Genetic Variation

1
Genetic Variation

• Ms. Garcia
• LN 17

Science Content Standards Genetics
2a. Students know meiosis is an early step in
sexual reproduction in which the pairs of
chromosomes separate and segregate randomly
during cell division to produce gametes
containing one chromosome of each type.
2
Overview of Meiosis

• Homologous chromosomes carry different versions
  of the same gene.
• Meiosis turns one diploid cell (2n) into four
  haploid cells (n).
• n is the number of chromosome pairs. In the
  figure n 1
• Gametes formed by meiosis have variation.

3
Dragon Meiosis
6
Diploid (2n) cell has ______ chromosomes.
Haploid (n) cells have ______ chromosomes.
3
3
In Dragon Meiosis n _____
4
Independent Assortment

• Homologous chromosomes line up in the middle
  during metaphase I.
• Whether one of the chromosome pairs is on the
  right or the left is random.
• Number of possible gametes is 2n, n haploid
  number of the organism.
• In this example n 2 22 4 possible gametes
• In humans n 23 223 about 8 million gametes.

5
Dragon Metaphase I
How the chromosomes line up is left up to
chance. How many possible gametes can result from
3 pairs of chromosomes?
n 3 23 8
6
Crossing Over

• Crossing over begins early in prophase I.
• In crossing over parts of the chromosomes trade
  places.
• For humans, an average of two or three crossover
  events occur per chromosome pair.
• Like independent assortment, crossing over is an
  important source of genetic variation.

7
Dragon Prophase I

• During prophase I homologous chromosomes pair up.
• The chromosomes are able to trade sections
  because they are so close together and have
  matching parts.
• They have the same traits.
• Crossing over creates new chromosomes.

8
Random Fertilization

• Random fertilization means that the sperm that
  fertilizes the egg is completely left up to
  chance.
• During fertilization a human egg (one of 8
  million possible combinations) combines with a
  single sperm cell (one of 8 million possible
  combinations) to produce a zygote.
• Even without considering crossing over, two
  parents can produce a zygote with about 64
  trillion (8 million x 8 million) diploid
  combination.

9
Summary

• What occurs during meiosis?
• What is independent assortment?
• During what phase does it occur?
• What is crossing over?
• During what phase does it occur?
• What is random fertilization?