KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.

1. KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity.

2. Sexual reproduction creates unique combinations of genes. • Sexual reproduction creates unique combination of genes or genetic variation. Three main sources of genetic variation are: • independent assortment of chromosomes in meiosis – the random placing of homologous chromosomes on either side of the equator during meiosis. • Crossing over during meiosis • random fertilization of gametes (more than 70 trillion possible combinations) • Unique phenotypes may give a reproductive advantage to some organisms. This photo shows only a small sample of the great genetic potential for variety in the human population.

3. Crossing over during meiosis increases genetic diversity. • Crossing over is the exchange of chromosome segments between homologous chromosomes. • occurs during prophase I of meiosis I • results in new combinations of genes Calculate: Fruit fly gametes each have four chromosomes, representing 24 or 16, possible chromosome combinations. How many chromosome combinations could result from fertilization between a fruit fly egg and a sperm cell? Answer: 256 or 16x16

4. Chromosomes contain many genes. • The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over. • Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage. • Genetic linkage allows the distance between two genes to be calculated. Predict: Suppose two genes are very close together on a chromosome. Are the genes likely to be separated by crossing over? Explain

5. parent cell DNA duplicates cell begins to divide daughter cells Binary fission is similar in function to mitosis. • Asexual reproduction is the creation of offspring from a single parent. • Binary fission produces two daughter cells genetically identical to the parent cell. • Binary fission occurs in prokaryotes (bacteria). • In order for bacteria to have genetic variation, they must do a process called conjugation (the transfer of DNA from one bacteria to another).

6. Hydra bud Yeast Some eukaryotes reproduce through mitosis. • Budding forms a new organism from a small projection growing on the surface of the parent. This is another type of asexual reproduction.

7. Other types of asexual reproduction: • Fragmentation is the splitting of the parent into pieces that each grow into a new organism. Examples of this include: flatworms and sea stars. • Vegetative reproduction forms a new plant from the modification of a stem or underground structure on the parent plant. Examples include: strawberries and potatoes • Some organisms can reproduce by sexual or asexual reproduction depending on the environmental conditions. Examples include: the sea anemone can reproduce by dividing in half, breaking into small pieces or by budding. It can also reproduce sexually by making eggs and sperm.

8. Asexual reproduction is an advantage in consistently favorable conditions. All offspring are genetically identical and if they are well suited to their environment, then genetic variation could be more harmful than good. In other words, if it ain’t broke, don’t fix it. • If environmental conditions change, then the more genetic variations, the better chance at least some individual wills survive thus sexual reproduction would be the advantage. Bacterial Growth • Environmental conditions determine what form of reproduction is most advantageous.