Part I Chapter Three The Genetic Code From One Cell to Many

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Part I
Chapter Three
The Genetic Code From One Cell to Many From
Genotype to Phenotype Chromosomal and Genetic
Abnormalities

• Heredity and Environment

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The Genetic Code

• Genes play a leading role in the drama of human
  development, yet they rarely take center stage.
  Genes are pervasive and powerful, but they are
  also hidden and elusive.

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What Genes Are

• DNA (deoxyribonucleic acid)
• Molecule that contains the chemical instructions
  for cells to manufacture various proteins.
• Chromosome
• a molecule of DNA that contains the instructions
  to make proteins
• Humans have 46 chromosomes (23 pairs), and about
  25,000 genes.
• Genome
• the code for making a human being
• Every person has a slightly different code, but
  the human genome is 99.5 the same for any 2
  people.

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What Genes Are

• Genes are as section of chromosomes and the basic
  unit for the transmission of heredity, consisting
  of a string of chemicals that code for the
  manufacture of certain proteins.

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The Beginnings of Life

• development begins at conceptioneach human
  reproductive cell or gamete, contains 23
  chromosomes, half of that persons 46
• gamete
• A reproductive cell that is, a sperm or ovum
  that can produce a new individual if it combines
  with a gamete from the other sex to make a zygote

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The Beginnings of Life

• Matching Genes
• conception occurs in the usual way
• zygote
• the single cell formed from the fusing of two
  gametes, a sperm and an ovum
• genotype
• An organisms entire genetic inheritance, or
  genetic potential

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The Beginnings of Life

• Male or Female?

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From One Cell to Many

• phenotype
• the observable characteristic of a person,
  including appearance, personality, intelligence,
  and all other traits

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From One Cell to Many

• New Cells, New Functions
• Gene-Gene Interactions
• occurs through cell differentiation, gene-gene
  (polygenic), and gene-environment interaction
• Multifactorial
• refers to a trait that is affected by many
  factors, both genetic and environmental
• The Human Genome Project is an international
  effort to map the entire human genome
• researchers have found that humans have only
  about 25,000 genes, 99 of which are present in
  the genomes of other creatures as well

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From One Cell to Many

• Additive Heredity
• an allele is a slight, normal variation of a
  particular gene
• some alleles are
• additive genes combine to make a phenotype

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From One Cell to Many

• Dominant-Recessive Heredity
• the interaction of a pair of alleles in such a
  way that the phenotype reveals the influence of
  one allele (the dominant gene) more than that of
  the other (the recessive gene)
• a special case of the dominant-recessive pattern
  occurs with genes that are x-linked, located on
  the x chromosome

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From One Cell to Many

• More Complications
• A small alteration in the sequence of base pairs
  or several extra repetitions in one triplet ma be
  inconsequential or may cascade to create a major
  problem

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From One Cell to Many

• Twins, Clones, Assisted Reproduction (ART)
• dizygotic (fraternal) twins
• result from two sperm penetrating two ova, and
  share 50 of their genes
• monozygotic (identical) twins
• originate from one zygote, and share 100 genes
• a clone
• originates from a live organism
• ART
• general term for the technique designed to help
  infertile couples conceive and then sustain a
  pregnancy

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From One Cell to Many

• Assisted Reproduction (ART)
• general term for the technique designed to help
  infertile couples conceive and then sustain a
  pregnancy

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From Genotype to Phenotype

• Scientist in many nations have studied thousands
  of twins, both monozygotic and dizygotic, raised
  together in the same home and raised separately
  in different homes

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From Genotype to Phenotype

• Genes affect every aspect of human behavior,
  including social and cognitive behavior
• Most environmental influences on children raised
  in the same home are not shared

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From Genotype to Phenotype

• Each childs genes elicit other peoples
  responses, and these responses shape development.
  In other words, a childs environment is partly
  the result of his or her genes.
• Children, adolescents, and especially adults
  choose environments that are compatible with
  their genes (called niche-picking), and thus
  genetic influences in adulthood

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From Genotype to Phenotype

• Carrier
• a person whose genotype includes a gene that is
  not expressed in the phenotypesuch an
  unexpressed gene occurs in half of the carriers
  gametes and thus is passed on to half of the
  carriers children, who will most likely be
  carriers, too
• Generally, only when the gene is inherited from
  both parents does the characteristic appear in
  the phenotype.

