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Chapter 16 Blog: Simple Patterns of Inheritance (Siddarth)

Page history last edited by Siddarth Santhebennur 13 years, 4 months ago

A.  Daily Blog

     So chapter 16 is pretty much about genetics in a simplified and condensed format (hence the name "Simple Patterns of Inheritance"). We started off talking about a guy we are all familiar with and is also known as the "Father of Genetics". That's right! It is is Gregor Mendel. As you may know, Mendel was famous for using pea plants as a way to examine the cross of specific characteristics and see which traits would be expressed. Why did he use pea plants compared testing on some other organism?  One advantage that was seen by using peas in his experiment was that they had multiple characteristics (seeds, pods, flowers, and stems) that could be observed when reproduced. Another important feature why Mendel used pea plants was because they self-fertilize. These plants contain both the male and female gametes. The third reason why Mendel used these plants was in order to "save money"...well it could be said in other words that pea plants could be manipulated easily. The first inheritance experiment began with a monohybrid cross (or single-factor cross). In monohybrid cross, the experimenter follows one specific character. In this case, Mendel could have viewed height, color of the seeds or anything of that nature. By determining that specific characteristics, Mendel saw that some of the F1 generation plants (which may not have been very tall) reproduced to form plants that were tall. Mendel came to the conclusion that there were two kinds of traits (an identifiable characteristic of an organism) that could be expressed: Dominant or Recessive. A Dominant gene would considered to the be the gene that would be expressed IF that gene is present. Recessive genes are considered to be masked over if the Dominant gene is present. As Mendel observed many generations, he saw a pattern that each parent plant was a carrier for 2 alleles (that were of the same version) except the were being spread out or segregated. He looked at height as an example and noticed that there was a ratio of 3:1 (tall to dwarf respectively). Mendel coined up with an idea that the two alleles of a gene separate (segregate) during the formation of eggs and sperms so that every gamete receives only one allele. Today, this idea is known as Mendel's law of segregation. A way to predict the outcome of simple genetic is the almighty Punnett Square. In order to use the Punnett Square, the parents genotypes are required. Each of the females gametes are placed on the left side of the square and the males gametes are placed on top. Once they are positioned correctly, you match up the respective columns gamete with the gamete found in that row. It is pretty simple with a 4-squared Punnett Square, but it does get much more complicated. We tend to look an individuals phenotype as either homozygous or heterozygous. The question at hand is: how do we really determine if the individual is either homozygous or heterozygous? A testcross is the answer to this question! In a testcross, the researcher crossed the individual of interest to a homozygous recessive individual and observes the phenotypes of the offspring. Similar to a monohybrid cross, a dihybrid cross is used to follow specific characteristics from one generation to another. The only difference is the amount of genes that are being followed. As you can tell, in a dihybrid cross there are two characters that are being followed. With all this research being done, Mendel found some similar results for all the test that he ran. This was how the law of independent assortment came to be. This law states that the alleles of different genes assort independently of each other during gamete formation. 


The Chromosome Theory of Inheritance is made up of five main principles: 


  1. Chromosomes contain DNA
  2. Chromosomes are replicated; this information is passed on from one generation to the next
  3. The nucleus of a diploid cell contains two sets of chromosomes, which are found found in homologous.  
  4. At meiosis, each chromosome pair segregates into one daughter nucleus, and its homologue segregates into the other daughter nucleus. During the formation of haploid cells, the members of different chromosome pairs segregate independently of each other.
  5. Gametes are haploid that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.

These principles give us a better understanding of independent assortment in humans. On our chromosomes, we have areas known as a loci which is the physical location of a gene. Each pair of a homologous pair contains a locus at a specific location which is identical to the other side. In meiosis, chromosomes replicate and form homologous pairs. In Anaphase I, the pair segregate and separate into their respective cells. The sister chromatids then separate during Anaphase II. This produces four cells that have different alleles in their cells. This then makes each cell unique since the


