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Welcome
What is a dihybrid cross?
Agenda
• Quiz
• Sex Linked Traits
• Mutations
Sex Determination
• Thomas Hunt Morgan – studied fruit flies in
the early 1900’s
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
XY
X
XX
Y
X
Y
X
X
X
X
XX
XX
Female
Female
XY
XY
Male
Male
50% Female; 50% Male
Sex Linkage
• Sex Linkage: the presence of a gene on a
sex chromosome (X or Y)
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
Fruit Fly Eye Color
• Fruit flies normally have red eyes
• A few males have white eyes
• Red is dominant;
white is recessive
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXR) x White-eyed male (XrY)
XR
XRX r
XR
XRXr
Xr
F1 generation –
all red-eyed
XRY
Y
RESULTS:
XRY
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXr) x Red-eyed male (XRY)
XR
Xr
XRX R
XRXr
F2 generation –
3 red-eyed and 1
white-eyed
XRY
XrY
** all white-eyed
where males…why?
XR
Y
RESULTS:
Morgan’s 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
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.
Welcome
What is an X-Linked trait?
Agenda
• Review sex linked traits
• Mutations
• Movie
Review
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?
Worksheet
Linkage Groups
• 2 or more genes that are on
the same chromosome are
“linked”
• Linked genes tend to be
inherited together
More Fruit Fly Experiment
• Gray, long-winged (GGLL) x black, short-winged (ggll)
• F1 generation = all heterozygous gray,
long-winged (GgLl)
Morgan’s Fruit Fly Experiment
• Cross F1 flies: GgLl x GgLl
• F2 generation
– If alleles on different chromosomes, they assort
independently and get a 9:3:3:1 ratio
– If alleles on same chromosome, get 3 gray, longwinged: 1 black, short-winged ratio
– Morgan saw roughly the 3:1 ratio
Morgan’s Fruit Fly Experiment
• Unexpected results
– Some gray, short-winged (Ggll)
– Some black, long-winged (ggLl)
Morgan’s 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
Chromosome Mapping
• Chromosome Map: diagram
that shows the possible genes
on a chromosome
• Made using crossing-over data
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