NATIONAL CENTER FOR CASE STUDYTEACHING IN SCIENCE Part IlIl - Molecular Genetics
ID: 199799 • Letter: N
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NATIONAL CENTER FOR CASE STUDYTEACHING IN SCIENCE Part IlIl - Molecular Genetics More than 70 years after Arthur Fox serendipitously discovered that humans vary in th eir sensitiviry to F related chemicals, researchers from the Natik together discovered the main bitter taste rece for PTC taste is e onal Institutes of Health, Stanford University, and che Universiry of btun gene for ITC sensitivity in humans. This gene is a member of a gene family that encodes ptors in the mouth called the TAS2R gene family (Kim et al., 2003). TAS2R38, the gene responsible st ensitivity, is on chromosome 7 and codes for a receptor protein responsible for bitter caste perception that binds PTC. The two common alleles differ by three SNPs, causing three amino acid changes. One common allele or haplotype has proline (P), alanine (A), and valine (V) for these three amino acids and therefore is called PAV. The other common allele is called AVI because the amino acids are alanine (A), valine (V), and isoleucine (). The PAV allele codes for a form of the receptor protein that binds PTC, conferring the albility to taste this chemical. The protcin product of the AVI allele cannot bind PTC, and thus this allele confers the inability to taste PTC. However, the fact that the AVI and PAV alleles are expressed equally at the RNA level and that the AVI allele contains no frameshifts or other mutations that would result in carly termination of the protcin suggests chat the A binds another bitter chemical that has not yet been identified (reviewed in Wooding, 2006). Other haplocypes presumabl showed functional responses in in viso studies and, thus, these alleles appear to code for functional proteins (Bafe et al VI-coded receptor protcisn y due to recombination between the three SNPs, occur but are rare. These rare haplotypes also Questions 1. What is a gene family? What mechanism gencrates them? 2. What is a SNP? What is a haplorype? How can SNP haplotypes be determined? 3. Assuming the PAV allele is the original one, what types of mutations (silent, missense, nonsense, etc.) have changed the PAV allele into the AVI alldle? Explain. Based on the information given, how do dise changes fit with what you have learned about the "functional enzyme hypochesis"? 4. You repeat Snyder's experiment, but instead of determining dhe ability of each family member to taste PTC, you determine what alleles they carry: 2 PAV (PAV/PAV), 2 AVI (AVI/AVI), or 1 PAV and 1 AVI (PAVIAVI). Using the table provided below, what are the possible outcomes for offspring if in the first column? Below the genotype, write the phenosype (taster or non-taster) of each kind of individual. Parent Genotype Combinations Possible s of Chiidren Both pazents PAV/PA Boch parents PAV/AVI One PAVIPAV, the other PAVIAV One PAV/PAV, the ocher AVI/AVI One PAVIAVI, the other AVI/AV Both parents AV/AVI To the Birter End" by R. Deborah Overath PageExplanation / Answer
1. A) A gene family can be defined as a set where there is presence of a number of genes which are similar to each other, that have been formed as a result of duplication the main or the original gene. An example would be the hemoglobin subunits of humans, where there are 10 genes that are arranged in two separate clusters present on separate chromosomes which are known as beta globin loci as well as the alpha globin loci.
B) the generation of a gene family is when there would be a number of duplications taking place on the initial gene or the ancestral gene. These duplications are succeeded by mutations. Following mutations there is observation of divergence in the ancestral gene. Further, the duplications may take place in a lineage or they could occur as a consequence of speciation.
2. A) SNP is referred to as single nucleotide polymorphisms. This implies that there would be variations that would take place in one nucleotide or a single nucleotide taking place at one particular position inside the genome, such that, every occuring variation would be observed in the population up to a particular appreciable degree. Example: this has to be greater than 1%. As a result of these SNPs, there would be differences in how susceptible we would be to a disease. This has been seen in a wide range of diseases- sickle cell anemia, cystic fibrosis as well as beta thalessemia.
B) haplotype could be identified as an allele group that is present in an organism, such that, they would be inherited along with each other from one parent. This is also referred to as a haploid genotype. Initially, this came to be known as the collection of DNA sequences present in close linked genes and hence it used to be referred to being conserved sequence. A conserved sequence would be the one that would be present in a lineage throughout generations.
C) SNP- haplotypes are referred to as haplogroups. Linked SNPs are known to be as the alleles that would be associated on a statistical manner. If statistical associations would be identified, identification of a few or specific sequences of haplotype would help in identification of other polymorphic sites that are located close to the chromosome.
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