Suggested Problems for Chapter 4

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Suggested Problems for Chapter 4 1,3,5,11,13,18,
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Mendels Postulates (Laws)

• Unit factors occur in pairs
• Genetic characters are controlled by unit
  factors that exist in pairs in an individual
  organism

2. Dominance and recessive forms of unit factors
exist Of the two forms observed for a given
character and controlled by two forms of a unit
factor, one form will dominate (dominant form)
over the other (recessive form)
3. Segregation of unit factor During gamete
formation, the paired unit factors separate (or
segregate) randomly so that each gamete receives
on or the other unit factor with equal likelyhood
4. Independent assortment of unit factor During
gamete formation, segregating pairs of unit
factors assort independent of one another
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Alleles

• Alleles are alternate forms of the same gene.
• Alleles contain modified genetic information
  which specifies an altered gene product.
• An allele that occurs most often, is dominant, or
  is arbitrarily designated as normal and is
  often referred to as the wild type allele.

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Alleles of the Seven Characters
Each character has two versions, or alleles
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Alleles

• Alleles are alternate forms of the same gene.
• Alleles contain modified genetic information
  which specifies an altered gene product.
• An allele that occurs most often, is dominant, or
  is arbitrarily designated as normal and is
  often referred to as the wild type allele.

• A given gene may have multiple alleles (ie. more
  than 2).
• New alleles arise from mutations of existing
  alleles.

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Allele Symbols

• For peas, we have used letters of the alphabet to
  designate genes and capital or lower case letters
  to designate the alleles (SS ss).
• Some times a superscript is used to designate
  the wild type allele of a gene that is named for
  the mutant allele. The W allele in the mouse
  or Wr in drosophila fruit flies are examples.
• Often when there is no clear dominant allele, a
  capital letter will be used to designate the gene
  and the various alleles will be designated as
  superscripts (examples R1/R2 alleles for flower
  color in four oclock plants, LM/LN alleles for
  blood groups).

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Dominant and Recessive
Not all alleles behave as either dominant or
recessive.
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Incomplete Dominance
1
2
1


Thus, an intermediate phenotype occurs and the F2
generation has a phenotype ratio that matches the
genotype ratio.
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Co-dominance
Both alleles are expressed in the heterozygote
(F1). How does the F2 behave?
121 ratio!
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Biochemical Basis for ABO Blood Types
X
H-enzyme
A-enzyme
B-enzyme
No enzyme
What about the Bombay phenotypes?
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Coat Color in Mice
Tan, extreme non-agouti, yellow, and agouti are
all alleles of the coat color agouti gene (A).
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• Agouti Coat Color in Mice
• Normal colour is agouti, mutant is yellow
• A agouti x agouti all agouti (4/4)
• B yellow x yellow 2/3 yellow 1/3 agouti
• C agouti x yellow 1/2 yellow 1/2 agouti

What do you notice about the ratios?
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Lethal Alleles
Agouti gene is pleiotropic ie it effects
several different systems.
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• Lethal alleles
• Many gene products are essential for life
• Mutations in these may be tolerated in
  heterozygous condition, but are lethal when
  homozygous (recessive lethal allele)
• Usually die during development, but it can last
  into adulthood
• Sometimes gives a distinctive phenotype to
  heterozygotes

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• Combinations of Inheritance Patterns
• The previous examples modify the F1 31 ratio and
  therefore will also modify a 9331
• For example a cross involving albinism (simple
  recessive) and ABO blood groups (multiple
  alleles)
• Cross two individuals heterozygous for
  albinism (Aa) and AB blood type (IAIB)

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Dihybrid Crosses with Modified Phenotypic Ratios
The product law of probabilities(when two
independent events occur simultaneously, the
combined probability of the two outcomes is equal
to the product of their individual occurrence)
applies here too.
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Dihybrid Crosses with Modified Phenotypic Ratios
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• Combining Modes of Inheritance
• Easy once you work out probabilities from
  individual crosses
• Can be solved with Punnett squares as well
• Dihybrid crosses are always a modification of the
  9331 ratio
• (note that 363121 still has 16 genotoypes)

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ABO Modifications The Bombay Phenotype
H substance gene, H/h
X
hh geneotype
Blood group gene, I/i
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• Epistasis
• Occurs when two loci control the same phenotypic
  character
• Can be one pair of genes masking another but
  could also be a complementary or cooperative
  effect
• If a pair of alleles prevent or override another
  pair, they have an epistatic effect, while the
  effected alleles are hypostatic to the first set

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Bombay Phenotype An Example of Epistasis
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Bombay Phenotype Altered ABO Ratios
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Bombay Phenotype Altered ABO Ratios
Again the ratios are altered, but are based on 16.
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Karyotype
46, XY Male
46, XX Female
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More Altered Paterns of Inheritance
Drosophila melanogaster
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X-Linked Inheritance
White eyes is rare even in males
Red is dominant
Note the ratio (211) is based on 4.
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X-Linked Inheritance
But again the ratio (1111) is based on 4.
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What Do You See?
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X-Linked Recessive Inheritance
Tends to be expressed in males and carried by
female, however, females can express the trait if
homozygous.
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Human X-linked Inheritance
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Human X-linked Inheritance
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X-Linked Recessive
Note that when X-Linked recessive inheritance
appears to begin with a male, a medical history
will reveal that the brother of the affected
males mother (uncle) also was affected.
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X-Linked Dominant
What pattern of inheritance would you expect?
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Dosage Compensation

• Females have two X chromosomes while males have
  only one.
• One X chromosome in each female cell is
  inactivated to prevent females from making twice
  as much X gene product as males.

How do we know this is true?
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Orange Coat Color Locus
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Effect of X-Inactivation

• Orange locus is on the X chromosome
• The O allele modifies black eumelanin to
  produce haeomelanin
• There are two alleles O and o

• OO gives an orange female cat
• OY gives an orange male cat
• oo gives a black female cat
• oY gives a black male cat
• Oo gives a tortoiseshell orange and black
  mottled female cat

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Orange Coat Color Locus
OO or OY
oo or oY
Oo
or OoY (sterile)
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Orange Coat Color Locus Mechanism of Mottling
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Random X-Inactivation Leads to Dispersed Paterns