Transcript X-linked genes

X-linked traits
Modes of Inheritance
Autosomal Inheritance
– Dominant vs. Recessive
X-linked Inheritance
– Genes only on X chromosome
– Dominant vs. Recessive
Codominance or Incomplete
Dominance
Cytoplasmic or Maternal Inheritance
Sex Determination
Pairs of
chromosomes match
Sex chromosomes
don’t always match
– X and Y
– Females: X and X
chromosome
– Males: X and Y
chromosome
Cannot live without X
chromosome
– Y chromosome
determines males
Females are XX
Possible gametes: all eggs will have
one X chromosome
X X
X
X
X
X
Males are XY
Half of male
gametes will have
one X
chromosome
Half of male
gametes will have
one Y
chromosome
X Y
Y
X
X
Y
Punnett squares predict sex of
offspring
Genotype: 1XX:1XY phenotype: 1 female:1 male
X-linked traits
X is a sex chromosome
It also has hundreds of
genes
– Very few have to do with
gender
Y does not have many of
these genes
– Very small chromosome
– “sex determining” gene
– “male fertility” genes
http://www.uni.uiuc.edu/~dstone/XY.jpg
History of X-linked genes
Thomas Hunt
Morgan @Columbia
University in 1910
– Two students:
Alfred Strutevant &
Calvin Bridges
Found white eye
male fly(mutant)
Normal(red eyes)
They noted something weird…
Mated white eyed
male w/ red eyed
female(normal)
F1 all had red eyes
F2 had 3:1 ratio
Only males had
white eyes
Why?
x
Male
Female
Male
http://www.nature.com/genomics/images/drosophila_200.jpg
More research
Morgan did find a white eyed female
Cross with red-eyed female
All the male offspring had white
eyes!!!
What was going on?
Explanation
Gene for eye color is
found on X chromosome
– Eye color gene is on
different chromosomes in
humans
Gene not found on Y
chromosome
– So males only have allele
of that gene
These genes called Xlinked traits or Xlinked genes
Color Blindness: X-linked
Inheritance
B: dominant normal
b:recessive color blind
allele
Father only has one
X chromosome to
contribute
Mom gives all normal
genes
All offspring are
normal
But all females are
now carriers
– Seem normal, but carry
disease allele
– Carry the hidden color
blind gene
– Create a punnett with a
carrier as a mother
X-linked Diseases
Certain genes linked to diseases
“+” means wildtype
– Stands for normal gene
– Found in “wild” populations
“M or m” stands for mutation that causes
disease
– M= means mutations is dominant over
wildtype
– M= means mutations is recessive to wildtype
M>+>m
Examples of X-linked diseases
Dominant X-linked disease (XM)
– Rett’s syndrome
– Vitamind D resistance
Recessive X-linked disease (Xm)
–
–
–
–
–
Infantile spasm syndrome
Duchenne Muscular Dystrophy
Hemophilia A and hemophila B
Two types of autistic disorders
Color blindness
Color Blindness
Lack to see certain colors well
Hard to distinguish them from other colors
Red/Green Color Deficiency: most common
– 99% of color deficiencies
Blue/Yellow Deficiency: very rare
– Very rare and way to test
Seeing only gray is extremely rare
In 8-12% of European males
0.5% of females
2 types of cells in eye
Rod cells
– Supersensitive in dim light
– Lets you see at night
– Doesn’t see in color
Cones
– Sees in bright light
– Distinguishes color
One absorbs red, green, and blue
Color Blindness
Red and green cones impaired or
don’t work at all
Can’t see red or green or see it
differently than normal
Ready to see colors?
Previous Answer:25
What about this
one?
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What about this
one?
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What about this
one?
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And this one?
Previous Answer:6
This one?
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This one?
That was a hard one.
Normal:5
Color blind may see 2