Chapter 15 The Chromosomal Basis of Inheritance

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Chapter 15 The Chromosomal Basis of
Inheritance
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Timeline

• 1866- Mendel's Paper
• 1875- Mitosis worked out
• 1890's- Meiosis worked out
• 1902- Sutton, Boveri et. al. connect chromosomes
  to Meiosis.

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Sutton

• Developed the Chromosome Theory of Inheritance.
• Mendelian factors or alleles are located on
  chromosomes.
• Chromosomes segregate and show independent
  assortment.

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Morgan

• Chose to use fruit flies as a test organism in
  genetics.
• Allowed the first tracing of traits to specific
  chromosomes.

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Fruit Fly

• Drosophila melanogaster
• Early test organism for genetic studies.

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Life Cycle

• Egg
• Larva
• Pupa
• Adult

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Reasons

• Small
• Cheap to house and feed
• Short generation time
• Many offspring
• Few chromosomes

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Fruit Fly Chromosomes
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Genetic Symbols

• Mendel - use of uppercase or lowercase letters.
• T tall
• t short
• Morgan symbol from the mutant phenotype.
• wild phenotype

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Examples

• Recessive mutation
• w white eyes
• w red eyes
• Dominant Mutation
• Cy Curly wings
• Cy Normal wings

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Morgan Observed

• A male fly with a mutation for white eyes.

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Morgan crossed

• The white eye male with a normal red eye female.

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The F1 offspring

• All had red eyes.
• This suggests that white eyes is a genetic
  _________?
• Recessive.

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F1 X F1 F2

• Morgan expected the F2 to have a 31 ratio of
  redwhite
• He got this ratio, however, all of the white eyed
  flies were MALE.
• Therefore, the eye color trait appeared to be
  linked to sex.

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Morgan discovered

• Sex linked traits.
• Genetic traits whose expression are dependent on
  the sex of the individual.

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Fruit Fly Chromosomes
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Sex linked traits

• Sex linked traits in humans will be covered in a
  few minutes.

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Morgan Discovered

• There are many genes, but only a few chromosomes.
• Therefore, each chromosome must carry a number of
  genes together as a package.

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Linked Genes

• Traits that are located on the same chromosome.
• Result
• Failure of Mendel's Law of Independent
  Assortment.
• Ratios mimic monohybrid crosses.

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Body Color and Wing type
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Example

• bb vgvg X bb vgvg
• (b linked to vg)
• (b linked to vg)
• If unlinked 1111 ratio.
• If linked ratio will be altered.

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Crossing-Over

• Breaks up linkages and creates new ones.
• Recombinant offspring formed that doesn't match
  the parental types.

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If Genes are Linked

• Independent Assortment of traits fails.
• Linkage may be strong or weak.

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Linkage Strength

• Degree of strength related to how close the
  traits are on the chromosome.
• Weak - farther apart
• Strong - closer together

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Genetic Maps

• Constructed from crossing-over frequencies.
• 1 map unit 1 recombination frequency.

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• Comment - only good for genes that are within 50
  map units of each other. Why?

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Genetic Maps

• Have been constructed for many traits in fruit
  flies, humans and other organisms.

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Sex Linkage in Biology

• Several systems are known
• Mammals XX and XY
• Diploid insects X and XX
• Birds ZZ and ZW

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Chromosomal Basis of Sex in Humans

• X chromosome - medium sized chromosome with a
  large number of traits.
• Y chromosome - much smaller chromosome with only
  a few traits.

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Human Chromosome Sex

• Males - XYFemales - XX
• Comment - The X and Y chromosomes are a
  homologous pair, but only for a small region at
  one tip.

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SRY

• Sex-determining Region Y chromosome gene.
• If present - male
• If absent - female
• SRY codes for a cell surface receptor.

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Sex Linkage

• Inheritance of traits on the sex chromosomes.
• X- Linkage (common)
• Y- Linkage (rare)

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Males

• Hemizygous - 1 copy of X chromosome.
• Show ALL X traits (dominant or recessive).
• More likely to show X recessive gene problems
  than females.

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X-linked Disorders

• Color blindness
• Duchenne's Muscular Dystrophy
• Hemophilia (types a and b)

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Samples of X-linked patterns
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X-linked Patterns

• Trait is usually passed from a carrier mother to
  1/2 of sons.
• Affected father has no affected children, but
  passes the trait on to all daughters who will be
  carriers for the trait.

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Comment

• Watch how questions with sex linkage are phrased
• Chance of children?
• Chance of males?

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Can Females be color-blind?

• Yes, if their mother was a carrier and their
  father is affected.

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Y-linkage

• Hairy ear pinnae.
• Comment - new techniques have found a number of
  Y-linked factors that can be shown to run in
  the males of a family.
• Ex Jewish priests

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Sex Limited Traits

• Traits that are only expressed in one sex.
• Ex prostate glands

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Sex Influenced Traits

• Traits whose expression differs because of the
  hormones of the sex.
• Ex. beards, mammary gland development, baldness

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Baldness

• Testosterone makes the trait act as a dominant.
• No testosterone makes the trait act as a
  recessive.
• Males have gene bald
• Females must be homozygous to have thin hair.

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Barr Body

• Inactive X chromosome observed in the nucleus.
• Way of determining genetic sex without doing a
  karyotype.

