How Genes Are Transmitted from Generation to Generation

1
How Genes Are Transmitted from Generation to
Generation

• Chapter 4

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Central Points

• Genes are transmitted from generation to
  generation
• Traits are inherited according to predictable
  rules
• Dominant, recessive, and X-linked traits follow
  these rules

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Case A A Familys Dilemma

• Alans mother died from Huntington disease (HD)
• HD caused by a mutant gene and one copy of gene
  will cause the disease
• Neurologic symptoms develop between ages 3050,
  progress slowly, fatal in 1020 years
• Genetic test available

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4.1 How Are Genes Transmitted?

• Gregor Mendel father of genetics
• Experiments with pea plants in 1800s
• Traits, distinguishing characteristics
• Specific patterns in the way traits were passed
  from parent to offspring

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Mendels Experiments

• Some traits disappeared in the first generation
  of offspring (all tall)
• Reappeared in 31 ratio (tallshort)
• Dominant trait present in the first-generation
  offspring (tall)
• Recessive trait absent in first generation but
  reappeared in the next generation (short)

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Traits Are Passed by Genes

• Factors or genes transmitted from parent to
  offspring
• Each parent carries a pair of genes for a trait
  but contributes only one gene to each offspring
• Separation of gene pair occurs during meiosis

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Genes

• Alleles variations of a gene
• Homozygous identical alleles of a gene
• TT or tt
• Heterozygous nonidentical alleles
• Tt

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Different Plant Heights
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Phenotype and Genotype

• Phenotype what an organism looks like
• tall or short
• Genotype genetic makeup
• TT, Tt, and tt
• Identical phenotypes may have different genotypes
• TT or Tt have tall phenotype

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Mendels Law of Segregation

• Two copies of each gene separate during meiosis
• One copy of each gene in the sperm or egg
• Each parent gives one copy of each gene

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Mendels Law of Independent Assortment

• Members of a gene pair segregate into gametes
  independently of other gene pairs
• Gametes can have different combinations of
  parental genes

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Sorting of Alleles
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Animation Segregation of alleles pea plants
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Human Traits Albinism

• Pigmentation dominant and lack of pigment
  recessive
• AA, Aa Pigmented
• aa Albino
• Both parents Aa, each child has 25 chance of
  being albino (31 ratio)

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Segregation of the Albino Allele
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Fig. 4-3a, p. 61
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Fig. 4-3b, p. 61
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Pedigree 1

• Shows all family members and identifies those
  affected with the genetic disorder

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Pedigree 2
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Pedigree Symbols
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p. 62
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p. 62
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Proband

• Person who is the focus of the pedigree
• Indicated by an arrow and the letter P

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Animation Pedigree analysis - predicting future
generations
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Animation Observing Patterns in Inherited Traits
(Crossing Pea Plants)
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Animation Observing Patterns in Genetic Traits
(genetic terms)
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Animation Chromosomes and Human Inheritance
(pedigree diagrams)
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4.2 Examining Human Pedigrees

• Determine trait has dominant or recessive
  inheritance pattern
• Predict genetic risk for
• Pregnancy outcome
• Adult-onset disorder
• In future offspring

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Three Possible Patterns of Inheritance

• Autosomal recessive
• Autosomal dominant
• X-linked recessive
• Autosomal on chromosomes 122
• X-linked traits on the X chromosome

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Autosomal Recessive

• Unaffected parents can have affected children
• All children of affected parents are affected
• Both parents Aa, risk of affected child is 25
• Equal affected male and female
• Both parents must transmit the gene for a child
  to be affected

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Consanguinity

• Individuals related to each other and indicated
  by double line between parents

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Autosomal Recessive Pedigree
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Autosomal Recessive Genetic Disorders
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Albinism

• A normal coloring a albinism
• Group of genetic conditions, lack of pigmentation
  (melanin) in the skin, hair, and/or eyes
• Normally, melanin in pigment granules inside
  melanocytes
• In albinism, melanocytes present but cannot make
  melanin
• Oculocutaneous albinism type I (OCA1)

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Cystic Fibrosis (CF)

• C normal c cystic fibrosis
• CF affects glands that produce mucus and
  digestive enzyme
• CF causes production of thick mucus in lungs
  blocks airways
• Develop obstructive lung diseases and infections
• Identified CF gene and protein (CFTR)