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From Genotype to Phenotype

• Addiction
• inherited biochemistry making people vulnerable
  to various addition
• any one can abuse drugs or alcoholbut genes
  create an addictive pull that can be
  overpowering, extremely weak, or somewhere in
  between

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From Genotype to Phenotype

• Visual Acuity
• New borns cannot focus more than 2 feet away
• Children see better each year until about age 8
• Many adolescents become nearsighted when eyeball
  shape changes
• Vision is more likely to improve than to worsen
  until age 40

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From Genotype to Phenotype

• Visual Acuity
• In middle age, the elasticity of the lens
  decrease and the eyeball shape change again, so
  that many people become farsighted and need
  reading glasses
• Among the old, eye diseases, including cataracts,
  are common
• About 10 percent of people over age 90 are blind

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From Genotype to Phenotype

• Nearsightedness and Genes
• If children have a vision problem it is most
  often myopia (nearsightedness)
• Nearsightedness is a symptom in more than 150
  genetic syndromes
• Caused by physical trauma or illness, such as the
  rubella virus, or poor nutrition (such as vitamin
  A deficiency
• These factors cause high nearsightedness, so
  severe that it can lead to blindness

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From Genotype to Phenotype

• Culture and Cohort
• genes are not the major cause of poor vision
• historical and multicultural research finds that
  environment also influences nearsightedness
• if diet is deficient of vitamin A

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From Genotype to Phenotype

• Practical Application
• developmental application of nature-nurture
  interaction
• family history of genetic problems
• someone inherited a problem
• alcoholism in the genes
• lack of outdoor play

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From Genotype to Phenotype

• Practical Application
• type 2 diabetes (adult-onset diabetes)
• a chronic disease which the body does not produce
  enough insulin to adequately metabolize
  carbohydrate (glucose) it typically developed in
  people aged 50 - 60today it often appears in
  younger people
• begins when a person is vulnerable and has more
  body fat than is ideal

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From Genotype to Phenotype
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Chromosomal and Genetic Abnormalities

• abnormalities caused by identifiable
  problemsthose with an extra chromosome or a
  single gene
• study of these problems is relevant to the study
  of development
• providing insight into the complexities of nature
  and nurture
• knowing their origins helps limit these effects
• information combats the prejudice that surrounds
  such problems

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Chromosomal and Genetic Abnormalities

• Not Exactly 46 Chromosomes
• a variable that most often correlates with
  chromosomal abnormalities is the age of the
  mother
• occur not only in the formation of gametes but
  also in their early duplication
• mosaic is having a condition (mosaicism) that
  involves having a mixture of cells, some normal
  and some with an odd number of chromosomes or a
  sense of missing genes

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Chromosomal and Genetic Abnormalities

• Down Syndrome
• a condition in which a person has 47 chromosomes
  instead of the usual 46, with three rather than
  two chromosomes at the 21st position
• people with Down Syndrome typically have
  distinctive characteristics, including unusual
  facial features, heart abnormities, and language
  difficulties

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Chromosomal and Genetic Abnormalities

• Abnormalities of the 23rd Pair
• humans have at least 44 autosomes and one X
  chromosome
• an embryo cannot develop without an X chromosome
• an odd number of X chromosomes impairs cognition
  and psychosocial development and sexual
  maturation
• if a child has three sex chromosomes instead of
  two he/she may seem normal until puberty

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Chromosomal and Genetic Abnormalities

• Dominant-Gene Disorders
• everyone carries genes or alleles that could
  produce serous diseases or handicaps in the next
  generation
• 7,000 single-gene disorders
• their dominant effects are apparent in the
  phenotype

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Chromosomal and Genetic Abnormalities

• Fragile X Syndrome
• a genetic disorder in which part of the X
  chromosome seems to be attached to the rest of it
  by a very thin string of molecules
• the actual cause is too many repetitions of a
  particular part of a genes code

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Chromosomal and Genetic Abnormalities

• Recessive-Gene Disorder
• most recessive disorders are not X-linked
• double recessive patterns are lethalone
  recessive gene is protective
• sometimes a person who carried a lethal gene has
  many descendants who marry each other the
  genetic disease then becomes common in that group

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Chromosomal and Genetic Abnormalities

• Genetic Counseling and Testing
• consultation and testing by trained experts that
  enable individuals to learn about their genetic
  heritage, including harmful conditions that they
  might pass along to any children they may
  conceive

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Chromosomal and Genetic Abnormalities

• Who Should Get Counseling, and When?
• genetic counseling
• consultation and testing by trained experts that
  enable individuals to learn about their genetic
  heritage, including harmful conditions that they
  might pass along to any children they may
  conceive

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Chromosomal and Genetic Abnormalities

• Is knowledge Always Power?
• Genetic counselors, scientist, and the general
  public usually favor testing
• having some information is better than having
  none
• high risk individuals (who might hear bad news)
  do not always want to know
• the truth might jeopardize their marriage, their
  insurance coverage, or their chance of parenthood

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Chromosomal and Genetic Abnormalities

• Coping with Uncertainty
• much is uncertain in genetic testing and
  counseling
• those who learn that they have a harmful dominant
  gene have new information, as well as new
  uncertainties
• interaction of genes and the environment makes
  development overt the life span unpredictable,
  even if the genes are known