On page 3, Dr. Moalem briefly
discusses penetrance. Penetrance is how frequently a gene expresses itself in population. There are different levels of penetrance:
high, complete, low, etc. Moalem uses the gene for dimples as an example of
high penetrance because that single gene is all that is required to develop
dimples. On the other hand, the gene for hemochromatosis has low penetrance
because the gene along with a variety of other circumstances is required for an
individual to exhibit symptoms of hemochromatosis.
Discuss the difference between
penetrance and expressivity of a gene. Combining your prior knowledge of
genetic phenotypic expression as well as outside research, connect both
penetrance and expressivity to Mendel’s work. Then, choose one of the diseases recently
studied in class (Huntington’s, Cystic Fibrosis, etc.) and discuss the disease’s
penetrance and expressivity. Why might genes exhibit different degrees of
penetrance? Be sure to relate your response to Big Idea 1 (The process of
evolution drives the diversity and unity of life).
(Christine Fanning, cfannin4@students.d125.org)
Expressitivity of a gene is the extent to which a genotype is expressed while penetrance is the frequency of the gene expression in a population. While different tiers of penetrance can be due to “modifiers, epistatic genes, or suppressors in the genome or modifying effect of the environment” the different degrees of expressitivity is due to “variation in the allelic constitution”.. Penetrance and expressitivity is shown through Mendel’s work and his Mendelian Law of Inheritance. For example, a plant flower is autosomal dominant for red (RR and Rr) and is white when the plant has 2 recessive gene (rr). So the (R) allele is dominant over (r). When penetrance varies, Rr and RR may be expressed as red but sometimes Rr may “skip” a generation even if that specific plant carries the gene for the R allele. When expressitivity varies, Rr is expressed as red but expression varies with different shades of red. The expression (color) would vary even if the genotype is the same. The change may be a result of environmental factors or variation in the genome.
ReplyDeleteHuntington’s disease (HD) is a neurological disorder and is autosomal dominant. The disease occurs due to a defective gene on chromosome 4 that codes for a protein huntingtin. Most who receive the dominant allele express the disease. However, the expressitivity varies. Researchers at Boston University School of Medicine (BUSM) have found that Huntington’s “primarily damages the brain in 2 areas” which are the striatum and the outer cortical regions. According to Myers, “There are tremendous differences in how people with [HD] are affected” and the researchers have noted that variation in the Huntington gene causes the difference. Also they have discovered that even though HD is only 1 genetic defect, the results are different with individuals. The research discovered the different levels of expressitivity of the HD ranging from cognitive disability to motor control issues.
Genes might exhibit different degrees of penetrance because of different environmental factors as well as the variation within the genome of an individual. For example, like Dr. Moalem wrote, “a gene that requires a host of other circumstances to really manifest...is considered to have low penetrance” (3). Using hemochromatosis as an example, if the environment requires more iron in the diet such as those “poorly nourished population living in harsh environments” (14) then hemochromatosis may develop in order for those individuals to survive and absorb the needed iron. However if the environment does not require any more excess iron the disease may not have developed; the manifestation of the gene would be dependent on certain circumstances thereby affecting the penetrance of the gene.
Big idea 1is related to the expressitivy and penetrance of genes because the different levels of each affect the diversity present today. Like with HD, the different levels of expression of the disease create variation of the disease. Some who have milder cases of HD would be more likely to survive and reproduce as compared to those who have extreme cases of HD. Likewise hemochromatosis has once served to protect those who have the disease from the bubonic plague. However, nowadays those individuals who carry it are more at a selective advantage. The difference of degree of expression of genes creates an infinite amount of possibilities for that gene. That causes natural selection to occur for that gene, showing which are selective advantages and disadvantages.
Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Penetrance and expressivity. http://www.ncbi.nlm.nih.gov/books/NBK22090/
Boston University Medical Center. Pathology of Huntington's disease identified. ScienceDaily. http://www.sciencedaily.com¬ /releases/2012/10/121017132023.htm
Miko, I. (2008) Phenotype variability: penetrance and expressivity.
(http://www.nature.com/scitable/topicpage/phenotype-variability-penetrance-and-expressivity-573#)
(Hervy Ong, hong3@students.d125.org)