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Animation Segregation of alleles cystic fibrosis
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Sickle Cell Anemia (SCA)

• S normal red blood cells s sickle)
• High frequency in areas of West Africa,
  Mediterranean Sea, India
• Abnormal hemoglobin molecules aggregate to form
  rods
• Red blood cells, crescent- or sickle-shaped,
  fragile and break open

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Normal and Sickled Cells
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Autosomal Dominant (1)

• Requires one copy of the allele (Aa) rarely
  present in a homozygous condition (AA)
• aa Unaffected individuals
• Affected individual has at least one affected
  parent
• Aa X aa Each child has 50 chance of being
  affected

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Autosomal Dominant (2)

• Equal numbers of affected males and females
• Two affected individuals may have unaffected
  children
• Generally, AA more severely affected, often die
  before birth or in childhood

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Autosomal Dominant Pedigree
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Autosomal Dominant Genetic Disorders
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Animation Chromosomes and Human Inheritance
(autosomal-dominant inheritance)
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Animation Chromosomes and Human Inheritance
(autosomal-recessive inheritance)
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Neurofibromatosis (NF)

• N Neurofibromatosis 1 n normal
• Many different phenotypes
• Café-au-lait spots, or noncancerous tumors in the
  nervous system can be large and press on nerves
• Deformities of the face or other body parts
  (rarely)
• NF gene has a very high mutation rate

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Neurofibromatosis
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Huntington Disease (HD)

• H Huntington disease h normal
• Causes damage in brain from accumulation of
  huntingtin protein
• Symptoms begin slowly (3050 years old)
• Affected individuals may have already had
  children (50 chance with one Hh parent)
• Progressive neurological signs, no treatment, die
  within 1025 years after symptoms

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Brain Cells of a Person with HD
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Adult-Onset Disorders

• Expressed later in life
• Present problems in pedigree analysis, genetic
  testing may be required
• Examples
• Huntington disease (HD)
• Adult polycystic kidney disease (ADPKD)
• Both examples are autosomal dominant

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Case A Questions

• Who should be tested?
• Who should know the results of the test?
• How should the test results be used?
• See the textbook for further questions on this
  case

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4.3 X-Linked Recessive Traits

• Genes on X chromosome X-linked
• Genes on Y chromosome Y-linked
• For X-linked traits
• Females XX, XX, or XX
• Males XY or XY
• Males cannot be homozygous or heterozygous, they
  are hemizygous for genes on X
• Distinctive pattern of inheritance

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X-Linked Recessive Inheritance

• Mother gives one X chromosome to offspring
• Father gives X to daughters and Y to sons
• Sons carry X from mother
• For recessive traits, XX and XY affected
• More males affected

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Pedigrees X-Linked Inheritance
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X-Linked Recessive Genetic Disorders
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Inheritance of X-Linked Disorder
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Animation Chromosomes and Human Inheritance
(X-linked inheritance)
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Duchenne Muscular Dystrophy (DMD) (1)

• XM normal Xm muscular dystrophy
• Most common form, affects 1/3,500 males
• Infants appear healthy, symptoms age 16 years
• Rapid, progressive muscle weakness
• Usually must use a wheelchair by age 12
• Death, age 20 from respiratory infection or
  cardiac failure

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Duchenne Muscular Dystrophy (DMD) (2)

• DMD gene on the end of X chromosome
• Encodes protein dystrophin that supports plasma
  membrane during contraction
• If dystrophin absent or defective, cells are torn
  apart
• Two forms DMD, and less-serious Becker muscular
  dystrophy (BMD)

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Cells of a Person with MD
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Hemophilia

• XH normal Xh hemophilia
• Lack of clotting factor VIII in blood
• Affected individuals hemorrhage, often require
  hospitalization to treat bleeding
• Hemophilia A most common form of X-linked
  hemophilia
• Females affected if XhXh, both parents must carry
  the trait

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Factor VIII

• 1980s, half of all people with hemophilia became
  infected with HIV
• Recombinant DNA technology now used to make
  clotting factors free from contamination

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Case B The Franklins Find Out More

• Alan and siblings concerned about inheriting HD
  gene for themselves and future children
• Who should be tested and why?
• How will it affect health insurance coverage?
• See the textbook for further questions on this
  case