Chapter 15 Chromosomal Basis of Inheritance

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Chapter 15 Chromosomal Basis of Inheritance
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1902

• Walter Sutton
• Theodor Boveri
• Etc
• Noted parallels between chromosomes and Mendels
  factors
• CHROMOSOME THEORY emerged
• Mendelian genes have specific loci on chromosomes
  and it is the chromosomes that undergo
  segregation and independent assortment (see page
  270)

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Figure 15.1 The chomosomal basis of Mendels laws
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Linked genes

• Number of genes in a cell is FAR greater than the
  number of chromosomes
• Linked genes are genes that are located on the
  same chromosome and that tend to be inherited
  together
• Inheritance patterns with linked genes tend to
  deviate from expected Mendelian ratios
• Morgan first to trace a gene to a specific
  chromosome

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Figure 15.4 Evidence for linked genes in
Drosophila
Pages 272 and 273
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Genetic recombination

• General term for the production of offspring with
  new combinations of traits inherited from two
  parents
• Organisms that have these are called
  recombinants

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Figure 15.5a Recombination due to crossing over
Page 274
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Figure 15.5b Recombination due to crossing over
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Geneticists can use recombination data to map a
chromosomes genetic loci

• Method discovered by Alfred Sturtevant
• Called a genetic map (linkage map) an ordered
  list of the genetic loci along a particular
  chromosome
• Assuming that cross-over possibility is
  approximately equal at all points on a
  chromosome, the further apart two genes are, the
  HIGHER the probability that a cross-over will
  occur between those two genes the higher the
  recombination frequency
• Map units are called centimorgans in honor of
  Thomas Hunt Morgan

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Figure 15.6 Using recombination frequencies to
construct a genetic map
b body color cn cinnabar eyes (brighter
red) vg wing size
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Figure 15.7 A partial genetic map of a
Drosophila chromosome
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Figure 15.8 Some chromosomal systems of sex
determination
Page 276
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Sex-linked genes

• Show up more often in the sex that has only one
  copy of the X- chromosome
• Human examples
• -Duchenne muscular dystrophy
• -Hemophilia

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X-Inactivation in Female Mammals

• Only one X chromosome stays active in females
  other becomes inactivated during embryonic
  development.
• The inactive X in each cell of a female condenses
  into a compact object a Barr body
• Barr body chromosomes are reactivated in the
  ovary cells that give rise to ova
• See page 278-279 and discussion of tortoiseshell
  cat
• XIST is a gene that is active ONLY on the
  Barr-body chromo

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Figure 15.10 X inactivation and the
tortoiseshell cat
Mary Lyon and the inheritance of inactivation
page 278
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Two types of mutations

• Gene mutations affect only one base or point on
  a gene
• a. substitution
• b. deletion
• c. insertion
• Chromosome mutations affect large sections of
  genes
• alterations in numbers of chromosomes
  nondisjunction resulting in aneuploidy
• a. monosomy
• b. trisomy
• c. polyploidy
• alterations in size of chromosomes
• a. deletion
• b. inversion
• c. translocation (reciprocal or
  non-reciprocal)
• d. duplication

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Gene MutationsSubstitution, Insertion, and
Deletion
Section 12-4
Deletion
Substitution
Insertion
Gene Mutations only affect ONE point of the code
-- often called Point Mutations
Go to Section
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Figure 15.13 Alterations of chromosome structure
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Figure 15.11 Meiotic nondisjunction
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Figure 15.14 Down syndrome
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Figure 15.x3 XYY karyotype
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Human Genetic anomalies

• Autosomal Dominant Genes body cells, not passed
  on to offspring
• Autosomal Recessive Genes body cells, not
  passed on to offspring
• X-linked recessive Genes sex cells, passed on
• Y-linked only in males
• Chromosomal Abnormalities if affects sex
  chromos, passed on
• Multifactorial genetic component (gene or
  chromosome) plus a significant environmental
  influence

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Final notes

• Not all of a eukaryotic cells genes are located
  in the nucleus
• Extranuclear genes are found in mitochondria and
  chloroplasts, too!
• These cytoplasmic genes do not display Mendelian
  inheritance because are not distributed evenly as
  happens in nucleus
• 1909 Karl Correns studied variegated leaves in
  tomato plants
• Mitochondrial DNA comes from Mom maternal
  inheritance, because mitochondria passed on by
  zygote all come from the cytoplasm of the ovum