Download Sex-Linkage (X-Linked Traits)

Sex-Linkage
(X-Linked Traits)
Drosophila melanogaster
Thomas Hunt Morgan – (the
father of modern genetics)
He found genes located on the X
chromosome that regulate fruit fly eye
color.
Genes on the X
chromosome are
said to be Xlinked
Thomas Hunt Morgan was studying evolution
and was looking for mutations in fruit flies
(Drosophila).
Morgan and his “F1s”
(His two daughters)
Morgan noticed that while most fruit flies
had red eyes, there were a few whiteeyed fruit flies.
He also noticed that ones that had white
eyes were all males.
Morgan’s
Experiment:
1) Morgan crossed a true-breeding, red-eyed
female with a white-eyed male, he got all redeyed flies in the F1 generation.
2) When he crossed two F1 red-eyed flies,
he got three red-eyed flies to every one
white-eyed fly. ALSO-the white-eyed flies
were male.
As stated by Morgan the gene for eye color
is located on the X chromosome.
R = red color
r = …white
complelely
color
dominant over the…
Sex-linked genes
• Use XR for dominant allele and Xr for
recessive. Y stands for Y chromosome
• Let’s look at some genotypes and
phenotypes:
• XRXR=red-eyed female
• XRXr=red-eyed female
• XrXr=white-eyed female
• XRY=red-eyed male
• XrY=white-eyed male.
Sex-linked genes
• Red female with white male: all red
• Heterozygous female with red male: 1:1
red:white males
Sex-linked genes
• Heterozygous female with white male: all
females red, all males white.
Male fruit flies
have white eyes
more often than
females
• Because females get a second chance at a
dominant gene, they’re usually red-eyed.
• Males only get one chance at a dominant.
• Males are more likely to express recessive
traits if the gene is on the X chromosome.
(hemophilia, color-blindness, muscular
dystrophy.)
Other interesting Drosophila variations:
Other Sex-linked genes
• Human examples of sex-linked traits
– Red-green colorblindness
Other Sex-linked Traits
• Hemophilia is another example of a sexlinked recessive trait.
• The different possible alleles are XH, Xh, and
Y
• Absence of the dominant allele is necessary
for trait expression
– XH XH is a normal woman
– XH Y is a normal man
– XH Xh is a woman who is a carrier
– Xh Y is an affected man
– Xh Xh is an affected woman
– Males cannot be carriers
A Sample Cross for Hemophilia
XH
Xh
XH
XH XH
X H Xh
Y
XH Y
Xh Y
Normal
male
Carrier
female
Sex-linked genes
• Hemophilia and history: Czar Nicholas II.
X-Linked Dominant Traits
• The presence of the dominant allele causes the
expression of the trait
• A female who inherits the dominant allele will
have the condition, but a male will have a more
severe form because he has no other X
chromosome to offset it
• An affected male who has children with an
unaffected female cannot pass the trait to his
sons (males get their X from mom)
X-Linked Dominance: CGH
• Congenital
Generalized
Hypertrichosis
• Extremely dense
hair growth
• “Werewolf”
syndrome
X-Linked Dominance
Xc
Xc
XC
XC Xc
X C Xc
Y
Xc Y
Xc Y
Affected
Male
Unaffected
Female
Only the daughters have the trait