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Sex linked traits & pedigrees
Sex determination
– Female and
males have same
autosomes.
– Females have
“XX” for sex
chromosomes.
• Gametes only X
– Males have “XY”
for sex
chromosomes.
• Gametes x or y
X
X
X
XX
XX
Y
XY
XY
Sex Linked Traits
 Sex linked traits are
located on the X sex
chromosome.
 Males:
 XR Y
 XrY
 Females:
 XR XR
 XR Xr
 XrXr
Males: ½ chance of
recessive phenotype
Females: 1/3 chance of
recessive phenotype
Cross a carrier female for white eyes with a male
that is white eye. What percent of the offspring
will have white eye?
Xr
Y
XR
XRXr
XR Y
Xr
Xr Xr
Xr Y
P = XRXr x XrY
Multiple Alleles
 Instead of
two versions
or alleles
there are 3
or more
alleles.

•IA, IB, i are the
alleles for blood
type.
•Possible
genotypes would
be
–IA IA, Iai
–IB IB , IBi
–IA IB
–ii
What blood types might result in a family in
which the mother is O and the father is AB?
i
i
IA
IA
i
IA
i
•P = IAIB x ii
50% A, 50%B
IB
IB
i
IB
i
• A pedigree is a type of family tree that
shows the sex of the individuals and the
phenotype of that individual for a single
trait
• Circles are female ,squares are male
• Filled in has the ‘affected’ phenotype,
open has the ‘unaffected’
• The trait examined can either be
dominant or recessive
• A person who has the trait but does
not express it is a “carrier”
• (heterozygous for a recessive
trait)
Pedigrees
Using Pedigrees
• A pedigree can be used to determine the genotype of individuals in
a family
• It can be used to track a, dominant, recessive , or sex-linked trait
• Pair share: is the trait dominant or receive? How do you know? If it
is recessive are there any carriers?
What does this pedigree tell us?
Review
1.
2.
3.
For a sex-linked trait
what are the odds that a
male will have the
recessive phenotype?
What is a pedigree?
Think back to the
karyotyping lab. Name a
disorder that can occur
due to trisomy.