Reduction-Division

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Each locus (position of a gene) is in the same position on homologues.

Humans have 23 pairs of homologous chromosomes:

a. First 22 pairs of autosomes

b. Last pair of sex chromosomes

LOCI

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Homologous Chromosomes

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Crossing Over

Crossing over may occur between non-sister chromatids at sites called chiasmata.

Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.)

Causes Genetic Recombination

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Genetic Recombination

variation

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Sex Chromosomes

XX chromosome - female

XY chromosome - male

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MEIOSIS I

Homologs separate

Meiosis I

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Prophase I

Nucleus & Nucleolus disappear

Spindle forms

Chromosomes coil & Synapsis (pairing) occurs

Tetrads form & Crossing over Occurs

TETRAD

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Metaphase I

Shortest phase

Tetrads align on the equator.

Independent assortment occurs – chromosomes separate randomly causing GENETIC RECOMBINATION

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Metaphase I

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Formula: 2n

Example: 2n = 4

then 1n = 2

thus 22 = 4 combinations

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Question:

In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

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Answer

Formula: 2n

Human chromosomes: 2n = 46

n = 23

223 = ~8 million combinations

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Anaphase I

Homologous chromosomes separate and move towards the poles.

Sister chromatids remain attached at their centromeres.

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Anaphase I