   What is a pedigree chart? The use of a pedigree chart is mainly intended for humans. Since we don't have the time to spare to watch humans grow and identify what genes are expressed, we use pedigree charts. A pedigree analysis is a way to observe an inherited trait over the course of a few generations in one family. This method has been commonly known to track disease within a family. A pedigree chart is more or less a family tree except it focuses on the passage of genes. As you already know, a Dominant gene is the gene that is expressed throughout the body. It is not possible for one to carry a Dominant gene as compared to a Recessive gene. In a pedigree chart, males are distinguished by squares where as females are distinguished by circles. If he/she is a carrier, then it can be denoted by a half shaded square/circle. If the person has the gene expressed then the square/circle would be completely filled. A simple way to determine if the gene is dominant or recessive is to start  off with the parents. If the parents are half-filled then the trait would be recessive. However, if one of the parent were Dominant for the gene, then this would be prominent in the rest of the pedigree since there is NO chance of a Dominant gene having a recessive affect on an individual. 


     Some gene are known to be passed by the sex of your parent. In other words, the gene could be located on the sex chromosomes. It could vary from your father to your mother, but it all depends on the gene itself. Sex chromosomes play a big role in determining your gender....BUT it is not the only thing that plays a role in gender. There are four main systems that apply for sex determination: X-Y System, X-O System, Z-W System, and the Haploidiploid System. The X-Y system applies for mammals. Males have one X and one Y chromosome whereas a female has two X chromosomes. The Y chromosome is the big factor in this system that helps determine whether the offspring is genetically male or female. The X-O System applies for many insects. Unlike the previous system, the Y chromosome is not the determining factor. The number of X chromosome is considered to be the factor because females have two X chromosomes and males have one X chromosome. In the Z-W System the male is Z-Z and the female is Z-W. Notice that in this system the males are the ones with two similar chromosomes and the females have two different ones. In the Haploidiploid system, the males are genetically haploid and the females are diploid.  In fruit flies, there was a test conducted to see which sex chromosome was the determining factor to whether the offspring's had white eyes or red eyes. Scientists let the flies reproduced until the second generation. They saw that the females were the key factor that gave the flies white eyes where as the males carried a Recessive gene which led to white eyes if expressed. 


     Why were are there oversimplifications in Mendel's model result? Well one way to look at it is that he used a very simple species or that the world is never perfect. The fact is that there is never a perfect cross between Dominant and Recessive alleles. There is a possibility where there is codominance (Pattern that occurs when the heterozygote expresses both alleles simultaneously) or incomplete dominance  (Pattern that occurs when the heterozygote has a phenotype intermediate to the phenotypes of the homozygotes). There are other possibilites such as sex-influenced inheritance (Pattern that occurs when an allele is dominant in one sex and recessive in the other), X-influenced inheritance ( Pattern of traits determined by genes that display a dominant/recessive relationship and are located on the X chromosome). There are always changes that occur and it will never be fully accurate either. It is possible that other factors that may play in our life that could possibly change the way our genes are expressed. For example, there are many environmental factors out there that could create a change in gene expression. 


     Probability in genetic crosses is determined by multiplying factor that are already given to you. An example of finding these factors is the usage of the a Punnett Sqaure. By multiplying the chances of occurrence and adding them up together gives you an idea whether it may or may not occur. It can be done simply by adding up all the multiplication factors and seeing which gene (in this case) is more likely to be expressed. These numbers only give probability and give only the percent chance of the that even to occur within your body.  

B. Useful Materials

Useful Video

This is a video that describes the basic principles behind Mendelian Genetics. It gives a brief history about Gregor Mendel and his experiments that he carried. I did find some funny videos regarding this topic...but they contained no useful information!

Useful Image

This is an example of a pedigree analysis. In this specific chart, scientists are tracking down "tasters" and "nontasters. In this case you can see that this trait is dominant because of the way the people who have it are shaded. It can be traced all the way back to  the mother in the P generation. 

Useful Article

Homozygosity mapping on a single patient-identification of homozygous regions of recent common ancestry by using population data

This is an interesting article on how scientist had trouble determining if someone was homozygous by decent or not. They use a series of algorithms that seem to bey very affective in getting the job done. They have also developed a simulation method find the probability of being homozygous by decent a way to determine connections to disease for a homozygous area by viewing the intact regions in that specific population. Pretty cool!


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