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6.6 Meiosis and Genetic Variation Mendel’s Laws • Law of Independent Assortment: allele pairs separate independently of one another during meiosis, therefore traits are inherited separately. (He discovered this using dihybrid crosses) • Law of Segregation: Organisms inherit one gene from each parent (a total of two copies). Organisms donate only one copy to their offspring (so the paired copy separates during gamete formation). 6.6 Meiosis and Genetic Variation Most traits do not follow a simple dominance pattern • The Punnett Square was to show the probability of results from a simple dominance situation. Homologous chromosomes are the set of chromosomes donated by your parents…the matching traits. One chromosome from your mom and one from dad. • One of each from the set is what you donate to your child. (Fertilization) • Fertilization is the fusing of haploid sex cells. 6.6 Meiosis and Genetic Variation Heredity patterns can be calculated with probability. • Probability is the likelihood that something will happen. • Probability predicts an average number of occurrences, not an exact number of occurrences. number of ways a specific event can occur • Probability = number of total possible outcomes • Probability applies to random events such as meiosis and fertilization. 6.6 Meiosis and Genetic Variation KEY CONCEPT Independent assortment and crossing over during meiosis result in genetic diversity. 6.6 Meiosis and Genetic Variation Sexual reproduction creates unique combinations of genes. • Sexual reproduction creates unique combination of genes. – independent assortment of chromosomes in meiosis – random fertilization of gametes • Unique phenotypes may give a reproductive advantage to some organisms. 6.6 Meiosis and Genetic Variation 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 6.6 Meiosis and Genetic Variation • 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. 6.6 Meiosis and Genetic Variation Genotype from phenotype? • No you cannot determine genotype from phenotype, especially if the trait is more dominant. • You could perform a testcross to determine an unknown organisms genotype. • Testcross: crossing an organism with an unknown genotype with an organism with a known phenotype to determine the genotype of the unknown organism.