Karyotype Lab

Chromatin vs. Chromosomes ChromatinChromosomes Contains DNA Loose in formation Genetic code is easily read Contains DNA Tight in formation ◦A.k.a. condensed Found in dividing cells

Homologous Pairs Can be identified because they have the same… ◦A. size ◦B. shape ◦C. length ◦D. gene locations ◦BUT, the DNA sequences at each gene location might be different!  For example, both chromosomes in a pair may have the gene location for eye color, but one might have the DNA sequence for brown eyes and the other blue!!!

There are 2 chromosomes in each pair. Where do the 2 members of each pair come from? ◦23 from MOM ◦23 from DAD Homologous Pairs

Chromosome number is constant from species to species. Humans, for example, have 46 chromosomes (23 pairs) in each body cell. Any change in chromosome number will cause a change in the amounts of proteins produced in a cell. These changes can have a considerable, detrimental effect on the individual.

The centromere is a region of DNA where the two identical copies of DNA, formed during replication, are attached. Each strand of replicated DNA is called a chromatid. The chromatids on a single chromosome are called sister chromatids.

Staining the chromosomes with a dye produces the banding patterns on the chromatids.

In order to study the chromosomes of a patient, a sample of cells must first be obtained from them. Chromosome analysis can be performed using mitotic (dividing) cells from a number of sources, including white blood cells or skin cells. Then, chemicals are added to stop the cells at a particular point in the process of cell reproduction when the double armed chromosomes can be easily seen, counted and organized into numbered groups. The scientist then attempts to find all the homologous pairs and organize them into a picture called a karyotype. Observing a karyotype can help us understand the nature of several genetic disorders. One of the most common times for a karyotype to be performed is during pregnancy.

Computer-assisted karyotype preparation is now commercially available. In this system a television camera and a computer are coupled to a microscope. As chromosomes in metaphase are located, the television camera records the microscopic image, and the image is transmitted to the computer, where it can be analyzed and processed into a karyotype. Either way, the scientist can look at the completed picture to see whether any of the chromosomes are missing or damaged or if there are any extra chromosomes.

What is a karyotype? Picture representation of all of the chromosomes in a cell organized into their homologous pairs. Reveals genetic information for that cell.

Notice that there are 22 pairs of homologous chromosomes. These are called autosomes. The 23 rd pair includes the sex chromosomes. If a Karyotype includes two X chromosomes (XX), the individual is a female. Males carry an X and Y chromosome (XY). Any individual with a Y chromosome is considered a male.

Male vs. Female

USING A KARYOTYPE TO DIAGNOSE CHROMOSOMAL ABNORMALITIES Amniocentesis Chorionic Villi Sampling

Amniocentesis 16 – 20 weeks in pregnancy Sample of cells taken from amniotic fluid Karyotype constructed to check for abnormalities Weeks to make karyotype Easily detects spinal chord injuries

Villi Sampling weeks Sample of cells taken from chorion Hours to make karyotype Can not detect spinal chord injuries

Testing Karyotypes constructed from cells in metaphase If there is a problem, genetic counselors work with parents to make decisions for the best welfare of the baby

PREPARATION OF A SAMPLE KARYOTYPE You will be given a sample chromosome spread. It will have a letter (A-F) printed at the top of the page. Record the letter of your karyotype on the page titled “Sample Karyotype.” Cut out each chromosome. It doesn’t have to be exact; a rectangular cut is best. Use the normal karyotype found in this lab as a guide to identify the number of each cutout chromosome. Carefully glue the homologous pairs of chromosomes next to each other, above the correct number of the chromosomes on the “Sample Karyotype” page. Use the information in the next section to determine the genetic syndrome that your sample karyotype indicates.

Abnormal Karyotype:Syndrome:Description: 46, XX or 46, XY with one chromosome #5 upper arm deletion Cri-du-chat Babies with the “cry of the cat” syndrome have a cry that sounds like that of a cat in distress, because the infant’s larynx or voice box is improperly developed. The cause of this condition is a deletion of about half of the short arm of chromosome number five. Cri-du-chat babies are severely mentally retarded and have a small cranium. The incidence of this syndrome is 1/100,000 live births. 47, XY or 47, XX with three copies of chromosome # 21 Down Syndrome or Trisomy 21 This syndrome is one of the most common causes of mental retardation. Down syndrome is marked by a number of characteristic features, such as short stature, broad hands, stubby fingers and toes, a wide rounded face, a large protruding tongue that makes speech difficult and mental retardation. Individuals with this syndrome have a high incidence of respiratory infections, heart defects and leukemia. The average risk of having a child with trisomy 21 is 1/750 live births. (Mothers in their early twenties have a risk of 1/1,500 and women over 35 have a risk factor of 1/70, which jumps to 1/25 for women 45 or older.) 47, XX or 47, XY with three copies of chromosome # 18 Edwards Syndrome or Trisomy 18 This syndrome produces severe mental retardation and a highly characteristic pattern of malformations such as elongated skull, a very narrow pelvis, rocker bottom feet, and a grasping of the two central fingers by the thumb and middle finger. In addition, the ears are often low set and the mouth and teeth are small. Nearly all babies born with this condition die in early infancy. The frequency of this syndrome is 1/5,000 live births. 47, XYY with 2 copies of the Y chromosome Jacobs Syndrome A chromosome aberration in which the individuals are not markedly affected. Although these males are tall and have a slightly higher risk for behavioral problems, many individuals with this syndrome live normal healthy lives. The incidence is about 1/1000 live male births. 47, XXYKleinfelter Syndrome Characteristics in this syndrome do not develop until puberty, and many of the symptoms seem to be related to low testosterone levels. Affected males are generally infertile, display poor sexual development and have some degree of subnormal intelligence. Most men with this syndrome appear relatively normal in other ways (in fact some cases have no obvious signs) but a degree of breast development occurs in about half of the reported cases. May display problems with learning and behavior. Occurs in about 1/1000 male births. 47, XXXXXX or Metafemal e syndrome Approximately 1 in 1000 females are born with three copies of the X chromosome. In most cases these females are physically and mentally normal, although there is a slight increase in sterility and mental retardation compared with the population at large. In rare cases, XXXX and XXXXX karyotypes have been reported, and problems of mental retardation are severe. 45, XO Only one X sex chromosome Turner Syndrome Individuals are visibly female. As girls, they appear normal although they are shorter and have a chunky build. At birth, the distinguishable characteristics include a thick fold of skin on either side of the neck. At sexual maturity, the secondary sex characteristics do not develop and no eggs are produced. There is no menstruation or breast development. The frequency is 1 / 2,500 live female births.

Cri – du - Chat #5 upper arm deletion PMH /

Down Syndrome Trisomy 21 PMH /

Edwards Syndrome Trisomy 18 PMH /

Jacobs Syndrome XYY

Kleinfelter Syndrome XXY PMH /

Metafemale Syndrome XXX

Turner Syndrome XO PMH /