INHERITING
A GENE ALBINISM
This is an
albino
skunk. The
cells are not
able to
produce the
protein that
causes
color.
Cells in the skin produce a blackbrown pigment called melanin.
The chemical
melanin is
produced by
specialized cells
in the epidermis
called
melanocytes.
The melanin leaves the
melanocytes and enters other
cells closer to the surface of
the skin.
Different
shades of skin
colors is
determined by
the amount of
melanin
deposited in
these
epidermal cells
Sunlight
causes
melanocytes
to increase
production
of melanin.
A tan fades because the cells break down the melanin.
Some
organisms,
such as the
octopus,
can
rapidly
change
from light
to dark.
They control the color by scattering the
melanin in the cell for a dark color,
and concentrating the melanin in the
center for light color.
Melanin is made by the melanocytes
by chemically changing the amino
acid, phenylalanin, into tyrosine and
then into melanin.
An enzyme is required
to change tyrosine into
melanin.
If the enzyme is not
present, then melanin
cannot be produced by the
melanocytes.
The result of no melanin is an albino.
The eyes of an albino appear pink
because there is no dark melanin
in the eye to absorb light.
The blood in the retina and iris
reflects red light, resulting in pink
The gene
that
produces
this
enzyme
is on
chromosome 9
If both the
genes produce
the enzyme
tyrosinase,
there is plenty
to convert
tyrosine to
melanin.
If neither
gene
produces
tryosinase,
no melanin is
produced
and…
The
crow is
an
albino
rather
than the
normal
black
What if one
gene is
normal and
one gene does
not produce
the enzyme?
The one normal gene produces
enough enzyme to make normal crow
color
This albino squirrel received one albino gene from
the father and one albino gene from the mother.
But what if a squirrel
gets a normal gene
from one parent and
an albino gene from
the other parent?
The one
functioning
gene produces
enough enzyme
to make
melanin for
normal
coloration.
Is it
possible
for two
normal
colored
cockatiels
to have an
albino
offspring?
Yes!
Remember
the albino
has two
genes for
albinism.
One gene
from the
father and
one gene
from the
mother.
To be albino,
both genes
must be
albino genes
A normal
colored bird
could have
one albino
gene and one
normal gene.
If the sperm of a normal colored male pigeon
has an albino gene and the ova it fertilizes has
an albino gene than the offspring will be albino.
The same
happens in
humans. A
normal
pigment
father and
mother can
have an
albino
offspring.
We can see this in a genetic “family tree”
called a pedigree. The circles are
females, the squares are males. The open
symbols are normal coloration, the black
symbols are albino.
The parents in the circle have
normal pigment.
Most of the offspring received
at least one normal gene from
a parent.
But one female offspring
received an albino gene from
both the mother and the father.
A Punnett square is a matrix to show
the genetics of a mating.
What is the probability
of an albino doe giving
birth to a “normal”
fawn if she has mated
with a “normal” male?
The
female
must
have two
albino
genes
(use small
“a” for
the albino
gene
- aa
Since the albino gene is
relatively rare, the male
probably has two normal
genes of color. (Capital
“A” stands for the
normal gene)
- AA
AA X aa
Next, add the possible sperm and
ova genes.
A
A
a
Aa
Aa
a
Aa
Aa
As long as there is one normal
gene, none of the offsprings will
be albino
A
A
a
Aa
Aa
a
Aa
Aa
Therefore, all offsprings will have
a normal and an albino gene.
A
A
a
Aa
Aa
a
Aa
Aa
An albino must get one
albino gene from the
father and one albino
gene from the mother.
Then how
could an
albino
female
penguin
have an
albino chick.
The “normal” colored
father must have one
“normal coloration gene
and one albino gene.
There is only one
way for two normal
colored parents to
produce an albino
offspring.
Both parents must
have one normal
gene and one albino
gene.
Both
parents
have one
gene for
normal
and one
gene for
albinism.
Aa X Aa
The
father’s
sperm is
50%
with
normal
gene and
50%
with
albino
gene.
Aa X Aa
A
a
50% of
the
mother’s
ova have
a normal
gene and
50% of
the ova
have the
albino
gene
Aa X Aa
A
a
A
a
The ova and
sperm may
combine to
form an
offspring
with two
normal
genes, a
normal gene
and an albino
gene, or two
albino genes.
Aa X Aa
A
a
A AA Aa
a Aa aa
Only the
offspring
with two
albino
genes will
lack
pigment.
Aa X Aa
A
a
A AA Aa
a Aa aa
Sometimes an albino is born and
there is no history of albinism in
the colony.
The color
gene in the
cell that
produced this
white flower
changed to an
albino gene.
A change in a gene is called a mutation.