and chromosome 1 tails (1 T), chromosome 2 heads (2 H) and chromosome 2 tails (2 T), and chromosome 3 heads (3 H) and chromosome 3 tails (3 T). We begin with a father and a mother, each with three pairs of chromosomes. We will indicate the chromosomes from the father with f and the chromosome from the mother with m. So the chromosomal compositions of mother and father are:
Father: 1 f H and 1 f T, 2 f H and 2 f T, 3 f H and 3 f T Mother: 1 m H and 1 m T, 2 m H and 2 m T, 3 m H and 3 m T
Each team of two has a "mother" and a "father," with each person representing either the father or the mother. To begin the game, each person flips a coin three times to determine which of the pairs of chromosome they contribute to the next generation. The first flip determines whether chromosome 1 H or T is contributed, the second flip determines which chromosome 2 is contributed, and so forth. Then the team writes down the chromosome composition of their "child." If done properly, you will see that there is one f and one m chromosome for each pair of chromosomes. Now compare the chromosome composition of your "child" with that of other teams. Are there any with the identical chromosome composition? What is the chance that we would get two children with the same chromosome composition?
Try This at Home: Explore Genetic Databases
There is an explosion of genetic information being accumulated in databases around the world. Much of the information is available to anyone with Internet access. We will introduce you to some of that huge amount of information in this exercise to help you get acquainted with the databases available for genetic studies.
In this exercise, you will first find a gene associated with a human genetic disease. Then we will search the DNA database for the gene. Finally, we'll use one of many tools available to analyze DNA sequences, called BLAST (Basic Local Alignment Search Tool). We will use the example of Marfan syndrome that we encountered in chapter 3, but we encourage you to follow along in this exercise with another disease or issue of interest to you.
First, open an Internet browser and go to the Entrez search and retrieval system of the National Center for Biotechnology Information of the National Library of Medicine at the National Institutes of Health site at http://www.ncbi.nlm.nih.gov/Entrez/. At this site you will see a link called PubMed, which links to the professional biomedical literature database, another labeled Nucleotide, which links to the DNA-sequence database GenBank, and many others.
We will first look up a genetic disease in OMIM, the Online Mendelian Inheritance in Man, by clicking on "OMIM." Then, in the space provided, type in a disease or an organ of the body that you are interested in or even a characteristic of interest like left-handedness. As an example, we will type in Marfan and click "Go," which gives us a listing of OMIM entry numbers. Each of these listings provides authoritative information about the disease. Genes can be identified with a set of upper case letters sometimes followed by a number. When we click on #154700, the listing gives information about Marfan syndrome. At the beginning it states that "all cases of the true Marfan syndrome appear to be due to mutation in the fibrillin-1 gene (134797)." Clicking on that number gives us the name for the gene and its acronym, FBN1. In the column on the left, the light orange box labeled "R" provides a link to the reference-gene sequence. The default display is a short summary of the sequence. If you want more detailed information, you can continued on next page
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