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Chapter 16 Blog: Simple Patterns of Inheritance (Marvi)

Page history last edited by Marvi Cruz 13 years, 1 month ago

A. Chapter Summary


Just a note: I'm going with a new style and that's kind of story format so I'm actually enjoying what I'm writing down rather than just feel like I'm copying it straight out of the book.


16.1 Mendel’s Laws of Inheritance
Once upon a time, there was a monk named Mendel, a man who wasn't able to become a teacher due to the fact that he was unable to obtain his license. So, while he tended to pea plants for eight years, he realized something about them; newer generations shared similar characteristics to older generations. He tapped his chin and continued to take care of these plants but with the intention of investigating the inheritance. Why plants?, some would question. With a thinned smile, irritated that he was pulled from his work, he told them, "Because, there are many types of characteristics; the seeds can be round, the flowers can be of various colors. Traits, I will call them. And, they are self-fertilizing, so, they are easy to produce.

"After all, why would I want to wait for a woman and man to reproduce?" he mocked, causing the person to only agree and for him to continue. "But, plants are easy to manipulate and I can cross plants--hybridization."

Day after day, Mendal continued to work on his studies, tending to the plants when needed. First, would be the monohybrid cross--only one trait would be followed. He gathered two plants, one tall the other small, and cross fertilized them. After some time, they began to create offspring, all tall. Through self-fertilization of the offspring, they produced three tall offspring and one small offspring. At these results, he began to test for other traits, such as color, types of seeds and so on. Immediately, he grabbed the journal he faithfully wrote in each day and wrote, "There are two types of traits: dominant and recessive and an individual carries two genes for a given character. With variant forms, they are known as alleles. Two of these alleles of a gene, that is, separate during a gamete formation meaning that a sperm and egg can only receive one allele."

After he closed his journal, Mendal gingerly felt the leaves of the third generation of plants--the ones that were born through self-fertilization--absent-minded until he came to another thought. A reoccurring pattern, he muttered to himself. Two parents must carry two alleles and the ones by an F1 plant will separate during the gamete formation. It struck him and, again, he opened to his journal, writing, "Two alleles of a gene will separate during the formation of eggs and sperm. Every gamete receives only one allele."

There was one day, as Mendel was flipping through the filled pages of his journal, when he devised a method to distinguish between two possibilities of having a tall plant with dominant phenotype being homozygous or heterozygous, that was, having the genotype TT or Tt. A test cross, he mumbled to himself. He would have to cross the individual of interest to a homozygous recessive individual and observe the phenotypes of the offspring. But enough of that, he began to flip through the other pages, absent-minded. His fingers continuously turned through the pages to come across the page filled with dihybrid crosses, indicating, the time he followed the inheritance of two different characters.

He picked up his fountain pen and began to practice, remembering what he once did. If the pea was yellow and round, it would be YYRR. If it was gree and wrinkled, it would be yyrr. Now, he thought to himself, if they were to cross, what would be the predictions for the F2 generation. Nine out of sixteen would be yellow and round, three would be green and round, three would be yellow and wrinkled while one would be wrinkled and green.

The ink began to smear onto his skin and he let out a curse as he wiped the residue on his robe. Again, he began to flip through the pages and came across another thing he wrote. The law of independent assortment. His eyes scanned the statement: alleles of different genes are able to assort independently during gamete formation. One specific allele, it continued, for one gene can be found in the gamete regardless of which allele for a different gene is found in the same gamete. He closed his journal, content with his progress and shut his eyes, his breathing still and paced.

Time went on, however, and Mendel lied in the ground, dirt covering his flesh that once walked the earth. His journal remained as well as his theories. But, now, while his body rotted, plants--be it ones that he did or did not grow--grow and thrive on the remains that they share with the worms, gnawing holes through tender, cold skin.



16.2 The Chromosome Theory of Inheritance

In an empty library, a biology book was left open on a table, the words and paragraphs hi-lighted with a obnoxiously loud yellow. Beside it laid a notebook, wide-ruled to be precise, with the chromosome theory of inheritance. In a barely readable script, it spelled out the five principles.  The first, chromosomes contain DNA, the genetic material; the second, chromosomes are replicated and passed from parent to offspring as well as cell to cell; the third, the nucleus of a diploid cell contains two sets of chromosomes that are found in homologous pairs; the fourth, at meiosis, one member of each chromosome pair separates into one daughter nucleus; and the fifth, gametes are haploid cells that combine to form a diploid cell during fertilization.

Below the scrawled writing was an arrow that pointed to the next page, or rather, to the edge of the paper. Almost like a deus ex machina, the page turned to another section completely about the physical basis of independent assortment. There a pea plant's pair of homologous chromosomes were drawn, the letters Tt under it, and to the side it says that it was heterozygous for height. Each chromatid carried an allele T or t for height (taller and shorter, respectively). The words meiosis popped up on the page and described how the homologues would pair up, each with two sister chromatids, would pair up and separate into two daughter cells; one now had two copies of the T allele while the other two copies of the t allele. Again, the sister chromatids were described to separate during meiosis II, producing four haploid cells and each cell had a copy of just one of the two original homologues.


Not too far from the information was a note that read in bold letters TEST. Beside it, they scrawled down, "Which I am screwed for."


16.3 Pedigree Analysis of Human Traits
"Hello, I will talk about how pedigree analysis is used to discover inherited traits through generations." As they stood in their spot, they came to the realization that the useless pleasantries could be skipped and they could delve straight into the topic rather than beat around the bush. "For example, an ancestor of a person could have had an abnormal allele that could have arose through a mutation. With pedigree analysis, one can determine whether or not the mutant allele is dominant or recessive as well as predict the likelihood of the individual being affected."

