Download Name Date__________ Period ______

Name ____________________________________________________________________________________ Date__________ Period ________
SEX-LINKED TRAITS & PEDIGREES
 Sex-Linked Traits
 As many of you know boys are different than girls. In humans sex is determined by the twenty third pair of chromosomes known
as “__________________________________________________”. If you have two x-shaped______________) chromosomes
you are destined to be a female. If you have X and Y-shaped (______________________) chromosomes you are destined to be a
male.
 Since the X and Y chromosomes carry different information, any genes found on the X chromosomes are referred to as
_________________________________________________________. Therefore, women will have _________alleles for these
genes because they have two (XX) chromosomes. On the other hand, men have only ____________allele for each of these genes
because they have only one X chromosome (XY)
 Since females have two X chromosomes and men only have one, they exhibit their X chromosomes differently. Females work like
every other trait we’ve been talking about – she has two alleles for the trait and however those two alleles are expressed
(dominant, recessive, etc) will determine her phenotype. Males, on the other hand, only have one X chromosome so whatever
alleles he has on that chromosome are expressed. This makes men more __________________________to sex-linked diseases.
 For sex-linked traits we list the genotype in a different fashion. We identify the individual as being male or female first, then write
what alleles are on those chromosomes. Sex-linked traits are only found on the X chromosome, therefore the letters are placed
above the X chromosome. The genotype for the female is_____________ and the male is________________.

Example of Sex-linked Cross:
In fruit flies, the gene for eye color is carried on the X chromosome, which is a sex chromosome (sex-linked). The allele for red
eyes shows complete dominance over the allele for white eyes. If a white-eyed female fruit fly is mated with a red-eyed male,
predict the possible offspring.
PRACTICE
1. In fruit flies, eye color is a sex linked trait. Red (R) is dominant to white (r)
a. What are the sexes and eye colors of flies with the following genotypes:
X R X r __________________ X R Y _________________ X r X r _________________
X R X R _________________ X r Y _________________
b. What are the genotypes of these flies:
white eyed, male ____________ red eyed female (heterozygous) ________
white eyed, female ___________ red eyed, male ___________
2. Show the cross of a white eyed female X r X r with a red-eyed male X R Y.
3. Show a cross between a pure red eyed female and a white eyed male.
a. What are the genotypes of the parents: ___________& _______________
b. How many are:
white eyed, male ____
white eyed, female ____
red eyed, male ____
red eyed, female ____
4. Show the cross of a red eyed female (heterozygous) and a red eyed male.
a. What are the genotypes of the parents? ___________ & ________________
b. How many are:
white eyed, male____
white eyed, female ___
red eyed, male ____
red eyed, female ____
5. Hemophilia is a sex-linked trait. A person with hemophilia is lacking certain proteins that are necessary for normal blood clotting.
Hemophilia is caused by a recessive allele so use “N” for normal and “n” for hemophilia. Since hemophilia is sex-linked, remember
a woman will have two alleles (NN or Nn or nn) but a man will have only one allele (N or n).
A woman who is heterozygous (a carrier) for hemophilia marries a normal man:
a.
What are the genotypes of the parents?
b.
Make a Punnett square for the above cross.
c.
What is the probability that a male offspring will have hemophilia?
d.
What is the probability of having a hemophiliac female offspring?
6. Can a color blind female have a son that has normal vision?
Color blindness is caused by a sex-linked recessive allele. *use N = normal vision and n = color blind
7. What parental genotypes could produce a bald woman?
Baldness is a sex-linked trait. *use H = normal hair, and h = bald
 PEDIGREES
In genetics, traits can be traced over several generations similar to a family tree. This family tree is called a Pedigree chart. Pedigree charts are
useful in gathering background genetic information that can be used for medical reasons. A key is helpful in reading pedigrees, since the various
symbols have specific meanings:
1.
Use the below pedigree chart to answer the following three questions.
Fast-witch muscles (F) show complete dominance over slow-twitch muscles (f).
a. Place the genotypes of each individual below its symbol.
b. What is the phenotype and genotype of individual #3 and 4?
c.
Can individual #8 or 9 be homozygous?
d. Explain the family relationship that #12 has with #2.
2.
Brown eyes (B) are completely dominant over blue eyes (b).
Label the genotype for each of the individuals below its symbol on the pedigree chart.
3.
Those affected have cystic fibrosis – which is caused recessive allele(f).
Label the genotype for each of the individuals below its symbol on the pedigree chart.
 Sex-Linked Pedigrees
4.
Hemophilia is a sex-linked trait. Non hemophilia (N) is completely dominant over hemopilia (n).
Label the possible genotypes for each of the individuals below its symbol on the pedigree chart.
5.
Color blindness is a sex-linked trait. Normal vision (N) is completely dominant over color blindness (n).
Label the possible genotypes for each of the individuals below its symbol on the pedigree chart.