By Chase Livengood

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3-3 MEIOSIS

• By Chase Livengood
• and Todd Montgomery

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The 2 Types Of Reproduction

• Asexual Reproduction
• Only one parent cell is needed.
• The structures inside the cell are copied.
• Then, the parent divides, making two exact
  copies.
• This type of cell reproduction is similar to
  binary fission.

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The 2 Types Of Reproduction

• Sexual Reproduction
• Two parent cells join together to form offspring
  that are different from both parents.
• The parent cells are called sex cells.
• Sex cells are different from ordinary body cells.
• Human body cells have 46, or 23 pairs, of
  chromosomes.

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Meiosis

• Sex cells are made during MEIOSIS.
• Meiosis is copying process that produces cells
  with half the usual number of chromosomes.
• Each sex cell receives one-half of each
  homologous pair of chromosomes (23).
• The new cell that forms when an egg cell (23) and
  a sperm cell (23) join has 46 chromosomes.

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The Steps Of Meiosis

• Step 1 Prophase 1
• The chromosomes form exact copies and the nuclear
  membrane disappears.
• Step 2 Metaphase 1
• Each chromosome is made up of two identical
  chromatids. Similar chromosomes pair up along the
  equator or MIDDLE of the cell.

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The Steps of Meiosis (cont.)

• Step 3 Anaphase 1
• The chromosomes separate from their partner
  chromosomes and move APART to the opposite sides
  of the cell.
• Step 4 Telophase 1
• The nuclear membrane reforms and the cell
  divides. TWO cells are formed. The paired
  Chromatids are still joined.

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Steps of Meiosis ( cont.)

• Step 5 Prophase 2
• The two daughter cells now contain one member of
  each pair of chromosomes. The chromosomes DO NOT
  copy again.
• Step 6 Metaphase 2
• Next, the chromosomes of the two daughter cells
  line up at the equator or MIDDLE of their
  respective cells.

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The Steps of Meiosis (cont.)

• Step 7 Anaphase 2
• The chromatids pull APART and move to the
  opposite sides of each daughter cell.

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The Last Step of Meiosis

• Step 8 Telophase 2
• The nuclear membrane reforms. Two additional
  cells form from the two daughter cells. There
  are now 4 new cells each with half the number of
  chromosomes as the original cell.

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Nondisjunction

• During meiosis, chromosome pairs usually
  separate. But in rare cases, a chromosome pair
  may remain joined during meiosis.

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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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Genes and Chromosomes

• Like all living things, humans are what they are
  because of the genes they inherit from their
  parents.
• ALL TRAITS ARE CONTROLLED BY GENES, WHICH ARE
  FOUND ON CHROMOSOMES.
• Each human has about 100,000 genes, located on 46
  chromosomes.
• The 46 chromosomes are arranged in 23 pairs.
• Each chromosome pair has matching genes for a
  particular trait (eye color, hair color, ear-lobe
  shape).

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Sex ChromosomesMale and Female

• Sex chromosomes carry the genes that determine
  sex (gender).
• The X and Y chromosomes are the sex chromosomes.
• In normal human males, all the body cells have
  one X chromosome and one Y chromosome (XY).
• Females have two X chromosomes (XX).
• THERE ARE NO REPORTED CASES OF BABIES BEING BORN
  WITHOUT AN X CHROMOSOME.

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Sex-Linked Disorders

• The Y chromosome does not carry all of the genes
  of an X chromosome.
• The genes for certain disorders, such as
  colorblindness, are carried on the X chromosome.
• These disorders are called sex-linked disorders.
• Because the gene for such disorders is recessive,
  men are more likely to have sex-linked disorders.

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SLD (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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