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Chapter 10.4
How Meiosis Introduces Variations in
Traits
AP Biology
Fall 2010
Objectives
• Describe the various processes that contribute
to genetic variation
Crossing Over in Prophase I
• Homologous chromosomes pair up
• Non-sister chromatids exchange segments in a
process called crossing over
Crossing Over in Prophase I
• Because alleles for the same trait can vary
between homologues, new combinations of
genes in each chromosome can result
– This is one source of genetic variation
• Crossing over leads to genetic recombination
in gametes
Metaphase I Alignments
• During metaphase I, homologous
chromosomes randomly line up at the spindle
equator
• During anaphase I, homologous chromosomes
(still duplicated) separate into two haploid
cells, each of which has a random mix of
maternal and paternal chromosomes
Metaphase I Alignments
• Every time a human sperm or egg forms, the
possible combinations of chromosomes equal
223 (8,388,608)
• Figure 10.7
Crossing Over in Prophase I
• Gene swapping isn’t pointless
– Genes vary and come in slightly different forms
• Alleles
– A number of the alleles on one chromosome will
not be identical to their partner alleles on
homologous chromosomes
Review
1. Be able to describe crossing over
Answers
1. Homologous duplicated chromosomes pair
up. Intimate contact encourages crossovers
at various intervals along length of non-sister
chromatids. Non-sister chromatids exchange
segments at cross over site. Crossing over
breaks up old combinations of alleles and
puts new ones together in homologous
chromosomes, mixes up maternal and
paternal information about traits.