3/2/2016
Autosomes v. Sex Chromosomes
More Complex
Inheritance Patterns:
Sex-Linked Traits
• Of the 23 pairs of chromosomes (46 total) in
humans…
• 22 pairs = autosomes
– ♀ and ♂ autosomes are the same type
• 1 pair = sex chromosomes
– ♀ = XX,
♂ = XY
X and Y chromosomes photographed
by electron microscope
What do you notice
about the X and Y
chromosomes?
Punnett Squares with Sex
Chromosomes
• Sex is determined by
the father of a child.
• Why?
– The father can give an X
or Y but the mother can
only give an X.
• Every time a couple has
a baby, there is a 50%
chance of having a boy
and a 50% chance of
having a girl.
X
X
X
XX
XX
Y
XY
XY
Sex-linked
Traits
• Sex chromosomes also control
traits beyond what sex you are
– Sex-linked traits = traits of
genes located on sex chromosomes
• Sex-linked traits are usually
located on the X chromosome,
tend to affect males more than
females. WHY??
…What do you know about alleles
and homologous chromosomes?
≈ 1,000 genes on X, ≈ 90 on Y
Colorblindness is a sex-linked trait
People with normal vision should see a light
image of a red/brown sailboat.
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3/2/2016
Sex-linked Traits in Humans
• ♂ more likely to show sex-linked recessive
traits because only one X chromosome
…If he inherits a recessive allele (on X), there is no
paired dominant allele to mask it
• A carrier has the recessive allele, but it is not
expressed
…If she inherits a recessive allele (on X), she has a
paired dominant X allele to mask it
– ♂ cannot be a carrier of sex-linked trait – he will
always express the trait
– Example: color blindness
Other sex-linked traits
• Baldness (some types)
• Muscular Dystrophy
• Hemophilia
• Because the X chromosome carries
alleles for sex-linked traits, X is used for
the alleles
– The traits are written as superscripts.
More colorblindness test
Color-blindness
• Ex: balding in humans- XB=not bald, Xb=bald
Punnett Squares for Baldness
bald dad x carrier mom
Is your father bald?
Will you be bald?
– Does a son inherit
baldness from his
dad?
XB
Xb
Xb
Y
XB X b
XB Y
Xb X b
bald dad x homozygous non-bald mom
• If mom is a carrier
for baldness (XB Xb),
son has a 50%
chance of being bald!
…So who should you
look at to see if you
will be bald?
XB
XB
XbY
Xb
Y
XB Xb
XB Y
XB Xb
XBY
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