Non-Mendelian Genetics

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Non-Mendelian Genetics

• Inheritance patterns that dont follow the rules

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A. Linkage

• 1. Image of Bateson and Punnett
• 2. tried to replicate work of Mendel using other
  traits of garden pea
• 3. traits investigated
• -flower color P (purple) and p (red flowers)
• -pollen size L (long) and l (short pollen)

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4. Parental cross

• a. pure breeding purple flowers with long pollen
  grains were crossed with pure breeding red
  flowered plants which possessed short pollen
• b. write down the genotypes of the parental
  generation
• c. write down the genotypes of the F-1 generation
• d. the F-1 generation were then test crossed
• e. expected outcome?
• f. actual outcome 7P_L_1ppL_1P_ll7ppll

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5. Explanation

• a. results far from anticipated 1111
• b. showed that alleles for purple color and long
  pollen traveled together and that red color and
  short pollen traveled together usually
• c. create punnett square for the test cross
• d. which parent determined the outcome of the
  offspring
• e. expected that equal numbers of each gamete
  type would be produced
• f. appears that alleles traveled as a unit most
  of the time-why

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g. linkage
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If crossing over happens, four different gamete
types are produced, but in very different numbers.
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Sample Problem

• In Drosophila melanogaster there is a dominant
  gene for gray
• body color and another dominant gene for normal
  wings.
• The recessive alleles of these two genes result
  in black body
• color and vestigial wings respectively.
• Flies homozygous for gray body and normal wings
  were crossed with flies that had black bodies and
  vestigial wings.
• The F1 progeny were then test-crossed, with the
  following results
• Gray body, normal wings 236
• Black body, vestigial wings 253
• Gray body, vestigial wings 50
• Black body, normal wings 61
• Would you say that these two genes are linked? If
  so, how
• many units apart are they on the chromosome?

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B. Mapping

• 1. in the example above crossing over occurred
  2/16 times or 12.5 of the time
• 2. if the above two genes had been found further
  apart on the chromosomes, would they have been
  more or less likely to rearrange themselves by
  crossing over?
• 3. further apart two genes are on a chromosome,
  the more likely they are to recombine by the
  process of crossing over
• 4. by convention, a 1 crossing over frequencey
  occurs when the two genes are 1 map unit apart
  from each other

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5. Sample problem-determine the order of the
following genes

• a. the frequency of crossing over between the
  following genes is listed below
• b. A and B 40
• B and C 20
• C and D 10
• C and A 20
• D and B 10

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6. Other points about mapping

• a map unit previously was called a Dalton
• the physical length of a map unit is not constant
• some regions of a chromosome experience crossing
  over more frequently than others
• more money was spent on fruit fly genetics
  determining order of genes on their chromosomes
  than was getting men to the moon

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C. Multiple allele inheritance-blood type

• 1. definitions of antigens and antibodies
• 2. agglutination
• 3. three alleles involved in blood type
• IA produces antigen A on the surface of the RBC
• IB produces antigen B on the surface of the RBC
• i produces neither of the antigens A nor B
• notice that the first two alleles are codominant
• An individual who doesnt possess the antigen
  does possess a circulating antibody against the
  antigen

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4. Table of genotypes/phenotypes
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5. Differing percentages by race
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6. Practice genetics problem

• If a man with blood type B, one of whose parents
  had blood type O, marries a woman with blood type
  AB, what will be the theoretical percentage of
  their children with blood type B?

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7. Rh factor
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D. Gene Interactions-dont appear as typical
9331 ratios

• Collaboration
• Two dominant alleles produce a phenotype that
  neither can produce alone
• Mutually dependent upon one another to produce
  the new phenotypes
• Comb types in chickens good example
• Single comb shown here

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e. Other comb types

• R produces rose combs
• P produces pea combs
• R_P_ produces walnut

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• f. RRpp (Wyandotte chicken) X rrPP (Brahma
  rooster)
• g. list the F1
• h. free range breeding to produce the F2

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Sample Problem

• A dominant gene, A, causes yellow color in rats.
• The dominant allele of another independent gene,
  R, produces black coat color.
• When the two dominants occur together (A/-R/-),
  they interact to produce gray.
• Rats of the genotype a/a r/r are cream-colored.
• If a gray male and a yellow female, when mated,
  produce offspring approximately 3/8 of which are
  yellow, 3/8 gray, 1/8 cream, and 1/8 black,
• What are the genotypes of the two parents?

