Skip to main content
Biology LibreTexts

9.E: Changes in Chromosome Number and Structure (Exercises)

These are homework exercises to accompany Nickle and Barrette-Ng's "Online Open Genetics" TextMap. Genetics is the scientific study of heredity and the variation of inherited characteristics. It includes the study of genes, themselves, how they function, interact, and produce the visible and measurable characteristics we see in individuals and populations of species as they change from one generation to the next, over time, and in different environments.

9.1

Make diagrams showing how an improper crossover event during meiosis can lead to: (a) an inversion or (b) a translocation.

9.2

Make a diagram showing how a nondisjunction event can lead to a child with a 47,XYY karyotype.

9.3

How many Barr bodies would you expect to see in cells from people who are: (a) 46, XY, (b) 46,XX, (c) 47, XYY, (d) 47,XXX, (e) 45,X, and (f) 47,XXY ?

9.4

Why can people survive with trisomy-21 (47,sex,+21) but not monosomy-21 (45,sex,-21)?

9.5

If Drosophila geneticists want to generate mutant strains with deletions they expose flies to gamma rays. What does this imply about gamma rays?

9.6

What would happen if there was a nondisjunction event involving chromosome 21 in a 46,XY zygote?

9.7

Design a FISH based experiment to find out if your lab partner is a 47,XXX female or a 47,XYY male.

9.8

What would Figure 9.18 look like if it also showed metaphase chromosomes from another cell?

Answers

9.1 a) As in Figure 9.7 homologous chromosomes pair during prophase I. The shaded boxes are regions of sequence similarity, for example Alu transposable elements. A crossover occurs between two of the Alu elements on the same chromatid leading to a chromosomal inversion.

b) A crossover occurs between Alu elements on different chromosomes leading to a chromosomal translocation. Note that the homologous chromosomes are not shown in this figure for simplicity.

9.2  As in Figure 9.12 there is a nondisjunction event during gamete formation. The larger X chromosomes are shown using open symbols and the smaller Y chromosomes are shown with shaded symbols. A second division nondisjunction event in the male parent leads to a zygote with an XYY karyotype.

9.3 a) 46, XY - zero Barr bodies, b) 46,XX - one, c) 47, XYY - zero, d) 47,XXX - two, e) 45,X - zero, f) 47,XXY - one.

9.4  Having a shortage of key proteins is usually more detrimental than having an excess.

9.5  Gamma rays are efficient at causing double strand DNA breaks, which are then more likely to rejoin and produce a deletion.

9.6  At the two cell stage, one of the embryo’s cells will be 45,XY,-21 while the other will be 47,XY,+21. As embryogenesis continues most of the monosomy-21 cells will die and the embryo will ultimately be made of mostly trisomy-21 cells. The child will be born with Down syndrome.

9.7  Obtain permission from the person (and ethical approval from the university), isolate some white blood cells, place the cells on a slide, denature the DNA, hybridize with fluorescent nucleic acid probes specific for the centromeres of the X chromosome and the Y chromosome, observe the results with a fluorescence microscope. If they are XXX there should be three X signals, if XXY, there should be two X signals and one Y in each cell nucleus.

9.8  The results would be similar to Figure 9.19b. There would be 47 chromosomes glowing blue. The centromeres of two of the chromosomes would be glowing green and the centromeres of three other chromosomes would be glowing red.

Contributors