Exercise 10 - Mendelian Genetics II: Human Inheritance

Students study Mendelian genetics by observing human traits that are recessive or dominant. Procedures for blood-typing are performed. Chromosome differences between the sexes are observed by viewing the Barr bodies in stained human epithelial cells. Several story problems are solved to illustrate basic principles of genetics.

EQUIPMENT	AMOUNT (Class of 24 with 8 groups)
Compound microscope	1/student
Rh viewbox, optional (CBS#70-0636)	1/lab

MATERIALS

SOLUTIONS

PREPARATION

One Week before Lab

1. Acetocarmine stain preparation: See Exercise #9 for instructions.

2. Saline preparation:

   0.85% NaCl

   0.85 g NaCl/100 ml water

   Store and dispense in dropping bottle.
   Shelf life: indefinite

3. 70% ethanol preparation:

   70% ethanol

   70 ml 95% ethanol/25 ml distilled water

   Mix alcohol and water; store and dispense in dropper bottles.

NOTES

1. Sometimes student blood gives little or no response to the antisera when test is done on glass slides. Special Dispos-A-slides can be ordered from CBS. If this happens, slides might have an oily film. They should be rinsed with 70% ethanol, acid washed (5 ml HCl/95 ml water), and rinsed with distilled water. An Rh viewbox can help to see the reaction.

2. Antisera stores well in the refrigerator. To prevent contamination, students should not touch the drop of blood with the tip of the antisera bottle. Instructor may want to dispense the antisera to conserve supplies.

3. NO LANCET SHOULD EVER BE USED TWICE. Caution students about transmission of viral diseases, such as AIDS and hepatitis, by sharing lancets. A widemouthed jar (16 to 32 oz) should be half filled with full-strength bleach. All lancets, toothpicks, and blood-typing slides should be put in the jar immediately after use. Pour off bleach and autoclave contents before disposing.

CLASSROOM SUGGESTIONS

1. Students enjoy this lab and show high interest in how their dominant-recessive traits rank with their classmates. If more than one lab section meets, a running total of the phenotypes, including frequency of blood types, can be kept for later discussion.

2. Another inherited human characteristic is the taste response to certain chemicals. Several supply houses have taste papers available. If the school schedule permits a holiday at this time, students can take a sample taste paper packet home to report family members' taste responses.

3. Occasionally, a student will become faint during the blood-typing procedure. To prevent injury from fainting and falling, students can work together in pairs. They can also assist each other in pricking the finger. They must not share lancets.

4. The major problem encountered in blood-typing is that students' slides may dry out. This can give the appearance of agglutination and be interpreted as a positive reaction for the test. With a few practice trials, the instructor can distinguish drying from agglutination. The results from the blood-typing in the lab are only tentative and should not substitute for a test performed by a certified medical technician.

5. Students have difficulty identifying and locating the Barr bodies. A DISPLAY of prepared slides (CBS#G504 and G506)* can give the student an idea of the location of the bodies in the cells. Not all cells in the female will show Barr bodies at the same time. The bodies are easily seen when the cells are in interphase.

6. This laboratory can be completed in two hours and can be combined with counting of phenotypes from fruit fly crosses in previous lab. Summarization might be delayed if surveys of blood types and phenotypes of family members are made.

ANSWERS TO CRITICAL THINKING QUESTIONS

1. Because the second mother who is type O, RH-, has anti-A and anti-B antibodies, the survival time of any fetal A, Rh+ blood will be quite short. Because of this rapid destruction of fetal A, Rh+ cells, the mother's immune system may not be stimulated to produce anti-Rh. In the case of the first mother, however, the fetal cells would have a normal survival time and would stimulate anti-Rh production.

2. ``John Doe'' is XXY. This is the result of non-disjunction, or failure of homologous chromosomes to move apart during Anaphase I of meiosis I or failure of sister chromatids to separate during Anaphase II. This would result in the production of gametes with either an extra chromosome or one missing. Fertilization of such gametes by a normal gamete will result in an atypical individual having either three homologous, as in John Doe's case, or only one, as in an XO individual.

3. An autosomal recessive mode of inheritance is indicated for this trait. Since neither parent expressed the trait, but their offspring did, the parents would be heterozygous for that trait. Without knowing the phenotypes of the grandparents, the only inference to be made is that one maternal grandparent at least carried the recessive allele, and also, that one paternal grandparent at least carried the recessive allele.

SUPPLEMENTAL MATERIALS

Bio Sci II, videodisc contains diagrams and photos applicable to this exercise. Dubuque, IA: Wm. C. Brown Publishers. See appendix.

Heredity, Health and Genetic Disorder, audio filmstrip. West Los Angeles, CA: Science Software Systems, Inc. #600-0075

Human Blood Groups, 12 minute video filmstrip. Burlington, NC: Carolina Biological Supply. #49-8386