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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Reasons

• Small
• Cheap to house and feed
• Short generation time
• Many offspring
• Few 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 XY and XX
• Diploid insects X and XX
• Birds ZZ and ZW
• Social insects haploid and diploid

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Homework

• Read parts of Chapter 15
• Lab Genetics of Organisms
• Chapter 48 part II
• Chapter 15 Homework
• Due in by Dec. 12th noon

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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 (very rare if exists at all)

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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 markers 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 gland

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Sex Influenced Traits

• Traits whose expression differs because of the
  hormones of the sex.
• These are NOT on the sex chromosomes.
• Ex. beards, mammary gland development, baldness

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Baldness

• Testosterone the trait act as a dominant.
• No testosterone 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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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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Variegation in African Violets
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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.

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Summary

• Be able to work genetics problems for this
  chapter.

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Happy Holidays and have a great winter break!