Transcript Sex-linked Traits - Down the Rabbit Hole

Sex-linked Traits
Sex Determination
Sex chromosomes – determines the sex of an
individual
Y
X
Males have X and Y
Two kinds of gametes
Female have two X’s
Only one type of gamete
Which parent determines the sex of the offspring?
XY
male
X
XX
Y
X
Female
X
It is the male that determines the sex of the offspring.
Sex Linked Traits
Traits controlled by genes located on sex
chromosomes (X or Y)
Most are X-linked
The X chromosome is larger and has more genes
A Y has very few genes
Show inheritance pattern that differs for
autosomal traits.
T.H. Morgan worked with fruit flies
• Fruit flies normally have red eyes.
• One day, in one of his cultures of fruit
flies, Morgan found a fly with white eyes.
• This white-eyed fly was male, so Morgan
mated it to red-eyed female fly to see if
he could determine how this particular
trait is inherited.
• When he mated it with a normal female,
the offspring all had red eyes. This was
expected in light of Mendel's
experiments.
• The trait reappeared in the F2 generation.
However, there was a twist because the
trait only reappeared in males.
• There were no female flies with white
eyes!
Males only have one copy of the X
chromosome (hemizygous), plus a Y
Female have two X chromosomes, and may be
homozygous or heterozygous for a trait
X-linked genes are never passed from father
to son. The Y chromosome is the only sex
chromosome that passes from father to son.
Females with one copy of the normal gene
and one copy of the mutated gene are
called carriers. They don’t show the trait.
Males are never carriers – if they have a
mutated gene on the X chromosome, it will
be expressed
X-linked disorders
 Recessive
 Red-green color
blindness
 Hemophilia
 Duchenne muscular
dystrophy
 Ichthyosis
 Dominant
 Hypertrichosis
Red-green color blindness
Can not distinguish red from green.
Normal red-green
Red-green colorblind
What a red-green colorblind person sees
Normal
Red-green colorblind
Hemophilia
X Chromosome Inactivation
While females have two X chromosomes,
only one is active.
The other is turned off.
This is random, in some cells the maternal X
is turned off and in others the paternal X is
turned off.
All descents of that cell will have same X
turned off
It is condensed and known as a
Barr Body.