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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• Parents are two true-breeding pea plants
• Parent 1 Yellow, round Seeds (YYRR)
• Parent 2 Green, wrinkled seeds (yyrr)
• These 2 genes are on different chromosomes.
• Draw meiosis to determine the resulting gametes
  of parent 1.
• How do the resulting gametes connect to Punnett
  squares?

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• F1 YyRr x YyRr
• What are the predicted phenotypic ratios of the
  offspring?
• ¾ yellow ¾ round
• ¼ green ¼ wrinkled
• ¼ (green) x ¼ (wrinkled) 1/16 green, wrinkled
• 9331 phenotypic ratio

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First Experimental Evidence to connect Mendelism
to the chromosome

• Thomas Morgan (1910)
• 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
• 3 pairs of Autosomes
• 1 pair of sex chromosomes

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Examples

• Wild type is most common,
• NOT dominant or recessive
• 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.
• Male ww x Female ww

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The F1 offspring

• All had red eyes.
• This suggests that white eyes is a _________?
• Recessive.
• F1 ww
• What is the predicted phenotypic ratio for the F2
  generation?

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F1 X F1 F2

• Expected F2 ratio - 31 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 gene are located on the sex
  chromosome

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Fruit Fly Chromosomes

• Female Male
• XX XY
• Presence of Y chromosome determines the sex
• Just like in humans!

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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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Sex-Linked Problem

• A man with hemophilia (a recessive, sex-linked,
  x-chromosome condition) has a daughter of normal
  phenotype. She marries a man who is normal for
  the trait.
• A. What is the probability that a daughter of
  this mating will be a hemophiliac?
• B. That a son will be a hemophiliac?
• C. If the couple has four sons, what is the
  probability that all four will be born with
  hemophilia?

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• Original Man - XhY
• Daughter - must get the dads X chromosome XHXh
  (normal phenotype, so shes a carrier)
• Daughters husband XHY (normal phenotype)
• A. daughter must get XH from the dad. 0 (50
  carrier, 50 homo dom.)
• B. son must get Y from dad. 50 chance to be
  hemophiliac
• C. ½ x ½ x ½ x ½ 1/16

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Linked Genes

• Traits that are located on the same chromosome.
• Result
• Failure of Mendel's Law of Independent
  Assortment.
• Ratios are different from the expected

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Example

• Body Color - gray dominant
• b - Gray
• b - black
• Wing Type - normal dominant
• vg - normal
• vg vestigial (short)

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Example

• bb vgvg X bb vgvg
• Predict the phenotypic ratio of the offspring

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Show at board

• bb x bb vgvg x vgvg
• ½ gray ½ black ½ normal ½ vestigial
• --------------------------------------------------
  ---¼ gray normal, ¼ gray vestigial,
• ¼ black normal, ¼ black vestigial
• 1111 phenotypic ratio

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Conclusion

• Most offspring had the parental phenotype. Both
  genes are on the same chromosome.
• What do you expect to happen if theyre on the
  same chromosome? (draw chromosomes)
• bbvgvg parent can only pass on b vg
• bb vgvg can pass on b vg or b vg

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bb vgvg - Chromosomes (linked genes)
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Crossing-Over

• Breaks up linkages and creates new ones.
• Recombinant offspring formed that doesn't match
  the parental types.
• Higher recombinant frequency genes further
  apart on chromosome

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If Genes are Linked

• Independent Assortment of traits fails.
• Linkage may be strong or weak.
• Strong Linkage means that 2 alleles are often
  inherited together.

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• Degree of strength related to how close the
  traits are on the chromosome.

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Genetic Maps

• Constructed from crossing-over frequencies.
• 1 map unit 1 recombination frequency.
• Can use recombination rates to map chromosomes.

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• Comment - only good for genes that are within 50
  map units of each other. Why?
• Over 50 gives the same phenotypic ratios as
  genes on separate chromosomes

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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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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 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)
• Immune system defects

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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

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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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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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Nondisjunction

• When chromosomes fail to separate during meiosis
• Result cells have too many or too few
  chromosomes which is known as aneuploidy

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Meiosis I vs Meiosis II

• Meiosis I all 4 cells are abnormal
• Meiosis II only 2 cells are abnormal

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Aneuploidy

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

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Types

• Monosomy 2N - 1
• Trisomy 2N 1

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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 need for ovary development.

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Homework

• Read Chapter 15 (Hillis 8)
• Genetics Lab Report today
• No class Feb. 4 and 5
• Chapter 15 Thurs. 2/7

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Other Sex Chromosome changes

• Kleinfelter Syndrome
• Meta female
• Supermale

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Kleinfelter Syndrome

• 2N 1
• Genotype XXY
• Phenotype male, but sexual development may be
  poor. Often taller than average, mental
  development fine, usually sterile.

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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 Down's 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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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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Cri Du Chat Syndrome

• Part of p arm of 5 missing.
• Good survival, but low birth weight and slow
  gain.
• Severe mental impairment.
• Small sized heads common.

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Cri Du Chat Syndrome
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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.
• Gametes are methylated to code as male or
  female.

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Result

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

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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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Chloroplasts

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

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Variegation in African Violets
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Variegated Examples
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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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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.