Chromosomal Basis of
Inheritance
Featuring fruit fly:
Drosophila Melanogaster
Basic Terms/Information about
Drosophila
My diploid number
is 2N = 8
Wild-type (+) refers to the most
common phenotype in fruit flies.
It’s usually dominant (but not
necessarily)
Mutants are
non wild-type
traits
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 forThe
white
eyes. The gene
Y
is found on the “X” chromosome only.chromosome
This is considered
Xis
shorter than the
linked.
X
Does
the
gene
These
are
the
for
eye Y
color
X and
exist
on the “Y”
chromosomes
chromosome?
of a male fly.
Why
Howor
is why
the Ynot?
chromosome
R
r
different from
the X?
XX
r
XY
What
Whatwould
wouldbebethe
the
phenotype
phenotypeofofthis
thisfemale
male fly?
fly?
No, because the
gene for eye color
is found on the
longer segment of
the X
chromosome. This
part is missing on
the Y
White
eye
Red eye
male
female
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
Writing X-linked Genotypes
What is the possible
genotype(s) for the
eye color of this fly if it
is a female?
XrRXXr R or XRXr
What is the possible
genotype for the eye
color of this fly if it is
male? XrRYY
Answer the above questions again for
this fly.
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?
Phenotypic Ratio
Genotypic Ratio
Red-eye
female:Red-eye
male
R
r
R
X X :X Y
2:2 reduced to 1:1
2:2 reduced to 1:1
X
XR
XR
Xr
XRXr
XRXr
Y
XRY
XRY
Sample Problems
Example 2: What would
the genotypes and
phenotypes be of the
F2 generation?
X
Genotypic Ratio
XR
Xr
XR
XRXR
XRXr
Y
XRY
XrY
XRXR : XRXr : XRY : XrY
1 : 1 : 1
Phenotypic Ratio
: 1
Red-eye female : Red-eye male : White-eyed male
2
:
1
:
1
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?
Take, for example, colorblindness (Xb)
If you have a normal female, what is her possible
XBXB or _____
XBXb
genotype(s)? _____,
If you have a colorblind female, what is her genotype?
______
XbXb
If you have a colorblind male, what is his genotype?
______
XbY
How many colorblind genes do males need to inherit to be
colorblind? _____
1 Females? _____
2
Who does the male inherit the colorblind gene from?
_____________________________________
His mother, who donates the X chromosome
Other traits and alleles of
Drosophila melanogaster
Wild Type Traits
(+)
Gray body = G+
Mutant Traits
Normal wings = N+
Shriveled, vestigial
wings = n
Black body = g
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?
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 G+gN+n and ggnn.
Surprising
Predicted Results!
Cross
G+gN+n x ggnn
G+N
G+n
gN+
gn
gn
gn
gn
gn
25% G+gN+n
25% G+gnn
25% ggN+n
25% ggnn
Actual Results
8.5%
8.5%
41.5%
41.5%
Why did this happen???
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
Mendel’s Law of
Independent Assortment .
unless cross over
segregates the linked genes
G+ G+
N+
n
g
g
N+
n
Homologous
Chromosomes
What are recombinants?
Recombinants are offspring that have different
phenotypes from those of the parents.
Let’s look back at our original cross
X
X
C
B
8.5%
D
8.5%
41.5%
41.5%
A
Which offspring (A-D) from this
cross are the recombinants?
How do we determine if two genes
are linked or if two genes are
located on different chromosomes?
Answer
Calculate the
recombination
frequency! If the
frequency is less
than 50%, it is
assumed that the
two genes are on
same chromosomes.
X
C
B
206
D
185
944
965
A
Total
Flies: 965 Frequency
+ 206
+ 185= +recombinants
944
2500 flies
Recombination
/ total flies =
Calculate
the= recombination
391/2500
= .156
=frequency
16%
for+this
Total number
of recombinants:
206
185 =cross!
391
Gene Mapping
Grey
Body
Black
Body
Normal
wings
Small
wings
• 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)
Using recombination frequencies,
create a linkage map for the
following a - c:
between 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
b
c