Midterm

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Midterm

• Friday - or - Monday?
• Covers Chapters 1,2 and 3.6
• If yed, it will mean moving Exam 2 to May 6.

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Sex Chromosomes

• ... X and Y chromosomes that determine the
  sex of an individual in many organisms,
• Females XX
• Males XY

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male
4
a
hemizygous condition where gene is present in
only one dose.
male
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X Linkage

• the pattern of inheritance resulting from genes
  located on the X chromosome.

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X-Linked Genes

• refers specifically to genes on the
  X-chromosome, with no homologs on the Y
  chromosome.

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P
x
Blue Female
Pink Male
Gametes
or
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Gametes
or
F1
Blue Female
Blue Male
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F1
x
Blue Female
Blue Male
Gametes
or
or
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Gametes
or
or
F2
Blue Female
Blue Male
Blue Female
Pink Male
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F2
Blue Female
Blue Male
Blue Female
Pink Male
3 1
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P
x
Pink Female
Blue Male
Gametes
or
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Gametes
or
F1
Blue Female
Pink Male
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Gametes
or
or
F2
Pink Female
Pink Male
Blue Female
Blue Male
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F2
Pink Female
Pink Male
Blue Female
Blue Male
1
1
1
1
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Sex Linkage to Ponder

• Female is homozygous recessive X-linked gene,
• what percentage of male offspring will express?
• what percentage of female offspring will express
  if,
• mate is hemizygous for the recessive allele?
• mate is hemizygous for the dominant allele?
• Repeat at home with female heterozygous X-linked
  gene!

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Sex-Linked vs. Autosomal

• autosomal chromosome non-sex linked chromosome,
• autosomal gene a gene on an autosomal
  chromosome,
• autosomes segregate identically in reciprocal
  crosses.

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X-Linked Recessive TraitsCharacteristics

• Many more males than females show the phenotype,
• female must have both parents carrying the
  allele,
• male only needs a mother with the allele,
• Few (or none) of the offspring of affected males
  show the disorder,
• all of his daughters are carriers,
• half of the sons born to these daughters are
  carriers.

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X-Linked Dominant

• Affected males married to unaffected females pass
  the phenotype to their daughters, but not to
  their sons,
• Heterozygous females married to unaffected males
  pass the phenotype on to half their sons and
  daughters,
• Homozygous dominant females pass the phenotype on
  to all their sons and daughters,

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Autosomal Dominant

• Phenotypes appear in every generation,
• Affected males and females pass the phenotype to
  equal proportions of their sons and daughters.

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Pedigree for Very Rare Trait
1/2
1/2
Recessive?
---gt Yes!
1/2 x 1/2 x ?
1/2 1/8
Autosomal?
X-Linked?
---gt Yes!
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Linkage

• Genes located on the same chromosome segregate
  together and are said to be linked.

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Independent Assortment
A
b
a
B
2n 4

A
A
a
B
b
B
a
b

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Meiosis Prophase I cont.
Chiasmata and crossing over
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MeiosisNo Cross Over
2n 1
A
a
Parent Cell
B
b
A
A
a
a
B
B
b
b
Daughter Cells Have Parental Chromosomes
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MeiosisWith Cross Over
2n 1
A
a
Parent Cell
B
b
A
A
a
a
B
b
B
b
Daughter Cells Have Recombinant Chromosomes
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Gamate Formation in F1 Dihybrids P GGWW x
ggww
F1 Genotype GgWw
G g W
w
alleles gametes
GW
Gw
gW
gw
probability
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How do you test for assortment of alleles?
F1 GgWw
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Test Cross

• phenotypes of the offspring indicate the genotype
  of the gametes produced by the parent in question.

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Test CrossGgWw x ggww
GW (.25)
gw (1)
GgWw (.25)
Gw (.25)
gw (1)
G gww (.25)
gW (.25)
gw (1)
ggWw (.25)
gw (.25)
gw (1)
ggww (.25)
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Test CrossGgWw x ggww
GW (.25)
gw (1)
GgWw (.25)
Gw (.25)
gw (1)
Ggww (.25)
gW (.25)
gw (1)
ggWw (.25)
gw (.25)
gw (1)
ggww (.25)
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Recombination Frequency

• or Linkage Ratio the percentage of recombinant
  types,
• if 50, then the genes are not linked,
• if less than 50, then linkage is observed.

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Linkage

• Genes located on the same chromosome do not
  recombine,
• unless crossing over occurs,
• The recombination frequency gives an estimate of
  the distance between the genes.

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Recombination Frequencies

• Genes that are adjacent have a recombination
  frequency near 0,
• Genes that are very far apart on a chromosome
  have a linkage ratio of 50,
• The relative distance between linked genes
  influences the amount of recombination observed.

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Dihybrid Cross
phenotype genotype gametes genotype

• yellow/round
  green/wrinkled
• GGWW x ggww
• GW gw
• GgWw

P F1
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Questions

• Dont rely on the answers in the back of the book
  to solve your problems
• Dont just solve them, but understand the
  principles needed to solve them.
• More practice questions are online. No answers
  are available, i.e. confirm your own answers

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a
b
f
c
e
d
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