Chapter 11 Complex Inheritance and Human Heredity

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Chapter 11Complex Inheritance and Human Heredity
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Key Definitions

• trisomy 21 condition in which an individual has
  three number 21 chromosomes, resulting in Down
  syndrome
• pedigree family tree that records and traces the
  occurrence of a trait in a family
• carrier individual who has one copy of the
  allele for a recessive disorder and does not
  exhibit symptoms

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The nucleus contains an information-rich genome
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Key Terms

• chromosome theory of inheritance generalization
  that genes are located on chromosomes and that
  the behavior of chromosomes during meiosis and
  fertilization accounts for inheritance patterns
• gene locus specific location of a gene on a
  chromosome
• genetic linkage tendency for alleles of genes on
  the same chromosome to be inherited together.

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Chromosome Theory of Inheritance

• Genes are located on chromosomes, and the
  behavior of chromosomes during meiosis and
  fertilization accounts for inheritance patterns

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Accidents affecting chromosomes can cause
disorders
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Chromosomal Mutations

• Structure (environmental factors)
• inversion
• translocation
• deletion
• duplication
• Number (nondisjunction)
• monosomies (2n - 1)
• Trisomies (2n 1)
• Poyploidy

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Changes in Chromosome Structure
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Changes in Chromosome Number
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All gametes have abnormal numbers of chromosomes.
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Two normal gametes two abnormal gametes.
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Monosomy

• Turner syndrome (female one X) 2.5 -5.5/10,000
  live births.
• Monosomy 7 (myelodysplasia and acute myelogenous
  leukemia)
• Mosaicism RARE! (monosomy 21)

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Trisomy 13 Patau Syndrome

• Rare for fetuses to go to term, so occurs in only
  1 in 6000 live births.
• Survival beyond 1st year is uncommon because of
  the multitude of anomalies present.

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Trisomy 13
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Trisomy 21 Down Syndrome

• 1/1,000 births for mothers below the age of 30
  and 1/50 for mothers over the age of 50.
• 50 of children with Down syndrome born with
  heart defects, some major.
• Upward slant to the eyes
• Enlarged tongue that tends to stick out.

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Down Syndrome Karyotype
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As a woman ages, the likelihood of her having a
baby with Down syndrome increases.
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Polyploidy

• More than two sets of chromosomes.
• Not seen in animals
• Commonly seen in plants
• Wheat, cotton, corn, sugar cane watermelons,
  bananas, and apples
• Triploids (3n)
• Tetraploids (4n)
• Pentaploids (5n)

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Abnormal Sex Chromsome Inheritance

• Turner syndrome (female one X) 2.5 -5.5/10,000
  live births.
• Kleinfelter Syndrome (male 2 or more X)

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Turner's syndrome

• Affect females
• One of the X chromosomes is absent or abnormal.
• 1/5000 births
• Cause the range of anomalies

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Kleinfelters Syndrome

• Affected males have an extra X chromosome
  resulting in XXY
• 1/2000 live births
• Normally sterile with subnormal intelligence

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Mendels principles apply to humans
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Modes of Inheritance

• Autosomal dominant
• Autosomal recessive
• Sex-linked recessive

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Autosomal Dominant Disorders

• Smaller number of disorders are of this type
• Does not skip generations
• Expressed every generation
• Equal number of ? and ?
• No one can express trait unless at least 1 parent
  does.

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Autosomal Dominant Disorders

• Dwarfism (achondroplasia) 1/25,000 births
• Huntingtons disease
• Neurofibromatosis

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Autosomal Dominant

• Neurofibromatosis
• 1 in 3,000

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Autosomal Dominant

• Huntington Disease
• 1 in 20,000
• Progressive degeneration of brain cells
• Typically appears in middle age
• Gene located on chromosome 4
• Many repeats of CAG triplet
• Normal 11-34 disease 42gt120

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Autosomal Recessive Disorders

• Most human genetic disorders are of this type.
• Skips generations
• Offspring expressing trait can have both parents
  nonexpressing
• Appears out of nowhere
• Equal number of ? and ?

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Autosomal Recessive Disorders

• Albinism
• Tay-Sachs
• Cystic fibrosis
• Sickle cell anemia

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Autosomal Recessive

• Cystic Fibrosis
• 1 in 20 are carriers
• 17-28 year life expectancy
• Tay-Sachs
• Phenylketonuria (PKU)
• Sickle Cell Anemia

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Cystic Fibrosis

• 1 in 20 Caucasians are carriers.
• 1 in 2500 children
• Failure of Cl ions to pass through plasma
  membrane
• Gene therapy moderately successful.

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Tay-Sachs Disease

• 1 in 3600 Jews of central and eastern European
  descent
• Slowed development, blind, seizures, paralysis.
• Most die by age of 3-4
• Results in buildup of fatty substance in brain.

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Phenylketonuria

• 1 in 5000 births
• Missing enzyme for normal metabolism of
  phenylalanine.