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Lyon Hypothesis

• Which X inactivated is random.
• Inactivation happens early in embryo development
  by adding CH3 groups to the DNA.
• Result - body cells are a mosaic of X types.

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Examples

• Calico Cats.
• Human examples are known such as a sweat gland
  disorder.

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Calico Cats

• XB black fur
• XO orange fur
• Calico is heterozygous, XB XO.

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Question?

• Why dont you find many calico males?
• They must be XB XOY and are sterile.

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Chromosomal Alterations

• Changes in number.
• Changes in structure.

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Number Alterations

• Aneuploidy - too many or too few chromosomes, but
  not a whole set change.
• Polyploidy - changes in whole sets of
  chromosomes.

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Aneuploidy

• Caused by nondisjunction, the failure of a pair
  of chromosomes to separate during meiosis.

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Comment

• Nondisjunction in Meiosis I produces 4 abnormal
  gametes.
• Nondisjunction in Meiosis II produces 2 normal
  and 2 abnormal gametes.

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Types of Aneuoploidy

• Monosomy 2N 1 (very rare)
• Trisomy 2N 1 (more common)

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Turner Syndrome

• 2N - 1 or 45 chromosomesGenotype X_ or X0.
• Phenotype female, but very poor secondary sexual
  development.

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Characteristics

• Short stature.
• Extra skin on neck.
• Broad chest.
• Usually sterile
• Normal mental development except for some spatial
  problems.

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Question

• Why are Turner Individuals usually sterile?
• Odd chromosome number.
• Two X chromosomes needed for ovary development.

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Other Sex Chromosome changes

• Kleinfelter Syndrome
• Meta female
• Supermale

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Kleinfelter Syndrome

• 2N 1 (2N 2, 2N 3)
• Genotype XXY (XXXY, XXXXY)
• Phenotype male, but sexual development may be
  poor. Often taller than average, mental
  development fine (XXY), usually sterile.
• More X more mental problems

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George Washington

• May have been a Kleinfelter Syndrome individual.
• Much taller than average.
• Produced no children.

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Meta female

• 2N 1 or 2N 2
• Genotype XXX or XXXX
• Phenotype female, but sexual development poor.
  Mental impairment common.

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Super male

• 2N 1 or 2N 2
• Genotype XYY or XYYY
• Phenotype male, usually normal, fertile.

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Trisomy events

• Trisomy 21 Downs Syndrome
• Trisomy 13 Patau Syndrome
• Both have various physical and mental changes.

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Question?

• Why is trisomy more common than monosomy?
• Fetus can survive an extra copy of a chromosome,
  but being hemizygous is usually fatal.

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Question?

• Why is trisomy 21 more common in older mothers?
• Maternal age increases risk of nondisjunction.

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Polyploid

• Triploid 3N
• Tetraploid 4N
• Usually fatal in animals.

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Question?

• In plants, even polyploids are often fertile,
  why odd polyploids are sterile. Why?
• Odd number of chromosomes cant be split during
  meiosis to make spores.

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Structure Alterations

• Deletions
• Duplications
• Inversions
• Translocations

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Result

• Loss of genetic information.
• Position effects a gene's expression is
  influenced by its location to other genes.

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Translocations
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Cri Du Chat Syndrome

• Part of p arm of 5 missing.
• Good survival.
• Severe mental retardation.
• Small sized heads common.

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Philadelphia Chromosome

• An abnormal chromosome produced by an exchange of
  portions of chromosomes 9 and 22.
• Causes chronic myeloid leukemia.

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Parental Imprinting of Genes

• Gene expression and inheritance depends on which
  parent passed on the gene.
• Usually caused by different methylations of the
  DNA.

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Example

• Prader-Willi Syndrome and Angelman Syndrome
• Both lack a small gene region from chromosome 15.
• Male imprint Prader-WilliFemale imprint
  Angelman

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Cause

• Imprints are "erased" in gamete producing cells
  and re-coded by the body according to its sex.

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Result

• Phenotypes don't follow Mendelian Inheritance
  patterns because the sex of the parent does
  matter.

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Why have parental imprinting?

• Method to detect that TWO different sets of
  chromosomes are in the zygote.

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Experiment

• Can fuse nuclei into mouse eggs.
• If male/male normal placenta, abnormal fetus
• If female/female abnormal placenta, normal
  fetus
• If male/female normal placenta and normal fetus

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Extranuclear Inheritance

• Inheritance of genes not located on the nuclear
  DNA.
• DNA in organelles.
• Mitochondria
• Chloroplasts

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Result

• Mendelian inheritance patterns fail.
• Maternal Inheritance of traits where the trait is
  passed directly through the egg to the offspring.

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Mitochondria

• Myoclonic Epilepsy
• Ragged Red-fiber Disease
• Lebers Optic Neuropathy
• All are associated with ATP generation problems
  and affect organs with high ATP demands.

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Chloroplasts

• Gives non-green areas in leaves, called
  variegation.
• Several different types known.
• Very common in ornamental plants.

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Variegated Examples
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Comment

• Cells can have a mixture of normal and abnormal
  organelles.
• Result - degree of expression of the maternal
  inherited trait can vary widely.

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Summary

• Know about linkage and crossing-over.
• Sex chromosomes and their pattern of inheritance.
• Variations of chromosomes and inheritance
  patterns.

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Summary

• Be able to work genetics problems for this
  chapter.