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

Chromosomes• 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-overMeiosis 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 cells2. 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 The earth is constantly changing in unpredictable ways changing in unpredictable ways

• The only way for species to The only way for species to survive the changes is for each survive the changes is for each generation to have a variety of generation to have a variety of offspringoffspring

• The best way to produce The best way to produce genetic variability is through genetic variability is through sexual reproductionsexual reproduction