HEREDITY

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HEREDITY

• 3-2 Sex-Linked Traits

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Introduction

• Some human traits occur more often in one sex
  than in the other.
• Usually, the genes for these traits are carried
  on the X chromosome, which is a sex chromosome.
• Traits that are carried on the X chromosome are
  called sex-linked traits because they are passed
  from parent to child on a sex chromosome.

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Introduction (continued)

• Unlike X chromosomes, Y chromosomes carry few, if
  any, additional genes (except for maleness).
• So any gene even a recessive one carried on
  an X chromosome will produce a trait in a male
  who inherits the gene.
• As a result, FEMALES ARE LESS LIKELY THAN MALES
  to express sex-linked traits.

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Male
Female
Y
X
X
X
Because a female has two X chromosomes, a
recessive gene on one X chromosome can be masked,
or hidden, by a dominant gene on the other X
chromosome.
There is no matching gene on the Y chromosome to
mask, or hide, the gene on the X chromosome.
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Hemophilia

• An example of a disorder caused by a sex-linked
  trait is hemophilia.
• Hemophilia is an inherited disease in which the
  blood clots abnormally slow or not at all.
• Hemophilia is also called bleeders disease.
• For a person with hemophilia, even a small cut or
  bruise can be extremely dangerous.

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Hemophilia
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Colorblindness

• Colorblindness is another sex-linked trait.
• Difficulty in distinguishing between the colors
  red and green is the most common type of
  colorblindness.
• More males than females are colorblind.
• A colorblind female must inherit two recessive
  genes for colorblindness, one from each parent.
• A colorblind male needs to inherit only one
  recessive gene.

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Male-Pattern Baldness

• Some traits that seem to be sex linked are
  actually not caused by genes on the X chromosome.
• For example, baldness is much more common in men
  than in women.
• So you might think that male-pattern baldness is
  a sex-linked trait.
• However, male-pattern baldness is a
  sex-influenced trait.

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Male-Pattern Baldness

• A sex-influenced trait is a trait that is
  expressed differently in males than it is in
  females.
• It is called male-pattern baldness because men
  who inherit one gene for normal hair and one gene
  for baldness tend to be bald, whereas women do
  not.

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HEREDITY

• 3-3 Human Genetic Disorders

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Introduction

• During meiosis, chromosome pairs usually
  separate. But in rare cases, a chromosome pair
  may remain joined during meiosis.
• The failure of a chromosome pair to separate
  during meiosis is known as nondisjunction.
• As a result of nondisjunction, body cells receive
  either more chromosomes or fewer chromosomes than
  normal.
• An abnormal number of chromosomes may result in
  certain genetic disorders.

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Down Syndrome

• Humans usually have 46 chromosomes, or 23 pairs
  however, nondisjunction may produce an extra
  chromosome.
• An extra chromosome in the twenty-first pair
  (Trisomy-21) is called Down Syndrome.
• People with Down Syndrome have various
  physical/mental problems however, many lead
  normal, active lives and make valuable
  contributions to society.

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Detecting Genetic Disorders

• Is there a way of knowing before a child is born
  whether he or she will have Down Syndrome or
  another inherited disorder?
• Fortunately (unfortunately?), the answer is YES!
  One method of diagnosing a genetic disorder is
  called amniocentesis.
• Amniocentesis involves the removal of a small
  amount of fluid from the sac that surrounds a
  baby while it is still inside the mother.

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Curing Genetic Disorders

• At present, there are NO CURES for genetic
  disorders.
• Scientists have isolated genes for certain
  disorders and have cured them in the
  laboratory. Treatment on humans leads to ethical
  questions (abortion, cost/expense, health
  insurance, etc).

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Detecting Genetic Disorders

• Some tests to detect a genetic disorder can make
  it possible to treat the disorder before birth.
• Other tests can be performed immediately after
  birth so that treatment can begin as soon as
  possible.
• Parents who are concerned that they might pass on
  a genetic disorder should consult a genetic
  counselor.