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Sickle-Cell Disease

• Controlled by incompletely dominant alleles.
• RBCs not biconcave (irregular hemoglobin)
• Sickling not evident unless dehydration or mild
  O2 deprivtion
• Trait affords protection for malaria

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Sickle-Cell Disease
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Sickle-Cell Disease
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Sex-linked Recessive Disorders

• Affects males (mostly)
• Affected males do not pass to sons.
• Trait skip generations
• Carrier females transmit trait to ½ of sons and
  ½ of daughters (never passed from father to son.)

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Sex-Linked Disorders

• X-linked recessive
• Color Blindness
• 8 of Caucasian men
• Duchenne Muscular Dystrophy
• 1 of every 3600 male births
• Hemophilia
• 1 in 10,000 males
• 2 types, A and B

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Sex-linked Recessive Disorders

• Red-green colorblindness
• Hemophilia

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PedigreesFamily trees that record and trace the
occurrence of a trait in a family.
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This pedigree at left shows the appearance of
colorblindness in four generations of a family.
(Sex-linked recessive)
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Earlobes are either free or attached. This
pedigree tracks the occurrence of attached
earlobes in three generations of a family.
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This pedigree reveals that free earlobes are
dominant, and attached earlobes are recessive.
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Section 11.2Complex Patterns of Inheritance
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Objectives

• Describe how alleles interact in intermediate
  inheritance.
• Describe inheritance patterns involving multiple
  alleles.
• Explain how polygenic inheritance can result in a
  wide range of phenotypes.
• Describe how environmental conditions can affect
  phenotype expression.

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Key Terms

• Incomplete dominance (intermediate inheritance)
  inheritance in which heterozygotes have a
  phenotype intermediate between the phenotypes of
  the two homozygotes.
• codominance inheritance pattern in which a
  heterozygote expresses the distinct traits of
  both alleles. (Blood type)
• polygenic inheritance combined effect of two or
  more genes on a single character. (height)

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Key points to remember!

• Mendel only worked with complete dominance and
  complete recessive-ness.
• These are two extremes of dominance
  relationships.
• Most allelic pairs do not show this type of
  relationship.

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An important concept!

• Genes only provide the POTENTIAL for developing a
  particular phenotypic characterstic.

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Complex Inheritance

• Incomplete Dominance
• Codominance
• Multiple Alleles
• Sex-Linked

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Incomplete Dominance (Intermediate Inheritance)
Because parent phenotypes can reappear in the F2
generation.
Why doesnt this support the blending hypothesis?
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Codominance

• Both alleles are expressed in the heterozygous
  condition.
• Both normal and abnormal hemoglobins are made.

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Multiple Alleles
Three alleles for human blood type. Any one
person has only two. Combinations of these
alleles result in six genotypes and four
phenotypes. Alleles IA and IB are codominant.
Allele i is recessive.
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Sex-linked Inheritance

• Involves X and Y chromosomes
• Why X?
• Dosage compensation
• Why Y?

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Sex Determination

• Specific gene on Y chromosome (SRY)

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Why Sex?

• Need for DNA repair.

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Why X and Y?

• X 100 genes
• Y 78 active genes
• One concerned with male development
• Most others with sperm production and fertility.

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Why X and Y?

• X and Y only pair at tips
• Evolutions has prevented close pairing
• No gene swapping between the two!
• With swapping, all would be males
• Y proofreads self using palindromes
• No need for a paired Y!
• Males persist!

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Sex-linked traits have unique inheritance
patterns.
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Inheritance of Sex-Linked Genes

• The sex chromosomes have genes for many
  characters unrelated to sex
• A gene located on either sex chromosome is called
  a sex-linked gene
• In humans, sex-linked usually refers to a gene on
  the larger X chromosome

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Sex-Linked Genes

• Follow specific patterns of inheritance
• Expression of a recessive sex-linked trait
• Female needs two copies of the allele
• Male only needs one copy of the allele
• Sex-linked recessive disorders are more common in
  males than in females

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Sex-Linked Genetic Disorders

• Some disorders caused by recessive alleles on the
  X chromosome in humans
• Color blindness
• Duchenne muscular dystrophy
• Hemophilia
• Sex-linked recessive disorders are more common in
  males than in females

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Sex-linked Disorders

• red-green color blindness
• hemophilia (a disease in which blood fails to
  clot normally)
• inherited as sex-linked (X-linked) recessive
  traits.

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Sex-linked Genes
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Inheritance Effects
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Inheritance Effects

• Epistasis
• Polygenic
• Pleotropic
• Environmental

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Epistasis

• One alleles hides the effects of another.
• Coat color in Labrador Retriever

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Polygenic Effects

• A single trait influenced by many genes
• Height and skin color
• Example - height of students in a large high
  school might range from about 125 cm to 200 cm,
  with students of every possible height in
  between.

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Polygenic Effects
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Polygenic Effects
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Pleiotropic Effects

• A single gene influences many phenotypic traits
• Sickle Cell Disease

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Polygenic vs. Pleiotropic
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Environmental Effects

• Tree leaves vary in size.
• Temperature - Siamese cats.
• Nutrition height.
• Exercise body shape
• Exposure to sunlight skin pigmentation

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Nature or Nurture?Genes or Environment?
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Take Home Lesson!

• The product of a genotype is generally not a
  single, rigidly defined phenotype but a range of
  possibilities influenced by environment.