inheriting a gene - albinism

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 black-brown 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

eyes.

The gene that

produces this

enzyme is on

chromo-some 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

a

Aa Aa

Aa Aa

As long as there is one normal gene, none of the offsprings will be albino A A

a

a

Aa Aa

Aa Aa

Therefore, all offsprings will have a normal and an albino gene.

A A

a

a

Aa Aa

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.

Aa X AaBoth parents have one gene for normal and one gene for albinism.

Aa X Aa

A

a

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

A a

Aa X Aa

A

a

A a

AA

aaAa

Aa

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 a

AA

aaAa

Aa

Only the offspring with two albino genes will lack pigment.

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.