Announcements

1
Announcements

• Exam 1 next week, 9/19, 9/20 in testing center.
  Covers chapters 1 through 4, with emphasis on
  material from lectures through 9/13, from Monk
  in the garden, and from lab. Part
  multiple-choice, part short answer - emphasis on
  problem-solving. No time limit, but must finish
  that day so choose a 2-3 hr. time block. Closed
  book bring calculator, 2 pencils and BLUE BOOK.
• Review sessions next week in lecture and in
  lab. Bring your questions!
• Problem set 2 answers due Friday, 9/13 at start
  of class. Also practice Ch.4 problems
  this week (but do not turn in) 1, 7, 16, 27, 31.
  
• 3. re. printing power point slide files when
  in computer room of Brooks, please choose
  handout when asked print what and
• print 6 slides/page. Do not print 1 slide per
  page.

2
Problem Set 2 due Friday 9/13 in class - show
all your work.

• The LM and LN alleles at the MN blood group locus
  exhibit codominance. Give the expected genotypes
  and phenotypes (with their ratios) of progeny
  from the following crosses
• a) LMLM x LMLN
• b) LMLN x LNLN
• A woman of blood group AB marries a man of blood
  group A whose father was group O. What is the
  probability that
• a) their 2 children will both be group A?
• b) one child will be group B and the other child
  group O?
• 3. In snapdragons, red flower color (R1) is
  incompletely dominant to white (R2) the R1/R2
  heterozygotes are pink. A red-flowered
  snapdragon is crossed to a white colored one.
  Determine the ratios of the flower colors in the
  progeny from a cross of an F1 with the red parent.

2 points each question part for 10 points total
3
Review of last lecture

• I. Chi-square revisited small deviation from
  expected yields small X2 value this correlates
  with high probability that deviation is due to
  chance and you should NOT reject your hypothesis
• II. Pedigree analysis- recessive vs. dominant
  traits
• - solving pedigree problems

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Solving Pedigree Problems

• Inspect the pedigree
• If trait is dominant, it will not skip
  generations nor be passed on to offspring unless
  parents have it.
• If trait is recessive, it will skip generations
  and will exist in carriers.
• Form a hypothesis, e.g. autosomal recessive.
• Deduce the genotypes.
• Check that genotypes are consistent with
  phenotypes.
• Revise hypothesis if necessary, e.g. autosomal
  dominant.

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Pedigree Example 2 p. 71, 26
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Outline of Lecture 7
In all crosses discussed so far, one of two
traits for a character has been dominant to the
other. ie. according to Mendels second
postulate of dominance/ recessiveness. Does the
expression of all genes occur in this way? ex.
Are there only two colors of hair for humans with
one clearly dominant to the other? NO

• I. Alleles alter phenotypes in different ways a
  variety of symbols are used for alleles
• II. Incomplete dominance - where neither allele
  is dominant
• III. Codominance - both alleles in a
  heterozygote are expressed
• IV. Multiple alleles of a gene are studied in a
  population
• V. Lethal alleles - recessive or dominant
• VI. Modification of the 9331 ratio

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I. Alleles - alternate forms of the same gene

• Wild-type allele - allele (form of gene) most
  frequently found in nature (normal) specifies
  normal phenotype and is usually dominant.
• Mutant allele specifies an altered phenotype.
• Mutation creates new alleles.

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Gene Symbol Conventions

• ebony body color mutation in Drosophila e
• Normal (wildtype) color is gray e
• e/e or / is homozygous wildtype
• e/e is homozygous ebony
• e/e or /e is heterozygous
• Other systems are also used, but symbol usually
  reflects the function of the gene, e.g. cdc,
  leu-, BRCA1

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II. Incomplete Dominance

• Neither of two alleles is dominant, e.g.
  snapdragon flower color
• R1 is red
• R2 is white
• Heterozygotes give an intermediate (blended)
  phenotype
• P1 cross gives pink flowers in F1
• F1 cross gives 121 redpinkwhite in F2

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III. Codominance

• When two alleles of a gene specify two distinct,
  detectable gene products
• MN blood group in humans LM, LN alleles
• MN locus codes for surface glycoprotein on red
  blood cells can detect immunochemically.
• LM LM gives M phenotype
• LM LN gives MN phenotype
• LN LN gives N phenotype
• LM LN X LM LN produces 1/4 LM LM, 1/2 LM LN,
  1/4 LN LN

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IV. Multiple Alleles ABO Blood Groups

• When 3 or more alleles present (allelic series)
  can only be studied in populations.
• A and B alleles code for glycoproteins on red
  blood cells which can be detected
  immunochemically
• mix blood sample with type A or type B antibodies
• look for clumping of RBCs
• O allele carries neither antigen

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ABO Blood Groups
A - A antigen only B - B antigen only AB -
Both A and B antigens O - Neither antigen
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ABO Genotypes and Phenotypes
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ABO, continued

• IA and IB are codominant
• Both IA and IB are dominant to IO
• All possible matings shown in Table 4.1
• Applications
• testing compatibility of blood transfusions
• disproving parentage of a child
• forensic science

15
Biochemical Basis of ABO

• A and B antigens are on carbohydrate groups bound
  to fatty acids (glycolipid) on RBC membrane
• The A and B alleles code for enzymes that
  differentially process the carbohydrate during
  synthesis of the glycolipid
• A enzyme adds N-acetylgalactosamine
• B enzyme adds galactose

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Complexity with ABO blood groups The Bombay
Phenotype
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Biochemical Basis of Bombay Phenotype

• h mutation prevents addition of fucose to form H
  substance.
• A and B enzymes no longer recognize structure,
  dont add A and B antigens.
• So individual is phenotypically O but can be
  genotypically A_, B_ or AB.
• H/h acts upstream of A and B in the pathway.

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Bombay Phenotype hh masks the expression of ABO
(Epistasis)
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The Secretor Locus also affects the expression of
the ABO blood type
About 80 of human population have the A and B
antigens present in various body secretions - not
only in blood. Genetics - dominant allele, Se
(Se/Se or Se/se)
In what societal application would the secretor
locus have significance?
Forensic science - ABO blood typing can be
performed on tissue samples other than blood.
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Multiple alleles - a second example

White locus in Drosophila - over 100 alleles may
occupy this locus. This results in an allelic
series of eye colors ranging from pure white, to
light buff to yellowish pink to deep ruby. (Table
4.3 in text)
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V. Lethal Alleles

• Recessive lethal if heterozygote is viable,
  homozygous mutants die common, e.g.
• Yellow (AY) in mice
• Manx (M) in cats
• Curly wing (Cy) in Drosophila
• Dominant lethal if either heterozygotes or
  homozygous mutants die rare, e.g. Huntington
  disease

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Mouse coat colors
Agouti x agouti Yellow x yellow Agouti x yellow
All agouti 2/3 yellow, 1/3 agouti 1/2 yellow, 1/2
agouti
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Effect of Dominant Yellow, Recessive Lethal in F2
AY is dominant to A AAY is yellow, but AYAY is
lethal
Results in 21 monohybrid ratio
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VI. Modified Dihybrid Cross First Consider Each
Trait on its Own
Ex. 2 humans heterozygous for albinism and are
blood type AB
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Modified Dihybrid Cross Next Consider Both
Traits Together