1. Sex-Linked Traits Practice ProblemTt t t
Sex-Linked Traits Practice Problem

2. Sex-linked Traits
Sex-linked traits discussed here are recessive and on the X chromosome.
Each parent can only give one sex chromosome:
Males – either an X or a Y.
Females – one of the two X chromosomes

3. Setting up the Cross
With sex-linked traits, any cross will have male (♂) and female (♀) chromosomes
with the traits written as superscripts.
Example: In cats, we represent black fur color with a b and yellow fur color with a B.
XbY and XbXb create black
XBY and XBXB create yellow
XBXb create tortoiseshell

4. Setting up the Cross
The only way to get a tortoiseshell- colored cat is for 2, X chromosomes to be present.
XbY and XbXb create black
XBY and XBXB create yellow
XBXb create tortoiseshell

5. Sex-linked Practice Problem In cats, black fur color is due to an allele b and yellow fur color to the allele B. The heterozygous condition Bb, results in a mixed pattern of black and yellow referred to as tortoise shell. These alleles are sex-linked. What kind of offspring would be expected to result if a black male is mated to a yellow female? Show the Punnett square. Could you ever have a cross that would result in any tortoise shell males? Explain. XB XB
THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE
♂ x ♀ 
XbY X XBXB Xb Y
XbY or XbXb black XBXb tortoise shell XBY or XBXB yellow

6. Sex-linked Punnett Square Practice Problem
XbY X XBXB
THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE
DROP THE LETTERS ON THE TOP, INTO EACH SQUARE XB XB Xb XB Xb XB Xb
MOVE EACH LETTER ON THE SIDE, INTO EACH SQUARE Y
X GOES BEFORE Y IN THE BOXES XB Y XB Y

7. Sex-linked Punnett Square Practice Problem
XbY X XBXB
WHAT DO THE RESULTS SHOW?
RESULTS: XB XB
XbY and XbXb create black
XBY and XBXB create yellow
PHENOTYPES: 50% tortoise shell
50% yellow
0% black Xb
XBXb
XBXb
XBXb creates tortoise shell
GENOTYPES:
50% XBXb
50% XBY Y
XBY
XBY
THEN WE CAN MAKE PREDICTIONS ABOUT THE OUTCOMES

8. Questions answered What kind of offspring could you expect?
50% yellow and 50% tortoiseshell kittens.

9. Dobby – my daughter’s male calico cat!
Questions answered
Could you ever have a cross that would result in any tortoise shell males? Explain.
If only two chromosomes are present, only females can be tortoiseshell.
BUT, if a mistake occurs and the cat has three sex chromosomes, like a XXY combination, you COULD get a male tortoiseshell. The male is often sterile, but it
happens (1:3,000 chance).
Calico-coloring in cats is also sex-linked
and follows the same rules.
Dobby – my daughter’s male calico cat!