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Sex Linked Traits | DAT
Sex Chromosomes
• Human chromosomes can be
mapped out in a chart called a
karyotype
• Everybody’s first 22
chromosomes are structurally
identical
o They appear as two
homologous X’s
• However, the structure of the 23rd
chromosome differs between
males and females
o It appears as two X’s in
females, and an X and a
Y in males
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Human Karyotype
For both males and females, chromosomes 1-22
appear as two homologous X’s. The 23rd
chromosome appears as two X’s in females, and
an X & Y in males.
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Genes carried on the 23rd
chromosome are said to be sexlinked
o Often, sex-linked traits
are carried on the Xchromosome and not the
Y-chromosome
Colorblindness
• Colorblindness is a sex-linked
trait that is carried exclusively on
the X-chromosome
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Furthermore, it is a recessive trait
o That is, someone will
only be colorblind if they
carry the recessive allele
and no dominant allele to
mask it
Suppose we have a female and a
male, neither of whom is
colorblind
But let’s suppose the female is
heterozygous for colorblindness
o That is, one of her Xchromosomes will carry
the recessive, colorblind
allele, and the other will
carry the dominant allele
o We will denote this as
XXC, where the C
denotes the recessive
colorblindness allele
Since she has the dominant allele
on one X-chromosome, the
colorblindness allele is masked
o Though she is not
colorblind, she is said to
be a carrier for the allele
So, the two parent’s 23rd
chromosomes will be denoted as
XXC and XY for the female and
male, respectively
If we want to predict whether or
not their offspring will be
colorblind, we put the sex
chromosomes into a Punnett
square and solve it
Punnett Square for Colorblindness
Inserting the parents’ sex chromosomes (green)
into the Punnett square, we solve as normal.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017
Sex Linked Traits | DAT
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Evidently, there are four possible
outcomes for the offspring
If the offspring is female (it
receives two X chromosomes), it
can receive:
o Two healthy X
chromosomes (red
square)
Making her noncolorblind
o One healthy X
chromosome and one Xchromosome with the
colorblindness allele
(blue square)
Since the
dominant allele
prevails, she will
not be colorblind
If the offspring is male (it
receives an X & Y chromosome),
it can receive:
o Healthy X and Y
chromosomes (orange
square)
Making him
noncolorblind
o A healthy Y chromosome
and an X-chromosome
with the colorblindness
allele (purple square)
Since the Ychromosome
doesn’t carry the
gene, the
dominant allele is
absent and so this
male only has the
colorblindness
allele
• He will be
colorblind
So, a female child has a 0%
chance of being colorblind and a
male child has a 50% chance of
being colorblind
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017