechanisms of Heredity

• Scientists now have a rough map of the order of 3
  billion nucleotide bases in the human genome, but
  scientists do not know what all of the base
  sequences mean.

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The Mechanisms of Heredity

• When the entire project is complete, scientists
  will know the genetic markers for every disease
  that can be inherited.
• This information may then help save or prolong
  millions of lives.
• For example, if hemophilia runs in your family,
  you will be able to take a genetic test to find
  out of you carry the gene.

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How Genes and Environment Interact

• Developmental psychologists call the interaction
  of genetics and the environment the G X E
  interaction.
• This term refers to the ways in which nature
  (represented by genetics and nature) combines
  with nurture (the environment) to produce a given
  outcome.

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How Genes and Environment Interact

• One way to understand the interaction between
  genes and the environment is to recognize that an
  individuals genetic material (genotype)
  establishes boundaries on the possible phenotype,
  or observable traits, that can occur.
• For any individual, the various possible
  phenotypic outcomes are the range of reaction for
  that genotype (Gottesman, 1963).

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How genes and Environment Interact

• The key with this is that genes set the
  boundaries for the range of reaction, but the
  environment determines which possible outcomes
  actually materialize

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How Genes and Environment Interact

• Another way to think about G X E interactions
  emphasizes how genetics can help protect children
  from the effects of the environment.
• Canalization refers to the way genes limit
  developmental outcome (Waddington, 1942, 1957).

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How Genes and Environment interact

• In other words, genes provide a strong buffer
  against environmental variations, limiting the
  effects that the environment can have.
• A more recent interpretation places less emphasis
  on genetic determination.
• This new model, called experiential canalization,
  gives more weigh to the role of the environment,
  particularly prenatal and early experience
  (Gottlieb, 1997).

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How Genes and Environment interact

• In this view, genetics provides a fairly wide
  range of possible development outcomes and it is
  the environment that plays a limiting role.
• For example, genetics endow all humans with the
  ability to perceive language sounds, but an
  individuals linguistic environment eventually
  limits the sounds they can distinguish.

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How Genes and Environment interact

• To add to the complexity of explanations to
  address genes and environment interaction,
  Probabilistic Epigenesis is an explanation that
  states that the specific environmental conditions
  will activate specific genes that lead to
  particular traits or behavioral outcomes.

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How Genes and Environment interact

• This means that every individual has many more
  potential developmental pathways than are ever
  realized, and it is the individuals environment
  that controls which genes are activated.
• An implication of probabilistic epigenesis is
  that a significant change in the environment will
  change which genes are expressed.

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How Genes and Environment interact

• What this means for human development is that we
  as humans have a vast amount of genetic
  potential, and only a small amount is expressed,
  Which portion is expressed depends on the childs
  environment.

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How Genes and Environment interact

• We as scientist do not have a thorough
  understanding of which environmental factors are
  most important in affecting gene expression such
  as intelligence and personality.
• What Probabilistic Epigenesis does is make a
  strong statement about the relationship between
  genes and the environment.

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Common Phenotypes associated with single pairs of
genes

• Dominant Phenotype characteristics
• Curly hair
• Normal hair
• Detached earlobes
• Ability to roll tongue
• Arched feet
• Dark skin
• Thick lips
• Dimpled cheeks
• Normal hearing and vision
• Farsightedness
• Normal color vision
• Types A and B blood

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Common phenotypes associated with single pairs of
genes

• Recessive phenotypes
• Straight hair
• Male pattern baldness
• Attached earlobes
• Inability to roll tongue
• Flat feet
• Blond hair
• Fair skinned
• Thin lips
• No dimples
• Nearsightedness
• Normal vision as it corresponds to farsightedness
• Type O blood

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Behavioral Genetics

• This is the branch of genetics that deals with
  inheritance of behavioral and psychological
  traits.
• These behavioral and psychological
  characteristics are not either or cases, where
  the inheritance of genes will produce definitive
  results but represent an entire range of
  different outcomes.

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Behavioral Genetics

• For example, extroversion and introversion, which
  occurs on a continuum where there are a few
  people on either extreme but the majority falling
  somewhere in between.

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Behavioral Genetics

• Many behavioral and psychological characteristics
  are distributed in this way, including
  intelligence and many aspects of personality.
• When phenotypes reflect the combined activity of
  many separate genes, the pattern is known as
  polygenic inheritance. Because so many genes are
  involved in polygenic inheritance, scientists
  usually cannot trace the effects of each gene.

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Behavioral Genetics

• Twin studies offer scientists a way to compare
  identical (monozygotic) and fraternal (dizygotic)
  twins to measure the influence of genetic
  inheritance.
• Adopted children are another source of
  information about heredity. In these cases,
  adopted children are compared with their
  biological parents and adoptive parents. The idea
  is that biological parents provide the childs
  genes but adoptive parents influence the
  environment.

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Behavioral genetics

• Twin studies and adoption studies are not
  foolproof.
• Parents and other people may treat identical
  twins more alike than they treat fraternal twins.
• With adoption studies, adoption agencies usually
  try to place children with people who are like
  the biological parents

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Behavioral genetics

• Scientists today are able to move beyond
  traditional ways of studying behavioral genetics.
• Today, it is possible isolate particulars
  segments of DNA in human chromosomes which serve
  as makers for identifying specific alleles ( an
  alternative version of a gene).

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Behavioral genetics

• New techniques have the potential to identify
  many different genes that contribute to complex
  behavioral and psychological traits but there are
  limits to the method, such as large sampling of
  children, which can be hard to get for rare
  disorders.

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Behavioral genetics

• Regarding psychological characteristics that are
  affected by heredity, research indicates that
  genetic influence is strongest in the
  psychological areas of intelligence,
  psychological disorders and personality.
• In the case of intelligence, identical twins
  scores on IQ test are more alike than fraternal
  twins (Goldsmith et al., 1999 Neiderhiser et
  al., 1999).

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Genetic Disorders

• Some children are affected by heredity in a
  special way they have genetic disorders that
  disrupt the usual pattern of development.
• Genetics can derail development in two ways.
• Disorders can be inheritedex. Sickle cell anemia
• Or eggs or sperm have more or fewer than the
  usual 23 chromosomes.

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Genetic Disorders

• Relatively few serious disorders are caused by
  dominant alleles.
• The reason why is due to the fact that if the
  allele for disorder is dominant, every person
  with at least one of these alleles would have the
  disorder.

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Genetic Disorders

• Individuals affected with these disorders
  typically do not live long enough to reproduce,
  so dominant alleles that produce fatal disorders
  soon vanish.
• An exception is Huntingtons disease, a fatal
  disease characterized by progressive degeneration
  of the nervous system.

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Genetic Disorders

• Huntingtons disease is caused by a dominant
  allele found on chromosome 4.
• Individuals who inherit this disease develop
  normally through childhood, adolescence, and
  young adulthood.
• However, during middle age, nerve cells begin to
  deteriorate, causing muscle spasms, depression,
  and significant changes in personality (Shiwach,
  1994).
• By the time symptoms of the disease appear,
  adults affected may have already had children,
  many of whom go on to develop the disease
  themselves.

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The Mechanisms of Heredity

• Even though it would be helpful to know this sort
  of information, what if your genetic
  predisposition were revealed to your employer or
  your health insurance company?
• Or better yet, what if the government could
  decide on who could or could not become parents
  based on this information, would this be a good
  thing? Should people with faulty genes be
  allowed to have children?