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Name: ___________________________________________________________ Per. _______ Date: _____________________________________ Recombination Review: Crossing-over and Independent Assortment Crossing-over Reading – Underline key terms and highlight important concepts in each paragraph. Introduction Recall that chromosomes come in pairs. Each chromosome pair has the same set of genes, but those genes may be different alleles. There can be many genes on a single chromosome. Pairs of chromosomes are called homologous chromosomes. This is a picture of a human karyotype, which is all the chromosomes present in a single cell. Humans have 23 homologous chromosomes, or a total of 46 chromosomes. For sexual reproduction to happen, each parent must make gametes (eggs or sperm) that contain half the number of chromosomes. That way, when the gametes come together during fertilization, the resulting baby will have the same number of chromosomes as the parent. The formation of gametes is called meiosis. Meiosis is a type of cell division in which the number of chromosomes in the daughter cells is reduced by half. It occurs only in certain special cells of sexually reproducing organisms. Two cell divisions occur during meiosis, and a total of four daughter cells are produced. Those daughter cells become eggs (in females) or sperm (in males). Genetic recombination can happen during meiosis because of the way that chromosomes line up and then separate into daughter cells. There are two ways genetic recombination occurs, crossing-over and independent assortment. Crossing Over Crossing-over is the exchange of genetic material between homologous chromosomes. It results in new combinations of genes on each chromosome. Crossing-over happens during meiosis when homologous chromosomes line up in pairs before being separated into different gametes. The chromosomes come very close to each other and swap segments of DNA. That is, the material breaks off and reattaches at the same position on the homologous chromosome. This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. Crossing-over occurs when homologous chromosomes pair up during meiosis. The chromosomes can exchange pieces of DNA or whole genes. 1 Crossing-over Practice In the circles below, which represent the 4 gametes that will be produced from meiosis, draw (including gene labels) the chromosomes that will end up in each gamete. Use information from the reading to help you with your diagram. You can also look at the animations at these websites: http://www.biostudio.com/d_%20Meiotic%20Recombination%20Between%20Linked%20Genes.htm http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation5.html Homologous Chromsomes Not Crossed Over Homologous Chromsomes Crossed-Over 1. How does crossing-over increase variation in gametes? 2. Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of existing alleles. Which category does crossing-over fall into, and why? 2 Independent Assortment Reading – Underline key terms and highlight important concepts in each paragraph. When cells divide during meiosis, homologous chromosomes are randomly distributed to daughter cells. Before they separate into daughter cells, homologous chromosomes line up in the middle of the cell (similar to metaphase in mitosis). But the order in which they line up is completely random. This is called independent assortment. It results in gametes that have unique combinations of chromosomes. Notice how the two different line-ups of chromosomes could result in different gametes. This is called independent assortment. In humans, there are over 8 million ways in which the chromosomes can line up during metaphase I of meiosis. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Genetic recombination can also occur during fertilization. In sexual reproduction, two gametes unite to produce an offspring. But which two of the millions of possible gametes will it be? This is likely to be a matter of chance. It is obviously another source of genetic variation in offspring. This is known as random fertilization. Because of these three genetic recombination processes, more possibilities for genetic variation exist between any two people than the number of individuals alive today. Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. In humans, over 8 million (223) chromosome combinations exist in the production of gametes in both the male and female. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations! 3 Independent Assortment Practice In the blank cell in the Metaphase I section, diagram another way the homologous chromsomes can line up in Metaphase I that will lead to genetic variation In gametes. In the four cells beneath each metaphase I cell that represent gametes after meiosis, draw (including gene labels) the chromosomes that will end up in each gamete 1. How does independent assortment increase variation in gametes? 2. Mechanisms of variation can either produce brand new genetic material (new alleles), or new combinations of existing alleles. Which category does independent assortment fall into, and why? 3. How is independent assortment represented in: a. Punnet Squares? b. Ugly Baby? 4 Extension: Mutation Outcomes Go to http://learn.genetics.utah.edu/content/variation/outcomes/ This website shows some examples of how mutations can lead to new or different traits in organisms. Choose 3 of the following to describe using the diagrams below: peas, cattle, cats, curly hair, or red hair. Read each tab carefully! What does the protein from this DNA do? What trait do we see as a result? What does the protein from this DNA do? What trait do we see as a result? What does the protein from this DNA do? What trait do we see as a result? Original DNA Mutated DNA: Original DNA Mutated DNA: Original DNA Mutated DNA: 5