To learn the mechanics of human pedigree construction and analysis.
Objectives:
1. To learn the mechanics of human pedigree construction and analysis.
2. To become familiar with one trait in your family and its mode of inheritance.
Assignment:
- 1.Each student will make a drawing of a family pedigree. You need to trace the trait for 3 generation. This includes grandparents, their children (parents, aunts and uncles), and their children (daughters, sons, cousins). Label your diagram and identify the people affected by the trait. You do not need to look at your family. You can use somebody else’s family.
- 2.Draw a pedigree of your family for a simple, human trait. If your family exhibits a genetic disorder, and you feel comfortable dealing with it, that is also a good source for your analysis. Begin now and choose a trait and start checking with your parents, grandparents, and as many close relatives as you can show in your pedigree for that trait. Don’t use a trait that everyone in your family has. Ideally, you’d want something that varies a bit throughout your family and is easy to spot and verify. You’ll want to carry your pedigree back at least three generations. Use a computer graphics program or draw the pedigree carefully. Guess the genotype for each of your members of a pedigree.
- 3.Evaluate the genetics of the trait you chose. Is it a trait which is influenced by only one gene pair or is it polygenic. What kind of pattern of inheritance does it follow: dominant/recessive, co-dominant, or incomplete dominance? Is the trait strictly influenced by genetics or are there environmental factors as well?
How to construct a human pedigree:
Generations are numberered from the top of the pedigree in uppercase Roman numerals, I, II, III etc. Individuals in each generation are numbered from the left in arabic numerals as subscripts, III1 , III2, III3 etc.T
Modes of inheritance. Most human genes are inherited in a Mendelian manner. We are usually unaware of their existence unless a variant form is present in the population whichcauses an abnormal (or at least different) phenotype. We can follow the inheritance of the abnormal phenotype and deduce whether the variant allele is dominant or recessive.
Dominant Trait.A dominant condition is transmitted in unbroken descent from each generation to the next. A typical pedigree might look like this:
Examples of autosomal dominant conditions include Tuberous sclerosis, neurofibromatosis and many other cancer causing mutations such as retinoblastoma
Autosomal Recessive.A recessive trait will only manifest itself when homozygous. If it is a severe condition it will be unlikely that homozygotes will live to reproduce and thus most occurrences of the condition will be in matings between two heterozygotes (or carriers). An autosomal recessive condition may be transmitted through a long line of carriers before, by ill chance two carriers mate. Then there will be a ¼ chance that any child will be affected. The pedigree will therefore often only have one ‘sibship’ with affected members.
Guide to Human Traits. Genetics is the study of the mechanisms of heredity. Stated another way, it is the study of the ways traits or characteristics are passed on from generation to generation. People differ from each other in the expression of these traits; thus, expressing their own individuality and uniqueness. We will examine in more detail a few easily observable human characteristics that are known to be controlled by simple gene systems that follow Mendelian patterns of inheritance.
Review the list below of these common, innocuous, human genetic traits. You can use anyone of them for your pedigree analysis project. It is important to remember that the more data you have, the more worthwhile the project. Try to have at least 3 generations. Less than that and you may not have enough information to determine genotypes. Again, contact your TA if you are having a problem.
Common Human Traits.
1. Tongue Rolling. Attempt to roll your tongue into a U-shape, in which the sides of your tongue are curled upwards. Tongue rollers carry a dominant gene R. Non-tongue rollers are homozygous recessive (rr).
2. Widow’s Peak. A dominant gene W causes the hairline to form a distinct downward point in the center of the forehead (like Richard Nixon). Baldness will mask the expression of this gene. If you have a Widow’s Peak, you have at least one dominant gene. No downward point of the hairline, and you are
homozygous recessive (ww).
3. Earlobe Attachment. The inheritance of a dominant gene E results in the free or unattached earlobe. If the lobe is attached directly to the head, the individual is homozygous recessive, and the ee genotype is present. Other genes, working alone or together, affect the size and shape of the earlobe and are not considered here. We are only looking at whether the earlobe is attached or not.
4. Hitchhiker’s Thumb. Some individuals can bend the last joint of the thumb backwards at about a 45 degree angle. These individuals are homozygous for a recessive gene, hh, but there is considerable variation in the expression of the gene. For our purposes, we shall consider those who cannot bend at least one thumb backwards about 45 degrees, are carrying the dominant gene, H.
5. Bent Little Finger. The dominant gene, B, causes the terminal bone of the little finger to angle toward the fourth (ring) finger. Individuals whose little fingers are straight possess the homozygous recessive condition, bb. Check for this characteristic by laying your hands flat on the desk and relaxing them.
6. Mid-digital Hair. The presence of hair on the middle segment of the fingers is caused by a dominant gene, M. The homozygous recessive condition, mm, results in the lack or absence of hair on the middle segments of the fingers. Examine your hands closely since the hairs may be small in length and light in color.
7. Facial Dimples. The inheritance of cheek dimples is controlled by a dominant gene, D. The homozygous recessive, dd gene condition, lacks the ability to express facial dimples.
8. Big Toe Length. The length of the big toe is governed by the dominant gene, H. Individuals whose big toe is shorter in comparison to the second toe possess the dominant gene. The inheritance of the homozygous recessive, hh, results in the big toe being longer than or equal to the second toe.
9. Index Finger Length. If your second finger (index finger) is shorter than your fourth finger (ring finger) on the same hand, you have a short second finger in relationship to the length of your fourth finger. The gene for short second finger, S’, is also sex-influenced in its expression (like baldness). It is dominant in males and recessive in females. That is, while all S’S’ individuals have short second fingers, S’S” males show short second fingers and S’S” females do not. The gene for long second finger, S”, is dominant in females.
10. Human Blood Type. Human blood type are an example of multiple alleles. There are two dominant genes and one recessive gene. The dominant gene IA codes for type A blood, while the dominant gene IB codes for type B blood. Type A blood may be homozygous dominant (IA IA) or heterozygous (IA i). Conversely, type B blood can also be homozygous dominant (IB IB) or heterozygous (IB i). Type O blood is recessive, so it would be caused by a homozygous recessive condition (ii). Type AB blood is interesting in that it is an example of co-dominance. Both the A and the B gene are simultaneously expressed. The genotype would be IA IB.
You will need to construct the pedigree on paper or computer but by long hand. You will be expected to gather all your data about the trait in your family. From that data, draw the pedigree according to the standard conventions. Once the pedigree is drawn, you must add a written discussion of the the pedigree and determine the pattern of inheritance of your trait. (Make sure it is a genetic trait)
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