Alterations to Mendel

1
Alterations to Mendel

• Incomplete or partial dominance
• Codominance
• Multiple alleles and Lethal alleles
• Gene interactions multiple genes
• Epistasis and complementation
• Effect of environment
• Extranuclear inheritance
• Sex-linked, sex-limited, sex-influenced
• Sex determination and Gene dosage
• Polygenics

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The sex chromosomes

• The rest of the chromosomes are autosomes.
• Sex chromosomes determine sex of individual.
• X and Y differ in size, X much bigger in humans
• But still synapse during meiosis, so still a pair
• In humans, fruit flies, XX female XY male.
• In several other organisms, other combinations of
  sex chromosomes determine sex.
• Because there are genes on sex chromosomes,
  inheritance of certain traits can be sex-linked.

3
Sex inheritance

• In humans, fruit flies, XX female XY male.
• Inheritance of sex is just like any other trait,
  except it involve inheritance of an entire
  chromosome.

Because there are genes on sex chromosomes,
inheritance of certain traits can be sex-linked.
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Sex Linkage and Determination

• Early 1900s, Thomas Hunt Morgan was doing
  classical genetics on fruit flies, looking for
  mutants and checking out the patterns of
  inheritance.
• He studied the white eye phenotype and discovered
  something odd

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What Morgan saw
Reciprocal cross produced a different
result Inheritance of eye color depended on sex
of the fly.
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Morgan and the F2 generation
When the all red eyed F1 heterozygotes were
crossed, close to a 31 ratio was observed, but
the traits were not evenly divided between the
sexes.
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Explanation

• The traits of sex and eye color did not assort
  independently (as the traits in peas did).
• The traits are linked.
• The gene is NOT PRESENT on the Y chromosome.

R red eye R white eye XX female XY male
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significance

• With regard to X linked traits, males have only
  one allele, not two. They are said to be
  hemizygous.
• Morgans work led to the understanding that genes
  are located on chromosomes because inheritance
  of certain traits corresponded to inheritance of
  a visibly different chromosome.
• Inheritance of X-linked traits results in typical
  crisscross inheritance mother to son.

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Crisscross inheritance
Carrier mother passes allele to son who expresses
it, passes allele to daughter who carries it,
etc. Hemophilia color blindness examples in
humans.
http//www.udl.es/usuaris/e4650869/docencia/segonc
icle/genclin98/temes_teoria/imatges_temes_teoria/i
mage3.gif
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Sex linked, limited, influenced

• Sex linked inheritance is when the allele is
  present on a sex chromosome (usually X).
• Sex limited when other genetic factors restrict
  expression to one sex
• Bulls dont give milk.
• Sex influenced other genetic factors modulate
  expression. Example pattern baldness
• Females must be homozygous recessive for trait
• Even then, hormonal factors
• restrict expression.

http//rwqp.rutgers.edu/images/dairy20cow.jpg
imsc.usc.edu/.../contemporary_sb_3.html
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Sex determination

• Different organisms have different chromosomal
  mechanisms for determining sex.
• XX/XO typically, the male has one copy.
• Nematodes, e.g. C. elegans
• XX/XY as in humans, fruit flies, XX female XY
  male.
• Heterogametic sex is the one that produces a
  mixture of gametes. Usually the male but
• Female can be heterogametic in some species
• Designation is ZZ/ZW where female is ZW

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More on sex determination

• Temperature affects sex determination in many
  reptile species
• Females result from low, high, or extremes of
  temperature.
• Hypothesis

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History

• Sex determination studies began in late 1800s
• Work in humans started around 1912, but didnt
  get it right until 1956.
• Keys to understanding sex determination in
  humans
• Improved karyotype methods
• Study of aneuploidy of sex chromosomes
• Aneuploidy is the wrong number of a particular
  chromosome.
• Aneuploidy results from non-disjunction

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Abnormalities in chromosome number result from
non-disjunction
Homologues fail to separate during Meiosis I.
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Abnormalities in chromosome number result from
non-disjunction-2
Sister chromatids fail to separate during Meiosis
II.
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Evidence for XX/XY

• 47, XXY Klinefelter syndrome
• male in appearance, but some feminization
  sterile.
• slow to learn, but not retarded.
• XXXY etc. similar, but more severe symptoms
• 45, XO Turner syndrome
• Monosomy, the only one occurring in humans
• female, sterile, short webbed neck, broad chest,
  short.
• majority arent born
• If XXY is still male and XO is female
• Y must be determinant of maleness

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About the Y

• Y chromosome has been shrinking.
• Now missing many of genes that X has.
• Two regions PAR and MSY
• PAR pseudoautosomal region
• Regions near p telomere and q telomere are
  homologous to X chromosome. Crossing over can
  occur there during meiosis. Because of this,
  genes in this location do not behave as
  sex-linked traits, thus said to be
  pseudoautosomal because they behave like genes on
  autosomes rather than sex chromosomes.

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Structure of Y
universe-review.ca/R11-14-Ychromosome.htm
Human Genome project has revealed much about the
Y chromosome.
http//www.asiaandro.com/1008-682X/4/259fig.jpg
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Male specific region Y (MSY)

• X-transposed region
• 99 identical to X chromosome region, but only 2
  genes the rest are not expressed.
• X-degenerative region
• Contains DNA related to X chromosome regions
• Several functional genes and pseudogenes
• Contains SRY that codes for testis-determining
  factor, necessary for maleness during
  development.
• Ampliconic region
• Highly similar or repeated genes, some related to
  male development and fertility.

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Evidence for SRY

• SRY contains gene for testis determining factor
• Crossing over in meiosis
• Males with two X chromosomes SRY found on one
• Females with X and Y SRY is missing from Y
• Transgenic mice
• Remove SRY from Y chromosome
• Mice are XY but are female
• Reciprocal experiment also done

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Article about Y chromosome

• http//images.google.com/imgres?imgurlhttp//www.
  txtwriter.com/onscience/OSpictures/Y2520chromosom
  e2520repair.jpgimgrefurlhttp//www.txtwriter.co
  m/onscience/Articles/ychromosome.htmlh927w504
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  47tbnw80prev/images3Fq3DY2Bchromosome26svn
  um3D1026hl3Den26rls3DGGLG,GGLG2005-29,GGLGe
  n
• Copy and paste