Mendel hypothesized that each trait is controlled by something called a “factor.” Scientists now use the term gene, instead of factor, to indicate a genetic factor that controls a trait. Mendel knew that two parents contribute to an organism’s inheritance. Therefore, each organism must have two factors for each trait. Because diploid organisms have two of each chromosome, they also have two copies of a gene for a given trait. Different versions of a gene for the same trait are called alleles. In this exercise, you will be using pop-it beads to represent individual genes. Because thousands of genes are found on each chromosome, genes found on the same chromosome will be represented by connected beads

Select six of the same colored bead to represent six different traits on a single chromosome and snap them together. Record, using colored pencils, the sequence on your paper. This represents one half of the chromosome pair.

To make its homolog, select another six beads of a different color. Why do they both have the same number of beads? What is the significance of the two colors? With your colored pencils, draw this on your paper.

Crossing over is the exchange of genes between pairs of homologous chromosomes. This occurs during Prophase I of meiosis, when the homologous pairs are still joined together with their replica at the centromere. Sometimes these chromosomes can break where they meet and exchange genes. As a result of crossing over, the offspring have a combination of alleles not found in either parent. Break off the top three beads of the chromosomes and exchange the pieces. (Note: these are not the same color.) With your colored pencils, draw the new chromosomes.

Procedure III: Nondisjunction: This occurs when one or more pairs of chromosomes fail to separate during cell division. When this occurs during meiosis, a pair of chromosomes stays together, and both members go into the same daughter cell instead of separating and going into different daughter cells. The other daughter cell does not get a copy of this particular chromosome at all. Using the beads, show a cell with 2N = 4. Use different lengths of beads to indicate the homologues. Use the same two colors for each pair. Indicate using the beads, the resulting gametes this organism should produce with normal disjunction. Draw. Now show the resulting gametes if nondisjunction occurs. Draw. How many chromosomes are now found in the gamete? (There are two possibilities.)

Procedure IV: Polyploidy: This is a condition in which cells have some multiple of the normal chromosome number. This occurs when chromosome do not separate normally during meiosis or mitosis. The 4N condition(2N + 2N) is known as tetraploid. The 3n condition(2N + N) is known as triploid. Using the beads from the previous procedure, show a triploid cell. Draw. Plant breeders often use a chemical, colchicine to encourage the formation of polyploid cells during mitosis. Why?

Procedure V:  Other Chromosomal Mutations

Deletion - Here one or more genes may be lost entirely from a chromosome during division. This may occur if homologous chromosomes cross over unequally during meiosis. Using your beads, connect five beads. Imagine that each bead represents a word. Make up a sentence with five words. ex. The dog bit the cat. Now remove one or two beads. If you remove two from the right side the sentence now reads: The dog bit. If you remove one from the middle it will now read The dog the cat. Using colored pencils, draw the original chromosome and then draw it with its deletion.

Insertion or Addition - In this case, a piece of a chromatid breaks off and attaches to its sister chromatid. The result is a duplication of genes on the same chromosome. Using the above five beads, construct a duplicate to represent the sister chromatid and break off the first two beads and insert them in front of the other chromatid. ex: the sentence, The dog bit the cat would read The dog the dog bit the cat if you added the second bead to it. If you added the last bead to the end of its chromatid, it would read The dog bit the cat cat. Using your colored pencils, draw the original chromatids and then draw them with the insertions.

Inversion - This occurs when part of a chromosome breaks out and is reinserted backwards. ex. if you have a five bead sequence of blue-red-yellow-green-black and it becomes blue-yellow-red-green-black after an inversion, the sentence The dog bit the cat would read The bit dog the cat. Using your beads, select a five color sequence and show an inversion. Using your colored pencils, draw the original and the inversion.

Translocation - this occurs when part of one chromosome breaks off and is added to a different chromosome. If you have a five bead sequence of blue-red-yellow-green-black that reads The dog bit the cat and another chromosome has a three bead sequence of white-purple-orange that reads I love biology and the orange bead is translocated in front of the blue bead, the sentence will now read Biology the dog bit the cat. Use your beads to show a translocation and then use the colored pencils to draw the original chromosome and its translocation.

Follow-Up Questions v v v

1. What is polyploidy?

2. How does it differ from nondisjunction?

3. What may result from polyploidy in plants?

4. What results if the offspring has trisomy of

chromosome 21?

5.What results if there is nondisjunction of the sex chromosomes?

6. What is amniocentesis? Can it detect chromosomal abnormalities?

8. What is a karyotype?

What is cri-du-chat? Why is it so named? What kind of chromosomal abnormality is it?

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j. kent popit genetics