Meiosis
Purpose: reduce # chromosomes
: ensure the 4 products have a complete set of chromosomes
: promote genetic diversity
Differences in Meiosis
1. Crossing Over (Genetic Recombination)
- Homologous chromosomes come together in a process called synapsis (the pairing of duplicated
chromosome) to form tetrads (paired set of homologous chromosomes, 4 chromatids)
-the chromatids break and reunite at the chiasmata.
 this exchanges the genetic information
 the process is known as Genetic Recombination or crossing over.
 results in genetic variation within the population
2. Independent/Random Assortment
- there is a random alignment of maternal/parental chromosomes at the metaphase plate
- the way the homologous chromosomes line up along the metaphase plate changes the DNA in the
daughter cells
Mistakes in Meiosis
- Mistakes during meiosis often result in an abnormal number of chromosomes.
-Nondisjunction: the failure of chromosomes to separate properly. This results in the addition of
deletion of 1 or more chromosomes from a gamete.
 Monosomy: a diploid cell with only 1 copy of the chromosomes
 Polysomy: diploid cell that contains an extra copy of a chromosome
- Although it doesn’t happen often in animals, it does happen in plants
Ex. seedless watermelon
Abnormal Chromosome Structure
- Alterations caused by improper crossing over
- A deletion occurs if a chromosome fragment fails to reattach do the homolog
-Duplication occurs when a chromosome fragment reattaches to a complete homolog
-Inversion occurs when the chromosome fragment reattaches to the correct homolg but in the reverse
order
-Translocation occurs when the chromosome fragment reattaches to a nonhomologous chromosome.