What is a dihybrid cross?

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Welcome

• What is a dihybrid cross?

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Agenda

• Quiz
• Sex Linked Traits
• Mutations

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Sex Determination

• Thomas Hunt Morgan studied fruit flies in the
  early 1900s

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Sex Determination

• Observed that one pair of chromosomes was
  different between males and females
• Large one named X chromosome
• Smaller one named Y chromosome
• XX female XY male

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XY
XX
X
Y
X
X
X
X
XX Female
XX Female
X
XY Male
XY Male
Y
50 Female 50 Male
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Sex Linkage

• Sex Linkage the presence of a gene on a sex
  chromosome (X or Y)

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Sex Linkage

• X-linked genes genes found on the X chromosome
• X chromosome carries more genes
• Y-linked genes genes found on the Y chromosome

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Fruit Fly Eye Color

• Fruit flies normally have red eyes
• A few males have white eyes
• Red is dominant
• white is recessive

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Morgans Fruit Fly Experiments

• Red-eyed female (XRXR) x White-eyed male (XrY)

XR
XR
XRXr
XRXr
RESULTS F1 generation all red-eyed
Xr
XRY
XRY
Y
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Morgans Fruit Fly Experiments

• Red-eyed female (XRXr) x Red-eyed male (XRY)

XR
Xr
RESULTS F2 generation 3 red-eyed and 1
white-eyed all white-eyed where maleswhy?
XRXR
XRXr
XR
XRY
XrY
Y
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Morgans Conclusions

• Gene for eye color is carried on the X chromosome
  eye color is an X-linked trait
• Y chromosome does not carry a gene for eye color
• Red-eyed XRXR, XRXr , XRY
• White-eyed XrXr, XrY

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• In humans colorblindness (b) is an example of a
  sex-linked recessive trait. A male with
  colorblindness marries a female who is not
  colorblind but carries the (b) allele.
• Using a Punnett square, determine the genotypic
  and phenotypic probabilities for their potential
  offspring.

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Welcome

• What is an X-Linked trait?

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Agenda

• Review sex linked traits
• Mutations
• Movie

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Review
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• In fruit flies red eye color (R) is dominant to
  white eyes (r). In a cross between two flies, 50
  of the male and 50 of the female offspring had
  red eyes. The other half of the males and females
  had white eyes.
• What are the phenotype, and all possible
  genotypes, of the offspring?

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Worksheet
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Linkage Groups

• 2 or more genes that are on the same chromosome
  are linked
• Linked genes tend to be inherited together

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More Fruit Fly Experiment

• Gray, long-winged (GGLL) x black, short-winged
  (ggll)
• F1 generation all heterozygous gray,
• long-winged (GgLl)

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Morgans Fruit Fly Experiment

• Cross F1 flies GgLl x GgLl
• F2 generation
• If alleles on different chromosomes, they assort
  independently and get a 9331 ratio
• If alleles on same chromosome, get 3 gray,
  long-winged 1 black, short-winged ratio
• Morgan saw roughly the 31 ratio

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Morgans Fruit Fly Experiment

• Unexpected results
• Some gray, short-winged (Ggll)
• Some black, long-winged (ggLl)

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Morgans Fruit Fly Experiment

• How were these alleles separated?
• Alleles were rearranged through
• crossing-over during meiosis
• Genes that are farther apart are
• more likely to be separated by crossovers

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Chromosome Mapping

• Chromosome Map diagram that shows the possible
  genes on a chromosome
• Made using crossing-over data

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Chromosome Mapping

• The percentage of crossing-over between the genes
  for 2 traits is equal to the distance between
  them on a chromosome
• This distance is measured in map units