Lab #10: Karyotyping Lab

Transcription

1 Lab #10: Karyotyping Lab INTRODUCTION A karyotype is a visual display of the number and appearance of all chromosomes from a single somatic cell. A normal human karyotype would reveal 46 chromosomes (22 pairs of autosomes and a pair of sex chromosomes). Extra chromosomes, missing chromosomes, or abnormal positions of chromosomal structures can cause problems with a person s growth, development, and body functions. Based on an individual s karyotype, doctors can determine whether a patient has certain genetic diseases and it also can help doctors to determine the sex of an unborn child. For example, an individual with an extra copy of chromosome 21 (trisomy 21) will be affected by Down syndrome. The method to detect chromosomal abnormalities is called karyotyping and involves the identification of different features of a chromosome, such as the size and banding pattern of chromosomes and the placement of centromeres. Figure 1. Example of a chromosome smear and identifying features of a chromosome. Karyotypes are created using condensed, duplicated chromosomes that have been arrested at metaphase. The chromosomes are then dyed with a stain that will cause a distinctive banding pattern, called G bands. Homologous chromosomes that have the same banding pattern, centromere position, length, and overall size are paired and analyzed for any abnormalities. To analyze the karyotypes, doctors look for the following signs: The presence of 46 chromosomes The presence of all of the homologous pairs of chromosomes (including autosomal and sex chromosomes) The presence of any missing or rearranged chromosomes 1

2 Figure 2. Example of a karyotype after staining and pairing based on size and structure. Activity: You will be creating a karyotype of an individual using simulated human chromosomes, which will be provided by the instructor. You will arrange chromosomes into a completed karyotype and interpret 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 affect their lives. 2

3 Questions: 1. What syndrome, if any, does your patient have? Name: Diagnosis: 2. What type of information can be obtained from a karyotype? 3. How many chromosomes are in a normal human karyotype? 4. How do karyotypes for males differ from those of females? 5. What is the term for three chromosomes instead of two? 6. What syndrome has an extra chromosome in the 21 st pair? 7. Normal sex cells (in gametes) contain half of the chromosomes found in somatic cells (non-sex cells). How many chromosomes are found in the human sperm or egg? 8. What characteristics distinguish one chromosome from another? 3

4 9. Describe the causes and characteristics of the following syndromes. a. Turner b. Klinefelter c. Patau d. Edward e. Jacob f. Triple X 4

5 Chromosomal Disorders DISORDER OR SYNDROME CAUSE Down Trisomy 21 Klinefelter Turner Patau Edward Jacob Triple X Male with extra X Monosomy X Extra chromosome 13 Extra chromosome 18 Male with extra Y Female with trisomy X CHARACTERISTICS Lower than average cognitive ability, mild to moderate developmental disabilities. An almond shape to the eyes, shorter limbs, poor muscle tone, a larger than normal space between the big and second toes, and protruding tongue. Health concerns for individuals with Down syndrome include a higher risk for congenital heart defects, gastroesophageal reflux disease, recurrent ear infections, obstructive sleep apnea, and thyroid dysfunctions. The principal effects are development of small testicles and reduced fertility. A variety of other physical and behavioral differences and problems are common, though severity varies and many boys and men with the condition have few detectable symptoms. Short stature, lymphoedema, broad chest, low hairline, low-set ears, and webbed neck. Girls with TS typically experience gonadal dysfunction with subsequent amenorrhea and infertility. Concurrent health concerns are also frequently present, including congenital heart disease, hypothyroidism, ophthalmological problems, and ontological concerns. Patau syndrome causes serious physical and mental abnormalities including heart defects, incomplete brain development, unusual facial features such as a sloping forehead, a smaller than average head (microcephaly), small or missing eyes, low-set ears, and cleft palate or hare lip,extra fingers and toes (polydactyly), abnormal genitalia, spinal defects, seizures, gastrointestinal hernias, and mental retardation. Due to the severity of these conditions, fewer than 20% of those affected with Patau syndrome survive beyond infancy. Most infants with the syndrome die within the first three months of life. The average life expectancy of the survivors is about 10 years. Kidney malformations, mental retardation, developmental delays, growth deficiency, feeding difficulties, breathing difficulties. Some physical malformations associated with Edwards syndrome include: a small head (microcephaly), abnormally small jaw (micrognathia), cleft lip/cleft palate, upturned nose, narrow eyelid folds, widely spaced eyes, webbing of the second and third toes, clubfoot, or Rocker bottom feet, and undescended testicles in males. Boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. Unlike most other chromosomal conditions (such as fragile X), there is usually no distinguishable difference to the naked eye between women with triple X and the rest of the female population. 5