Download Sex-Linked Traits

Sex-Linked Traits
Sex-Linked Diseases you will
understand:
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Colorblindness
Hemophilia
Muscular Dystrophy
Chromosome Review
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_________ are the individual units found
on each chromosome that determine
hereditary traits.
GENES
Each body cell has ____ chromosomes.
46
Each egg or sperm has ___chromosomes
23
This means that there are ____ PAIRS of
chromosomes in normal body cells.
23
Sex Chromosomes:
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MALES
Have ______ sex
chromosomes.
X and a Y
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FEMALES
Have _____ sex
chromosomes.
X and X
Basic Chromosome Differences:
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X and Y are called
“SEX”
chromosomes.
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The other 22 pairs
of chromosomes are
called “BODY”
chromosomes or
AUTOSOMES.
Sex-linked traits:
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Since only men inherit Y chromosomes, they
are the only ones to inherit Y-linked traits.
Men and women can both get the X-linked
ones since they both inherit X chromosomes.
In women, a recessive gene on one X
chromosome is often covered up in
phenotype by a dominant gene on the other
X.
This explains why women are usually carriers
of X-linked traits, but rarely ever get them
themselves.
X-Linked GENES:
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In humans, at least 320 genes are x-linked
These include:
HEMOPHILIA
RED-GREEN COLORBLINDNESS
CONGENITAL NIGHT BLINDNESS
DUCHENNE MUSCULAR DYSTROPHY.
Basic Differences Between X and Y
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The Y chromosome
is much SHORTER
than the X.
So, it cannot hold as
many GENES on it
as X can.
Colorblindness:
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Some 10 million American men - fully 7
percent of the male population - either
cannot distinguish RED from GREEN,
or see red and green differently from
most people.
This is the most common form of color
blindness, but it affects only 4 percent of
women!
Why are so many MEN color-blind?
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Colorblindness
is so prevalent
in men because
it is carried on
the X
chromosome.
Men only have
1 copy of the X!
Color vision comes in 3 colors:
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Red, green, and blue.
Amazingly, we are all born colorblind!!
The cones don’t begin to function until a
baby is about 4 months old.
More than 95% of all variations in
human color vision involve the red and
green receptors in men’s eyes.
It is very rare for anyone - male or
female - to be “blind” to the blue end of
the spectrum.
Colorblind tests:
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Children are usually tested at 4 years.
Red-green colorblindness is a
RECESSIVE condition passed on the X
chromosome.
All girls whose fathers are colorblind will
at least CARRY the gene for
colorblindness.
In order for a girl to actually be redgreen colorblind, she must have a
mother who is a carrier AND a father
who is colorblind. (0.64% of American
girls)
ISHIHARA TEST: What numbers do you
see revealed in the pattern of dots below?
NORMAL VISION:
NORMAL VISION: