Why Sex? Life’s response to an ever changing environment
The Earth is constantly changing Change is unpredictable Living things must respond to change to survive –variety is the only way to prepare for change
Sexual Reproduction Produces Variety Three steps to variation During Meiosis: –Cross-over –Independent assortment Fertilization –Combining two genomes
Chromosome s Each chromosomes has the genes for several traits eye color = trait There can be different “alleles” for each trait –Black eyes, white eyes = alleles Individuals have two of each chromosome Alleles can be matching or different
Cross-over Meiosis part one: 1.Each chromosome is copied 2.Copies are put together 3.Chromosomes are paired 4.Cross-over: chromosomes line up & swap alleles
Cross-over = variety Imagine one chromosome Has genes for 3 traits Two alleles for each trait = 8 possible combinations of traits
Independent Assortment Meiosis part two: 1.Chromosome pairs are separated into two cells 2.Each cell divides again one chromosome in each of the 4 cells
Independent assortment = More variety Imagine a cell with 2 chromosomes Each chromosome has genes for three traits with two alleles = 8 possible arrangements Two chromosomes with 8 possible traits 8x8= 64 possible combinations of traits
Fertilization = even more variety Imagine 2 cells combining Each cell can have one of 64 possible combinations Possible combination:64x64 =4,096 Humans have 23 Chromosomes 8 Million possible gametes! 64 Trillion possible zygotes!!! From just 2 parents no wonder you’re nothing like your siblings :~)
Summary The earth is constantly changing in unpredictable waysThe earth is constantly changing in unpredictable ways The only way for species to survive the changes is for each generation to have a variety of offspringThe only way for species to survive the changes is for each generation to have a variety of offspring The best way to produce genetic variability is through sexual reproductionThe best way to produce genetic variability is through sexual reproduction