Karyotyping Activity

Introduction

This exercise is a simulation of human karyotyping using digital images of chromosomes from actual human genetic studies. You will be arranging chromosomes into a completed karyotype, and interpreting your findings just as if you were working in a genetic analysis program at a hospital or clinic. Karyotype analyses are performed over 400,000 times per year in the U.S. and Canada. Imagine that you were performing these analyses for real people, and that your conclusions would drastically effect their lives.

G Banding

During mitosis, the 23 pairs of human chromosomes condense and are visible with a light microscope. A karyotype analysis usually involves blocking cells in mitosis and staining the condensed chromosomes with Giemsa dye. The dye stains regions of chromosomes that are rich in the base pairs Adenine (A) and Thymine (T) producing a dark band. A common misconception is that bands represent single genes, but in fact the thinnest bands contain over a million base pairs and potentially hundreds of genes. For example, the size of one small band is about equal to the entire genetic information for one bacterium.

The analysis involves comparing chromosomes for their length, the placement of centromeres (areas where the two chromatids are joined), and the location and sizes of G-bands.

Your assignment

This exercise is designed as an introduction to genetic studies on humans. Karyotyping is one of many techniques that allow us to look for several thousand possible genetic diseases in humans.

You will evaluate 3 patients' case histories, complete their karyotypes, and diagnose any missing or extra chromosomes.

Patient Histories

Patient A

Patient A is the nearly-full-term fetus of a forty year old female. Chromosomes were obtained from fetal epithelial cells acquired through amniocentesis. Complete Patient A's Karyotype.

Patient B

Patient B is a 28 year old male who is trying to identify a cause for his infertility. Chromosomes were obtained from nucleated cells in the patient's blood. Complete Patient B's Karyotype.

Patient C

Patient C died shortly after birth, with a multitude of anomalies, including polydactyly and a cleft lip. Chromosomes were obtained from a tissue sample. Complete Patient C's Karyotype.

Patient A's Karyotype

Place the first four chromosomes in the partially completed karyotype on the following page by cutting each one out and placing it next to its homologous chromosome.

1 2 3 4 5

Patient B's Karyotype

Follow the same procedure as above and complete patient B’s karyotype.

1 2 3 4 5 6 7 8

Patient C's Karyotype

Follow the same procedure as above and complete patient C’s karyotype.

1 2 3 4 5 6 7 8 9

Teacher’s answer guide.

Patient A's Karyotype

Place the first four chromosomes in the partially completed karyotype on the following page by cutting each one out and placing it next to its homologous chromosome. Note that the 5^th chromosome is an extra one. Place it next to the homologous chromosomes

1 2 3 4 5

Ans: 2 16 12 5 21

Patient B's Karyotype

Follow the same procedure as above and complete patient B’s karyotype. Note that there is an extra Y chromosome.

1 2 3 4 5 6 7 8

Ans: 21 11 14 XX/XY 3 20 10 XX/XY

Patient C's Karyotype

Follow the same procedure as above and complete patient C’s karyotype. Note that there is an extra 13 chromosome.

1 2 3 4 5 6 7 8 9

Ans: 12 3 11 13 9 XX/XY 20 15 13

Making a diagnosis

The next step is to either diagnose or rule out a chromosomal abnormality. In a patient with a normal number of chromosomes, each pair will have only two chromosomes. Having an extra or missing chromosome usually renders a fetus inviable. In cases where the fetus makes it to term, there are unique clinical features depending on which chromosome is affected. Listed below are some syndromes caused by an abnormal number of chromosomes.

Diagnosis	Chromosomal Abnormality
Normal # of chromosomes	patient's problems are due to something other than an abnormal number of chromosomes.
Klinefelter's Syndrome	one or more extra sex chromosomes (i.e., XXY)
Down's Syndrome	Trisomy 21, extra chromosome 21
Trisomy 13 Syndrome	extra chromosome 13

What observations can you make regarding patient A’s karyotype?

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2. What diagnosis would you give patient A? Explain your answer.

What observations can you make regarding patient B’s karyotype?

What diagnosis would you give patient B? Explain your answer.

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What observations can you make regarding patient C’s karyotype?

What diagnosis would you give patient C? Explain your answer.