Discussion Board #7 PDF

Preview

Summary

discussion board...

Description

Which of the following statements describes genomic imprinting? A. It is proof that crossing-over has occurred during prophase of meiosis I B. It explains the observation that the phenotype of a heterozygous individual can depend on which parent provided the mutant allele C. It may explain the transmission of Huntington's disease D. It is observed mostly in females because of the egg has more cytoplasm than the sperm According to Mendel’s laws of inheritance, the sex of the parent passing down the allele is insignificant. Whether the allele is passed down from the mom or dad, it should have the same effect. However, scientists have identified several traits that depend on the sex of the parent. The variation in phenotype based on the sex of the parent that the allele is from is called genomic imprinting. It is an exception to Mendel’s laws. Imprinting genes are usually on autosomes. Genomic imprinting takes place during the formation of gametes and causes the silencing of alleles in certain genes. The genes are imprinted differently in male and female sex cells. As a result, only one allele from the mom or dad of an imprinted gene is expressed in offspring (Campbell, 308). A is incorrect. Genomic imprinting does not prove that crossing-over occurred in prophase of meiosis I. Genetic recombination or the formation of offspring with traits that differ from the parent proves that crossing over took place. Crossing over produces recombinant genes. Crossing over takes place when replicated homologous chromosomes are paired in prophase I. Proteins cause an exchange of DNA. End portions of nonsister chromatids change places. In an experiment with flies, crossing over in dihybrid female fly produced recombinant eggs and then offspring. The recombinant phenotype is present in offspring when crossing over occurs in the parent. The genes will be expressed the same no matter which parent passed it down unlike in genomic imprinting (Campbell, 300). Since genetic recombination not genomic printing shows that crossing over took place in meiosis I, choice A is false. B is correct. In genomic imprinting, only one copy of a gene is inherited and the other gene is silenced through methylation. Methyl groups are attached to cytosine nucleotides of the allele of the mom or dad to inactivate it. The silencing occurs depending on who the allele came from. In maternal imprinting, the allele from the mom is silenced and in paternal imprinting, the allele from the dad is silenced. The phenotype of a heterozygous individual can depend on which parent the mutant allele was inherited from because of genomic imprinting. A heterozygous human can thus express a normal or mutant allele since only one parent’s allele will be expressed. This was observed when heterozygous offspring were produced by mating normal mice with dwarf mice homozygous for the recessive mutation. The dwarf phenotype was only expressed when the father passed down the mutant allele. Therefore, it is true that genomic imprinting explains how the phenotype of a heterozygote can depend on the parent that contributed the mutant allele and choice B is the correct answer (Campbell, 308). C is incorrect. Huntington’s disease is a genetic neurodegenerative disease that is caused by a lethal dominant mutation in an allele. The disease is dominantly transmitted disorder that does not against Mendel’s laws. Since the effect of the lethal dominant allele is not obvious until a human is 35 to 45 years old, the allele is usually passed onto offspring. If a parent has the lethal

dominant allele, there is a 50% chance that the child will inherit the allele. The allele for dwarfism is expressed when the individual is homozygous dominant or heterozygous. Huntington’s is inherited whether the parent with the lethal dominant allele is male or female. Alleles are not silenced when passing on the disorder. Genomic imprinting does not have anything to do with the transmission of Huntington’s disease (Campbell, 285). The question asks what describes genomic imprinting. The statement about genomic imprinting is not true and so choice C cannot be the right answer. D is incorrect. Organelle genes are passed down by the mom. The cytoplasm in eggs include these extranuclear genes. The genes in the cytoplasm are maternally derived and can be passed down to offspring. For example, in plants, all the plastids of the zygote all come from the egg’s cytoplasm. Mitochondrial DNA is also maternally inherited in a similar fashion. Most of the zygote’s mitochondria comes from the egg’s cytoplasm. The mitochondria provided by the sperm is eliminated through autophagy. The extra cytoplasm in the eggs does not facilitate more genomic imprinting in females, but provides offspring with extranuclear genes. Genomic imprinting takes place in both males and females (Campbell, 309). Choice D’s statement regarding genomic imprinting is not true, so choice D is incorrect. Genomic imprinting is an amazing phenomenon. In the real world, scientists are still trying to learn more about it. The article I chose discusses how genes that go through genomic imprinting are expressed from only one parental allele. Imprinted expression is important during fetal development in mammals, since the genes involved in genomic imprinting usually regulate the process. In a recent study, complex functions of imprinted genes in the brain were found. These genes also impact behavior and neuronal function. The significance of proper expression of imprinted genes in stem cells was also discovered. The article demonstrates how intricate and important imprinted genes are (Plasschaert and Bartolomei). I agreed with Christopher Lau’s explanations and found his real life connection to be very interesting. My article focused on imprinting in mammals, so it was interesting to learn about imprinting in plants. Christopher’s article focused on imprinting in angiosperms. The scientists discovered that certain expressed genes in embryos influence the quantity of resources transferred from a mom to the offspring at the expense of the half siblings. When the allele of a gene for a placental growth factor is expressed, the embryo demanded more nutrients than the half siblings. When it wasn’t expressed, the demand remained the equal to the half siblings (Moore). Imprinted genes help provide nutrition to the embryo in plants. Genomic imprinting plays an important role in embryo growth in both mammals and plants.

Works Cited 1. Taylor, Martha R., et al. Campbell Biology. Tenth Edition (Chapter 14 and 15. Pearson Education, Inc., 2013. 2. Plasschaert, Robert N., and Marisa S. Bartolomei. “Genomic Imprinting in Development, Growth, Behavior and Stem Cells.” Development (Cambridge, England), The Company of Biologists, May 2014, www.ncbi.nlm.nih.gov/pmc/articles/PMC3994769/.

3. Moore, T., & Haig, D. (1991). Genomic imprinting in mammalian development: a parental tug-of-war. Trends in Genetics, 7(2), 45-49....