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A U.S. soldier from
the Vietnam War who
had been placed in
the Tomb of the
Unknown Soldier at
Arlington National
Cemetery was
identified through
DNA fingerprinting.
The soldier could have
been one of four
individuals. A DNA
sample from his body
was analyzed. The
DNA from the
parents was also
analyzed.
The diagram on the slide shows the DNA fingerprint pattern
similar to the one that was actually done. Find the match
between the soldier’s DNA fingerprint and those of his parents.
Below are the karyotypes of two individuals
What is the difference?
Below are two additional karyotypes.
What is the difference?
What We Have Learned
If you are female, the 23rd pair of chromosomes
are homozygous
XX
If you are male,
the 23rd pair of
chromosomes are
heterozygous
XY
Based on what you know about the
determination of sex in humans,
Which parent determines the sex
of the offspring?
The offspring of
any mating
between humans
will have a 50:50
chance of having
2 X chromosomes,
XX, which is
female, or having
one X and one Y
chromosome, XY,
which is male.
Drosophila (fruit flies), inherit sex
chromosomes in the same way as
humans.
Some traits are controlled by genes located on sex
chromosomes and are called sex-linked traits.
The alleles for sex linked traits are written as
superscripts of the X? and Y ? chromosomes.
Because the X and Y chromosomes are
not homologous, the Y chromosome has
no corresponding allele to the one on the
X chromosome.
Remember that any recessive allele on the X
chromosome of a male will not be masked by
a corresponding dominant allele on the Y
chromosome.
Since the Y chromosome has no corresponding
allele to the one on the X chromosome any trait
carried on the X chromosome will be expressed in
males.
In fruit flies red eyes (R)
are dominant to white
eyes (r).
XRXR
XRXr
XRY
XrY
Based on what we know about sex linked
inheritance of eye color in fruit flies, what
would be the genotype of the female fruit fly
below?
X? X?
Create a Punnet Square that shows the cross of a
male fruit fly with white eyes with a homozygous
dominant red eyed female.
XR
Xr
Y
XR Xr
XR Y
XR Xr
XR Y
XR
For Women there are three possibilities
XX
Normal Vision
Color blindness in
humans is also a
sex linked trait
that is carried on
the X chromosome.
XC X
Normal Vision
but a carrier for color
blindness
XC X C
Color Blind
. . . But for males
there are only
2 possibilities
XY
XC Y
Color Blind
Normal Vision
Remember that since
the Y chromosome
has no corresponding
allele to the one on
the X chromosome
any trait carried on
the X chromosome
will be expressed
in males.
X
If a color blind man
marries a women
with normal vision,
what are the chances
of one of their
children being color
blind?
X
XC
XC X
XC X
Y
XY
XY
Woman who is a
carrier for color
blindness marries a
normal vision man,
what are the chances
of one of their
children being color
blind?
XC
X
X
XC X
XX
Y
XC Y
XY