Meiosis

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Meiosis
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Heredity and Variation

• Heredity
• Is the transmission of traits from one generation
  to the next
• Variation
• Shows that offspring differ somewhat in
  appearance from parents and siblings
• Genetics
• Is the scientific study of heredity and
  hereditary variation

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Breeding in English Shepherds
Father
Mother
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Breeding in English Shepherds
Offspring
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Genes A simple definition

• Genes
• Are the units of heredity
• Are segments of DNA
• Each gene in an organisms DNA has a specific
  locus on a certain chromosome
• Sexual organisms inherit one set of chromosomes
  from the mother and one set from the father

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The Shallow End of the Gene Poolby the Austin
Lounge Lizards

• My Daddy was a man of letters My Mama was a
  head of state And when they got their
  chromosomes together They gave me all of their
  recessive traits I'm an embarrassment to
  evolution My disposition is unstable and cruel
  My blood's a catastrophic blend 'Cause I'm from
  the shallow end of the gene pool
• My IQ is a negative number I've got a sense of
  humor no one can stand And my development was
  seriously encumbered By playing music with the
  Lounge Lizards band I'm a social director's
  nightmare At dinner parties I just babble and
  drool It's not easy making friends When you're
  from the shallow end of the gene pool

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Asexual reproduction budding in Hydra Asexual
reproduction results in clones
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Chromosomes

• Homologous chromosomes
• Are the two chromosomes composing a pair
• Have the same characteristics
• May also be called autosomes
• Sex chromosomes
• Are distinct from each other in their
  characteristics
• Are represented as X and Y in mammals
• Determine the sex of the individual, XX being
  female, XY being male X and X are homologous in
  female
• A diploid cell
• Has two sets of each of its chromosomes
• In a human has 46 chromosomes (2n 46)

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Figure 13.x3 Human female karyotype shown by
bright field G-banding of chromosomes
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Figure 13.x5 Human male karyotype shown by
bright field G-banding of chromosomes
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Hairy Ears a sex linked trait
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Chromosome number in humans if there was no
meiosis prior to reproduction

• First Generation - Sperm and Egg with 46
  chromosomes each
• Second generation 92 chromosomes
• Third generation 184 chromosomes
• Fourth generation 368 chromosomes
• Etc.

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Sexual life cycles
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Interphase Prior to Meiosis
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Prophase I
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The synaptonemal complex binding together four
homologous chromosomes
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Anaphase I
Metaphase I
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Telophase I and Cytokinesis
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Prophase II
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Anaphase II
Metaphase II
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Telophase II and Cytokinesis
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Sexual reproduction leads to genetic variation
via

• Independent assortment during meiosis
• Crossing over during meiosis
• Random mixing of gametes (sperm and egg)