Chromosomal Basis of Inheritance

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Chromosomal Basis of Inheritance

• Featuring fruit fly
• Drosophila Melanogaster

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Basic Terms/Information about Drosophila
My diploid number is 2N 8
Wild-type () refers to the most common phenotype
in fruit flies. Its usually dominant (but not
necessarily)
Mutants are non wild-type traits
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Sex-Linkage or (X-linked)
In fruit flies, (R) is the dominant wild-type
gene for red eyes, and (r) is the recessive,
mutant gene for white eyes. The gene is found on
the X chromosome only. This is considered
X-linked.
The Y chromosome is shorter than the X
Does the gene for eye color exist on the Y
chromosome? Why or why not?
These are the X and Y chromosomes of a male fly.
How is the Y chromosome different from the X?
No, because the gene for eye color is found on
the longer segment of the X chromosome. This part
is missing on the Y
R
r
r
XX
XY
White eye male
Red eye female
What would be the phenotype of this female fly?
What would be the phenotype of this male fly?
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Sex-Linkage or (X-linked)

• When genes are sex-linked, we include the X and Y
  as part of their genotype. For example, the
  allele for red eye is not R but is written as
  XR. How would you write the allele for white eye?

Xr
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Writing X-linked Genotypes

• What is the possible genotype(s) for the eye
  color of this fly if it is a female?
• What is the possible genotype for the eye color
  of this fly if it is male?

XRXR or XRXr
XrXr
XRY
XrY
Answer the above questions again for this fly.
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Sample Problems

• Example 1 What is the F1 genotypic and
  phenotypic ratio of a female true-breeding
  wild-type fly for red eyes crossed with a
  white-eyed male?

X


XR
XR
XRXr
XRXr
Xr
Phenotypic Ratio Red-eye femaleRed-eye male
22 reduced to 11
XRY
XRY
Genotypic Ratio XRXrXRY 22 reduced to 11
Y
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Sample Problems

• Example 2 What would the genotypes and
  phenotypes be of the F2 generation?

X


XR
Xr
XRXR
XRXr
XR
Genotypic Ratio XRXR XRXr XRY XrY 1
1 1 1
XRY
XrY
Y
Phenotypic Ratio Red-eye female Red-eye male
White-eyed male 2
1 1
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X-linked disorders

• Definition diseases or disorders whose genes are
  found on the X-chromosome, but not on the Y.
• Ex hemophelia (Xh), color blindness (Xb),
  muscular distrophy (Xm)
• If the disorder is recessive, more males than
  females will tend to have the disorder.

Why?
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Take, for example, colorblindness (Xb)

• If you have a normal female, what is her possible
  genotype(s)? _____, or _____
• If you have a colorblind female, what is her
  genotype? ______
• If you have a colorblind male, what is his
  genotype? ______
• How many colorblind genes do males need to
  inherit to be colorblind? _____ Females? _____
• Who does the male inherit the colorblind gene
  from? _____________________________________

XBXB
XBXb
XbXb
XbY
1
2
His mother, who donates the X chromosome
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Other traits and alleles of Drosophila
melanogaster
Wild Type Traits () Mutant Traits
Gray body G Black body g
Normal wings N Shriveled, vestigial wings n
Body-color and wing-type are NOT located on the
sex chromosome, so they are considered
autosomal Things to think about
independently.How would you confirm or test that
these mutant traits are recessive?
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Example 3

• In flies, grey bodies (G) and normal-wing size
  (N) are dominant to black bodies (g) and small
  wing size (n).
• Predict a cross between GgNn and ggnn.

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Predicted Cross GgNn x ggnn
Surprising Results!
GN
Gn
gN
gn




gn
gn
gn
gn
25 GgNn
25 Ggnn
25 ggNn
25 ggnn
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Actual Results
8.5
8.5
41.5
41.5
41.5
41.5
Why did this happen???
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Linked Genes

• The genes for body color and wing size are
  linked, meaning they are found on the same
  chromosome.
• They will most likely be inherited together
  and will not undergo Mendels Law of .
• cross over segregates the
  linked genes

g
g
G
G
Independent Assortment
n
N
n
N
unless
Homologous Chromosomes
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What are recombinants?

• Recombinants are offspring that have different
  phenotypes from those of the parents.
• Lets look back at our original cross

X
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X
C
B
D
8.5
8.5
41.5
41.5
41.5
41.5
A
Which offspring (A-D) from this cross are the
recombinants?
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How do we determine if two genes are linked or if
two genes are located on different chromosomes?
Calculate the recombination frequency! If the
frequency is less than 50, it is assumed that
the two genes are on same chromosomes.
Answer
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X
C
B
D
185
206
944
965
A
Calculate the recombination frequency for this
cross!
Total Flies 965 206 185 944 2500 flies
Recombination Frequency recombinants / total
flies 391/2500 .156 16
Total number of recombinants 206 185 391
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Gene Mapping

• Genes are mapped on a chromosome based upon the
  recombination frequency.
• For ex. the distance between the genes for body
  color and wing type is therefore 16 map-units
  apart (16 frequency)

Black Body
Grey Body
Small wings
Normal wings
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Using recombination frequencies, create a linkage
map for the following a - cbetween genes b and
a 10.5 between genes c and a 48 between
genes c and b 37.5
10.5
37.5
48
a
c
b