Laboratory Exercise - Human Genetics, Part 2
Karyotype Analysis
Introduction
Karyotypes are prepared using cells from amniocentesis, chorionic villi sampling, or white blood cells.
Cells are photographed while dividing. cells are normally stained so that banding patterns appear on the chromosomes. The bands make it easier to identify the chromosomes. Banding patterns are not visible in the photograph below due to the staining technique.
Pictures of the chromosomes are cut out and arranged in pairs according to size and banding patterns.
Karyotypes can be used to determine if there is an abnormality in chromosome number or structure.
Construction of a Karyotype
You will be provided a photograph of a white blood cell in late prophase. The cell was taken from a patient with a gentic abnormality. You will construct a karyotype and determine the abnormality.
Prepare a page to be used for the karyotype by drawing five horizontal lines on a blank piece of paper. The lines will be used to line up the chromosomes; the centromeres of each chromosome will be placed on the lines when they are lined up.
With scissors, cut out each chromosome and place them on the lines as follows:
Line Chromosomes 1 1 - 5 2 6 - 12 3 13 - 18 4 19 - 22 and XX or XY Identify the genetic abnormality that your patient has. Identify the sex of the patient.
Abnormality ___________________________ Sex __________
Suppose that you are a genetic counselor and must explain to the patient or their family how the abnormality arose.
List all of the possible kinds of gamete combinations that could produce the abnormality. For example, monosomy 14 could be produced by a sperm that does not have a chromosome 14 and a normal egg. It could also be produced by a normal sperm and an egg that does not have a chromosome 14.
Write all of the abnormal gametes from your list above in the tables below.
Abnormal Eggs Type of Nondisjunction
Abnormal Sperm Type of Nondisjunction The following is a list of ways that nondisjunction of the sex chromosomes can occur during meiosis.
1. During meiosis 1 in females.
2. During meiosis 2 in females.
3. During meiosis 1 in males.
4. During meiosis 2 in males (Y chromosome only)
5. During meiosis 2 in males (X chromosome only)
For each of the abnormal gametes listed in the tables, determine all of the types of nondisjunction that could have produced it. It may be helpful to refer to your answers to "Laboratory Exercise - Human Genetics, Part 1".
Some Common Single-Gene Characteristics in Humans
The table below lists some human genetic characteristics that are determined by a single gene. The dominant phenotype and its genotypes are written on the left. The recessive phenotype and genotype is on the right. Circle your phenotype (dominant or recessive) for each of these characteristics. Some of the characteristics are described below the table. If you are unsure what the characteristic looks like, find somebody else in the lab that has the characteristic or consult your instructor.
Dominant Recessive Bent little finger (AA or Aa) Straight Little Finger (aa) No hitchhiker's thumb (BB or Bb) Hitchhiker's thumb (bb) Left thumb over right (CC or Cc) Right thumb over left (cc) Hair present on hand (DD or Dd) No hair on hand (dd) Widow's peak (EE or Ee) Straight hairline (ee) Freckles (FF or Ff) No Freckles (ff) Unattached earlobes (GG or Gg) attached earlobes (gg) Able to taste PTC (HH or Hh) Inability to taste PTC (hh) Normal color vision (XAXA, XAXa, or XAY) Colorblind (XaXa or XaY) Bent Little finger - The tip of the little finger is bent toward the next finger.
Hitchhiker's thumb - The tip of the thumb can bend backward more than 45 degrees.
Choose a characteristic that you have a dominant phenotype from the table above and write the possible genotypes below.
Characteristic______________________ Possible genotypes___________
What phenotypes would you look for in your parents or siblings to prove that your genotype is one or the other?