Tuesday, April 7, 2009
Genetics and gene expression
On page 47 of the book, Carroll shows William Castle's experiment of selection on coat color in rats. Castle states that many genes were modifying the fur patterns of the rats and creating a continuous gradation of variation. Describe the process of gene inheritance and expression that allows for this modification of fur patterns in rats in great detail. (Hint: DNA replication enzymes, promoters, meiosis, linked genes, law of independent assortment, law of segregation, DNA translation, transcription, RNA, ribosome, etc.) How can genes that are naturally selected pass on to other generations?
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The complex workings of gene inheritance and expression are what allow for such diverse fur patterns in rats and other species. When genes are passed down from parents to offspring, the dual copies of their genetic material split (meiosis) and only one set from each parent is sent on to the offspring, as of Gregor Mendel’s Law of Segregation. Those sets then assort themselves in the sperm and egg without interference by the other sets and then combine in a random sequence in the zygote, as of Mendel’s Law of Independent Assortment. This mixing and matching of genes, along with the added effect of genetic crossing over and mutation, can create a wide range of phenotypes. This is because during transcription and translation, each different genetic set makes its own RNA, and sends its own messages to the ribosomes, causing in the end the display of a unique phenotype.
ReplyDeleteGenes that are naturally selected are most often ‘fit’ genes. These genes are guaranteed to be passed on as they grant the individual in question a selective advantage over his counterparts. In the example of the mice above, a mouse with naturally selected dark colors is more likely to go unnoticed by predators in dark environments while an ‘unfit’ albino mouse would be spotted and eaten, eliminating it from the gene pool. The surviving black mouse would then be able to reproduce and propagate his naturally selected black color line while the white mouse’s line would end. The naturally selected genes’ can figuratively beat the less fit genes to ensure that they get passed on.
William Castle’s work with hooded rats focused on the variation of certain traits within a population and demonstrated how small variations could emerge with many different combinations. Through selective breeding, pigmentation and different patterns could be realized in the hooded rat populations.
ReplyDeleteAccording to Mendelian Inheritance, sexual reproduction, and cellular operations, several factors affect the outcome of different phenotypes in successive generations. According to Mendel, in sexual reproduction, a sets of gametes with 23 chromosomes for humans come from each parent. Upon these chromosomes, different alleles of genes exist, that in combination with the second set of chromosomes can determine what character traits are inherited. He further discusses the idea of how traits can attain dominance over another recessive trait. Mendel’s law of independent assortment is the idea that there is an independent segregation of each pair of alleles during gamete formation. Mendel’s law of segregation is the idea that there is a separation of alleles into separate gametes.
Other relationships exists as well such as codominance, incomplete dominance, and various levels in between these different degrees of dominance. The relationships between alleles and genes also spread to many significant situations. For example Multiple alleles can exist on a single gene, such as the determinants of ABO blood groups. Four different combinations exist as either A, B, AB, or O type blood. According to the A substance and B substance carbohydrates, or one that has neither, different blood types are determined. Another example of a particular case in inheritance is pleiotropy. In pleiotropy, the idea of how genes can have several different phenotypic effects is recognized. For example if certain pleiotropic alleles determine a certain disease, such as sickle-cell disease, many different obvious symptoms are noticed. Epistasis is an other factor in inheritance. A certain combination of alleles can determine either negate or allow the combination of a different set of alleles. This epistatic gene regarding fur color, can allow for multiple different colors by either allowing the prior combination of different alleles for fur color, or negating this combination to produce a white fur color rather than the traditional dominant and recessive combinations. There is also the case of polygenic inheritance, the opposite of pleiotropy, where several genes can factor the phenotypic outcome of one characteristic or trait. For example human skin pigmentation have a large variety of diverse outcomes due to numerous genes affecting the phenotypic outcome. Throughout all these different gene relationships in determining a certain phenotypic outcome, several of these relationships in determining fur color could exist in the different species of rats. As one possibility, Tympanoctomys barrerae, a red viscacha rat from Argentina has twice as much genetic material than its closest relatives, demonstrating that such an organism with numerous chromosomes can have different relationships and combinations that affect the diversity of fur coloration.
The environment itself has a great impact on inheritance and transfer of selective genes. Nutrition can stimulate faster growth or height or some other different characteristic. The climate can affect skin pigmentation. Exposure to wind or other environmental factors affect the outcome of a phenotypic outcome. As well, as Marc Moylan stated before, in the case of mice living on dark, lava environments, dark colored mice have a selective advantage over lighter colored mice on that environment and this ensures prolonged survival, maintaining this phenotypic outcome in the population. Likewise, lighter colored mice that live on sandy floors have a better chance of survival and of passing on their phenotypic outcome onto the next generation over darker colored mice. Overall the idea that environmental factors can affect the expression of a certain trait greatly diversifies the possible characteristics of an organism. The combination of environmental and genetic factors collectively influence phenotype.
There is also the idea of alterations in chromosome structure. A breakage in a chromosome can lead to a deletion, when a fragmenting chromosome that lacks the connective and transfer properties of a telomere used in meiosis, will lose a section of the chromosome. Up to several different genes could be lost due to the fragmentation. As well there could also be a duplication, where an extra segment of genetic material can be placed within the chromosome. In inversion, a chromosomal fragment could possibly reattach itself to the original chromosome at a different location. Translocation is the combination of a certain chromosome fragment on another nonhomologous chromosomes. These alterations of chromosome structure can also factor in rat fur coloration although these conditions would most likely result in many complications.
Following through the process of Meiosis and sexual reproduction, other factors in rat fur coloration can be recognized. First there is the possibility of alternative arrangements of two homologous chromosomes. When the homologous chromosome pairs line up for separation at the metaphase plate, many different arrangements of chromosomes are possible, especially with a larger number of chromosomes in that particular organism. A daughter cell of meiosis can either receive the maternal or paternal chromosome between a numerous amount of chromosomes, developing genetic diversity that affects the passing of genes between successive generations. This process stems from the law of independent assortment of chromosomes. As well crossing over can occur between two homologous chromosome pairs to create recombinant chromosomes with different genes from both the maternal and paternal chromosomes. This occurs in prophase I where similar genes align and portions then can cross over. Another factor that affects the outcome of a phenotypic trait is random fertilization, where a zygote is combined between different genders. This exponentially increases the diversity of chromosome arrangements especially with larger numbers of chromosomes and crossing over to create immense genetic diversity.
In different gene expressions, certain mutations could possibly alter the outcome of a certain trait. Through the processes of translation through first transcription of an mRNA, then editing, and then translation of the mRNA by ribosome, mutations of the DNA sequences could lead to different variations of phenotypes that could possibly be advantageous to the organism. In point mutations and base pair alterations, mutations in the genetic material of organisms could be passed on to successive generations that could either have a positive or negative affect. In base-pair substitutions, base pairs could be replaced with another possibly changing the coded amino acid. The idea is that a mutation that may create variation of fur color that advantageously aids the organism in survival, such as darker fur of mice in black lava flows and lighter fur of mice in sandy deserts. When survival is increased, the continuation of the specific trait is increased that can lead to what traits are actually passed on to successive generations. Eventually when ligher furred mice are excluded from the genetic pool of variations for mice living in darker environments, the spread of this particular trait is greatly increased.
http://www.absoluteastronomy.com/topics/William_E._Castle
http://en.wikipedia.org/wiki/Mendelian_inheritance
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mutations.html
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mutations.html
Biology sixth edition, Campbell and Reece