Past the sea of confused but intent students, they continued by turning their slide show to show an example of a pedigree. The slide showed several circles that represented women, squares that represented men--some shaded completely or halfway or not at all. They took a deep breath and continued, "As you can see, the pedigree can show that the person can be affected with the mutant allele but, not necessarily, have the disease, depending on what it is. These blank circles and squares indicate--in this case, the shaded have the dominant allele--that the gene is recessive that they do not have it at all. This pedigree, in this case, is showing for a recessive allele.

"In this one,"--the screen flipped to one where it was purely blank or purely shaded--"this is searching for the dominant allele. You either have it or you don't. If you have any questions at this point, ask me."

Most raised their hands. Obviously, the slides and short explanation wasn't enough of a lesson.


16.4 Sex Chromosomes and X-linked Inheritance Patterns
"What are you doing?"
"You should get ready for dinner."

All of the comments were dismissed as the student continued to lie on the couch, watching the television, specifically programs that could have possibly helped them with understanding the concepts of sex determination in animals. There were few things that they learned that they repeated to themselves in order not to forget. There was an X-Y system in mammals; males had one X and one Y chromosome while females had two X chromosomes. The X-O system were in insects and was determined by the ratio between X chromosomes and its sets of autosomes; 1/2 ratio deemed it male while a 2/2 or 1 will be deemed female.

Soon, the screen began to fill with bugs and the student continued to watch with disinterested eyes. Fruit flies, they said to themselves. As they continued to watch, there was a scientist, Morgan, that saw that the white-eye trait could have arisen from a mutation, causing the fruit fly's eyes to become white instead of red. He had made crosses to analyze the outcome and found that all the F1 offspring had red eyes, they were then mated to one another. In the F2 generation, there were no such thing as white-eyed females out of all of the flies that were reproduced. The student shifted their position, honestly impressed and continued to listen about what could have happened. Apparently, the white-eye allele was only possible within the males because the allele they had was w on their w chromosome rather than w+ or ww+, thereby providing evidence that genes were found on chromosomes.

As it delved further into Mendel, the student found themselves drifting to sleep into a dreamland where fruit flies came to eat them alive, their eyes of red and white mocking and laughing at them.

16.5 Variations in Inheritance Patterns and Their Molecular Basis

Okay, I'm tired of making stories. The fire has died, so, I'll just be blunt.

The reason why Mendel's model can result in oversimplicifcation is because of the fact that his model doesn't take in account incomplete and codominance. Mendel's model shows that there are only two possible results: dominant and recessive. When you get the dominant allele, that's what you only see. For example, if the dominant color is red, the flower will only be red and there's no such thing of having pink or red with white. There's also such things as mutant alleles and such.

There is a term known as the norm of reaction, it refers to the effects of the environmental variation on a phenotype.  Because of the environment, variation in organisms continuously change and grow. It can also be quite dramatic when considering environmental influences on inherited diseases; genetic disease phenylketonuria (PKU), a rare inherited disease.


B. Useful Material


So, usually, I wouldn't actually do my blog until much later on during the week, however, I feel as though I need to share a newly found link. That and the fact that I just realized that my past blogs have been done wrong since I've only put in one link and one PubMed (durr). So, I have to correct my wrongs--again. All right, so, I've been doing some research on this experiment that I've talked about to several professors about inheriting intelligence. In class, we learned about inheriting traits such as looks (for example, peas that were round or green or wrinkled or yellow). However, how can one possibly gain intelligence? Obviously, if my mother learned something through experience how would I and my brother gain that intelligence if it was gained through experience?


What happened in this experiment was fairly simple to test whether intelligence could be inherited. The scientists who participated in this experiment were to teach rats how to avoid a lighted exit (one that would shock them). As the rats went through experiments, the scientist bred them for thirty-four generations and through each generation, they began to have less errors in choosing the wrong door. If that wasn't enough, another scientist carried out the same experiment with trained rats (and with a control group of untrained rats). But, the result was same in both groups--the rats learned from their errors quicker than in previous generations.


These experiments, however, were disregarded by the scientific community. Major sigh. I'm not saying let the scientific community do this on humans (y'know, the same lighted exit maze business). But, I'm just asking if anyone else finds this weird? Because I definitely do.


So, I looked up more links on inheriting intelligence but nothing really led up to the study like this study. While I was reading it, they stated the scenario of a set of twins. If you were to place them in the exactly same environment through their entire life, they would have the same IQ. But, obviously, humans can't live the same exact life as someone else (that would be plain creepy).


So, this article defies the study that was done on the rats. I mean, let's put ourselves in that position. Let's say your mother or father went through this maze with an electrocuted exit. They got shocked a few times before they found the correct exit. Now, what's the probability of you not taking the electrocuted exit because of the fact that they already took it?


Unfortunately, we can't do that with humans because that's inhumane.


I'm not too sure what this article is supposedly pointing out, the PubMed one, that is. And it talks about how, in a family background, they bought education in Minnesota but not Sweden. There was one line that really caught my attention (I think it's somewhat important, that is). "Patterns of genetic influence on educational outcomes were similar in these two regions, but patterns of shared environmental influence differed markedly." I'm pretty sure that the article was hinting at how, because of the fact that there were environmental changes, that the influence of education was different. Meaning, if there was a difference in how a person was raised in a different environment that could affect their overall inherited intelligence. 





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