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2. Epistasis

• a. the dominant allele of one gene overshadows
  the expression of the dominant allele of a second
  gene
• b. C is the color gene in fowl
• c. cc produces a white fowl
• d. I is epistatic over the C allele of the color
  gene
• e. any bird who possesses I in his/her genotype
  is going to be white
• f. ii will be colored if the dominant allele C is
  present

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• g. IICC (white leghorn chicken) X iicc (white
  Wyandotte rooster)
• h. show the F1 and F2 results

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Sample Problem

• In Leghorn chickens colored feathers are due to a
  dominant gene, C
• White feathers are due to its recessive allele,
  c.
• Another dominant gene, I, inhibits expression of
  color in birds with genotypes CC or Cc.
  Consequently both C-I- and cc-- are white.
• A colored cock is mated with a white hen and
• produces many offspring, all colored.
• Give the genotypes of both parents and offspring.

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3. Multiple gene inheritance

• a. so far been discussing traits that are
  governed only by one gene
• b. far from the truth
• c. most phenotypes that we are aware of are
  distributed in a bell-shaped curve like human
  height
• d. often multiple genes affect such traits
• e. height in plants might be affected by three
  genes each possessing two alleles
• f. the dominant allele of each gene might add 1
  cm to basic height of plant
• g. the recessive allele of each gene would not
  affect the basic 10 cm height
• h. aabbcc X AABBCC
• i. F1 generation selfs itself
• j. 1/64 6/64 15/64 20/64 15/64 6/64 1/64
• k. the more genes affecting a trait, the
  smoother is the bell curve
• l. the environment also affects phenotype
  smoothing off the curve even more

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Human Pigmentation
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E. Sex Determination in humans

• 1. human condition there are 23 pairs of
  homologous chromosomes
• 2. 22 pairs of autosomes-nonsex chromosomes
• 3. one pair of sex chromosomes that exist in two
  different configurations

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• 4. X and Y
• XX female and XY is male
• 5. Concept of Barr Body and determination of
  genetic sex
• 6. Females are mosaics when it comes to traits
  carried on the X chromosome as either one or the
  other X chromosome can be active

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7. XO Turners syndrome

• -a. appear as a female phenotypically
• -b. the female pattern is the default pattern
  set to develop as female unless something
  triggers a change-
• -c. but doesnt develop into the adult pattern
• -d. hips and breasts remain immature
• -e. no Barr bodies
• -f. mental function is not impaired
• -g. 1/5000 affected
• -h. approximately 15-20 of aborted fetuses

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F. Sex Linkage

• 1. genes that are carried on the X chromosome are
  said to be sex-linked
• 2. since the male only has one X chromosome, he
  only has one allele present for that trait
• 3. if he has the allele, it is expressed no
  matter what
• 4. males are genetically weaker than females for
  this reason
• 5. the female has a back up copy of the allele if
  the first one is defective
• 6. another interesting point is that a male
  inherits his sex-linked traits from his mother
  ie male pattern baldness
• 7. example is hemophilia
• a. the trait for hemophilia is inherited as a
  sex-lined recessive trait
• b. express the alleles as
• XH for normal and Xh as the allele that
  represents the recessive state
• c. example

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Practice Problems

• Red-green color blindness is inherited as a
  sex-linked recessive. If a color-blind woman
  marries a man who has normal vision, what would
  be the expected phenotypes of their children with
  reference to this character?
• Suppose that gene b is sex-linked, recessive, and
  lethal. A man marries a woman who is heterozygous
  for this gene. If this couple had many normal
  children, what would be the predicted sex ratio
  of these children?