Name:_____________________________________
Genetics – Test Cross
1. In fruit flies, red eyes are dominant over sepia (brownish) eyes. Being the great genetic student that you
are, you happen to have a culture of pure red eye and pure sepia eye flies in your laboratory. While
working in your lab late one night, a cute, fuzzy, and fantastically friendly, red eyed fruit fly came in for a
crash landing on your banana. Wanting (naturally) to know more about your new friend, you decide to
run a test-cross on your little, buzzing buddy.
a. Give the phenotypes of the flies in your test-cross:____rr (tester)___X____R? (unknown)___
b. If all of the offspring turn out to be red-eyed (all 347 of them!!!) what would the genotypes of the
flies used in your test-cross? (Use “R” and “r”)
____rr_____X____RR______ Diagram the cross:
Genotypic ratio= ___1Rr:0________
Phenotypic ratio=___1 Red-eyed:0_______
c. If about 179 of the 347 show up with sepia eyes, what was the
actual genotype of your new found friend? ____Rr (1/2 will be the
recessive form)_____
r
R Rr
r
Rr
R Rr
Rr
Genetics – X Linked Genes
**In fruit flies, eye color is a sex linked trait. Red is dominant to white.**
2. What are the sexes and eye colors of flies with the following genotypes?
X R X r __f = red___
X R Y __m = red___
X R X R __f = red___
X r Y ___male = white_____
X r X r __f = white___
3. What are the genotypes of these flies:
white eyed, male ____ X r Y ____ red eyed female (heterozygous) __ f = red __
white eyed, female ____ X r X r ___ red eyed, male ___ X R Y ___
4. Show the cross of a white eyed female X r X r with a red-eyed male X R Y
5. Show a cross between a pure red eyed female and a white eyed male.
6. What are the genotypes of the parents:
XR
Y
Xr
XRXr
XrY
Xr
XRXr
XrY
Xr
Y
XR
XRXr
XRY
XR
XRXr
XRY
___ X R X R __ and ____ X r Y _____
How many are:
white eyed, male
white eyed, female
__0__
__0__
red eyed, male
red eyed, female
__2 of 4__
__2 of 4__
7. Show the cross of a red eyed female (heterozygous) and a red eyed male.
8.
What are the genotypes of the parents?
XR
Xr
XR
XR XR
XRXr
Y
XRY
XrY
___ X R X r ___ & _____ X R Y ____
How many are:
white eyed, male
__1 of 4 = 25%__
white eyed, female
__0__
red eyed, male
__1 of 4 = 25%__
red eyed, female
__2 of 4 = 50%__
Math: What if in the above cross, 100 males were produced and 200 females. How many total red-eyed flies
would there be? ___50 red-eyed males + 200 red-eyed females = 250 red-eyed flies_____
Human Sex Linkage
9. In humans, hemophilia is a sex linked trait. Females can be normal, carriers, or have the disease.
Males will either have the disease or not (but they won’t ever be carriers)
X H X H = female, normal
X H Y = male, normal
X H X h = female, carrier
X h X h = female, hemophiliac
X h Y= male, hemophiliac
Show the cross of a man who has hemophilia with a woman who is a carrier.
Xh
Y
X
Y
H
XH
XHXh
XHY
Xh
XhXh
XhY
XH
XHXH
XHY
Xh
XHXh
XhY
What is the probability that their children will have the disease? __50% = ½ the
males & females________
10. A woman who is a carrier marries a normal man. Show the cross. What
is the probability that their children will have hemophilia? What sex will a child in the family with
hemophilia be?
25% of their children, only ½ the males
11. A woman who has hemophilia marries a normal man. How many of their
children will have hemophilia, and what is their sex?
½ of their children will have hemophilia, all males will have it.
X
Y
H
Xh
XH Xh
XhY
Xh
XHXh
XhY