Name______Per____

Ch. 7 STUDY GUIDEDUE______

Chromosomal Alterations

Read and annotate the following article to answer the questions and complete the coloring diagrams on the back.

Permanent changes in chromosomes known as mutations may be passed to the offspring of a mating pair if they exist in cells that produce sperm or egg cells.

One kind of mutation affects only a single gene, while other types of mutations involve the rearrangement of several of them. For instance, pieces of chromosomes may be lost or exchanged between nonhomologous chromosomes. When altered chromosomes are passed to offspring, variation increases.

The diagram (on back of page) displays four different types of alterations that can occur in chromosomes. Focus on the first two alterations, entitled Deletion and Inversion.

The first chromosomal alteration we will discuss is deletion, which is illustrated in the upper left portion of the diagram. You should begin by coloring the normal chromosomes with genes A to G using seven distinct colors. When gene deletion occurs, a portion of the chromosome is lost, usually from the end. In our diagram, the chromosome that has undergone deletion is missing gene A. The remainder of the chromosome should be colored with the same colors that were used in the normal chromosome. A deletion sometimes results in the loss of an important gene, with severe consequences to the organism.

We will now turn to the second chromosomal alteration, called inversion. You should continue to use the same colors for the genes. We will now show how inversion differs from deletion.

We will start again by looking at the normal chromosome, which contains genes A through G (A to G). When an inversion takes place, a segment of chromosome turns around 180°. Notice that genes C and D have inverted, so that the sequence of genes in the altered chromosome is different.

At first glance, it may seem that the chromosome is not affected because the genes are present, but the position of a gene in a chromosome is very important. For example, a gene may be separated from its nearby regulatory gene as a result of inversion, so its rate of expression may be altered, or it may cease to be expressed at all. Scientists believe that chromosomal inversion may be a factor in developing cancer cells.

We will now focus on a third type of alteration called Translocation, in which two chromosomes are involved. Continue your colored as you read below.

A translocation involves the movement of a chromosomal segment from one chromosome to another. The two chromosomes involved are nonhomologous, which means that they are chromosomes from different chromosomal pairs. Begin by coloring genes A through G with the colors you used above, and then color the genes of the second chromosome, genes H to N, with different colors. (Note: if you do not have enough different colors, feel free to use green stripes, green dots, red squiggles, etc. to differentiate the gene colors.) Now take a look at the point at which translocation has taken place. Genes F and G from the first chromosome have moved to the second chromosome, and genes M and N have moved from the second chromosome to the first. Chromosomes 1 and 2 are now considerably different than they were originally. Certain translocations have been linked to cancer, and abnormal gametes can result from this alteration.

The final type of chromosomal alteration that we will consider is Duplication. Focus on the lower right portion of the diagram. The same colors that you used for the genes labeled before should be used here.

In duplication, a chromosomal segment doubles itself. For instance, here we see the normal chromosome with genes A to G on the left, but genes D and E appear twice after duplication occurs in the abnormal chromosome.

Duplication occurs when a broken segment of one chromosome attaches to its homologous chromosome. One effect of the repeating genes may be duplicate proteins in an individual. For example, there are two alpha chains in hemoglobin molecules in human red blood cells. The two molecules may result from a single gene that duplicated in an ancient ancestor so that the modern descendent now produces two proteins instead of one. Therefore, duplication may be a factor in evolution.

Questions:

  1. What are mutations?
  1. What is a deletion mutation? What could be the result?
  1. Explain what an inversion mutation is.
  1. What is one abnormality that may result from a chromosomal inversion?
  1. What is a translocation mutation?
  1. Duplication mutations are thought to possibly be a factor in ______, which is when species change over a long period of time.
Biology Student
EQ: Do I carry deadly genes? / Enduring Understanding
Nucleic acids transfer genetic information from generation to generation.
/ Karyotype
Study Guide / Broad Brush Knowledge
chromosomal disorders
Concepts Important to Know and Understand
Heredity
Targeted Skills
analysis / Core Objectives
9. Interpret the role of genetics in determining heredity and as it applies to biotechnology.

Background Information

Problems in the number of chromosomes (called chromosomal abnormalities) can be detected in an organism. In order to do this, cells from the organism are grown in a laboratory. After the cells have reproduced a few times, they are treated with a chemical that stops cell division at the metaphase stage. During metaphase, the chromosomes are at the best length for identification. Each chromosome has two identical chromatid pairs attached at the centromere. The appearance of each chromosome resembles an X-shape. The cells are treated further, stained, and then placed on a glass slide. The chromosomes are observed under the microscope where they are counted, checked for abnormalities, and photographed. The photograph is then enlarged, and the chromosomes are individually cut out. The chromosomes are identified and arranged in homologous pairs. Homologous chromosomes are identical, or matching, chromosomes. One chromosome in a homologous chromosome pair comes from the mother, the other from the father. The arrangement of homologous chromosome pairs is called a karyotype. Humans have 46 chromosomes, 23 pairs. A human karyotype would show 23 pairs of homologous chromosomes, lined up from largest to smallest.
The most common chromosomal abnormalities are caused when the chromosomes do not separate properly during meiosis (called nondisjuction). A monosomy is when only one homologous chromosome is present in the organism and a trisomy is when the organism has three copies of a homologous chromosome. The following are examples of three of the more common nondisjunction chromosomal abnormalities.
If there is a nondisjunction at chromosome 21, the result could be trisomy 21 (3 #21 chromosomes) also called Down Syndrome.
If there is a nondisjunction at chromosome 23, the result could be trisomy 23 with XXY also called Klinefelter’s Syndrome.
If there is a nondisjunction at chromosome 23, the result could be monosomy 23 with XO also called Turner’s Syndrome.

Define

1. karyotype ______

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2. homologous chromosome ______

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3. centromere ______

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4. chromatid pair ______

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5. chromosomal abnormality ______

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6. nondisjunction ______

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7. two types of nondisjunction

a) monosomy ______

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b) trisomy ______

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8. Define Down Syndrome ______

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9. Define Klinefelter syndrome ______

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10. Define Turner syndrome ______

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Directions

Use the attached pictures to complete the table below.

Figure / Number of Autosomes / Number of Sex Chromosomes / Gender / Normal or Abnormal / Type of Abnormality / Which Chromosome has the Abnormality?
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2
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5

Figure 1Figure 2

Figure 3

Figure 4Figure 5