DNA Sketch Model

As we work further into the human body and system, we must work to understand one of the most key components to human function: DNA structure and function. To do this, we were given instruction about the basic components to DNA and asked to fit them together in a sketch that would demonstrate how it fit together to form a strand of DNA. The pictures included below are the "finished product" of this activity.

As you can see in the last picture, a nucleotide is shown. A nucleotide is the basic component of DNA, composed of dioxyribose, phosphate, and one Guanine, Cytosine, Adenine, or Thymine. Notice the "or" in that list. A nucleotide only contains one of the four listed, and can be found int the acronym GCAT. These then form a hydrogen bond with the opposite strand; the only possible combinations of pairings are Guanine with Cytosine, and Adenine with Thymine. Once the strands are bonded, they align themselves in a double helix shape, forming the common DNA perception. There you have it: the basics of DNA!

Pedigrees and Genetic Counseling: In Sickness and Health

For this post, we are going to create a family pedigree for a couple who wishes to have children. We will evaluate the pedigree to predict the risk of their offspring having Hemophilia or Myotonic Dystrophy. We are also going to evaluate this situation to further understand the reasoning behind the risk prediction.

Shown below are the letters that Greg and Olga sent in regarding their families.

To set a basic understanding for this pedigree, I am going to define a few symbols shown. Each circle on the pedigree represents a female family member, while each square represents a male family member. Shapes completely filled in with purple represents family members affected by Hemophilia, while shapes filled in completely with blue represent family members affected by Myotonic Dystrophy. Shapes that are not colored in represent family members who are unaffected by either disorder.  Shapes that have a large X threw them represent deceased family members. The pedigree outlined in red represents Greg's side of the family while the pedigree outlined in green represents Olga's family.

Myotonic Dystrophy is classified as an"autosomal dominant disorder", meaning that a disease can be inherited through only one parent, if the gene is dominant. This means that the disorder is passed down directly, without skipping generations. It also means that if the disorder is not inherited by a child, it can no longer be passed down through that child. Because  Myotonic Dystrophy is autosomal dominant disorder, and the gene is not present in either Greg or Olga, they are not carriers of the gene and cannot pass the disorder down to their offspring.

When viewing the pedigree, on might wonder about the possibility of Greg's aunt or uncle being homogeneous for the Myotonic Dystrophy gene. Being that only one parent carried the gene, only one abnormal gene was inherited, causing the aunt and uncle to be heterogeneous for the Myotonic Dystrophy gene. You might also question how it can be determined that Greg's cousin does not have Myotonic Dystrophy, being that symptoms sometimes don't appear until after the age of fifty. Being that neither of the cousin's parents were affected by this gene, it can be determined that it would be impossible for the cousin to have Myotonic Dystrophy.

Now, let's discuss the matter of Hemophilia inheritance. As discussed, Myotonic Dystrophy is classified as an "autosomal dominant disorder", but what about Hemophilia?

Another type of classification of gene inheritance is that of an "autosomal recessive disorder", meaning that two abnormal genes are required in order to be affected by the disorder. If a person only inherits one abnormal gene, they become carriers of the disorder, however, they are not affected. This is the type of disorder that has the ability to skip generations. Is Hemophilia an autosomal recessive disorder?

It is important that we discuss another concept, that of consanguinity. Consanguinity is simply where two individuals from the same biological background mate. Consanguinity increases the probability of an individual involved to become a carrier of an autosomal recessive disorder. Again, this recessive trait can skip generations until it has the opportunity to create offspring that is homogeneous with the autosomal recessive disorder gene.

 When tracing the pattern of inheritance of the Hemophilia disorder, we begin to see a pattern. It becomes clear that only male individuals within the family inherited this disorder.  Hemophilia is a sex-linked disorder, indicating that Hemophilia is not an autosomal recessive disorder.

Sex-linked disorders are often classified in X-linked recessive inheritance. This means that specific disorders, such as Hemophilia, are attached to the X chromosome. Due to the fact that males do not pass down the X chromosome directly to their male offspring, the X-linked recessive inheritance is passed down through the female offspring, then on to the male grandchildren. Males do not receive the X chromosome from their father because the father passes down the Y chromosome down to the son while the mother passes down the X chromosome. The fathers can, however, pass down their X chromosome to their female offspring. This causes men to more likely be affected by diseases passed down through X-linked recessive inheritance. In order for females to display a sex-linked trait such as Hemophilia, she must inherit a defective X chromosome from both parents.

Why don't we apply the concept of carrying an  X-linked recessive gene to the pedigree of Greg and Olga? If we evaluate the family's history, we can see that Olga's mother, grandmother, and cousin were most definitely carriers of Hemophilia, while there is only a possibility of Olga being a carrier. If we evaluate Greg's family, we can see that Greg's mother, grandmother, and great-grandmother were definitely carriers while there is only the possibility of Greg's aunt and cousin being carriers. Greg is unaffected. Being that Olga has the possibility of being a carrier, there is a 50% chance that this trait would be passed down to her offspring. If the trait was passed down to a son, the son would be affected by the disorder. If the trait was passed down to a daughter, the daughter would only be a carrier. Again, there is only a 50% chance of Olga being a carrier, which means that there is the possibility that the trait will not be passed down to the offspring under any circumstance.

Human Chromosomes and Genetic Diseases

In this post, I am going to discuss how genetic diseases are inherited through human chromosomes while investigating some specific diseases. Before we begin, it is important to understand the basics of chromosomes. We have already discussed chromosomes frequently in previous posts, however, we have not yet labeled the specific components as of yet, which is very helpful in understanding genetics.

So how are genetic diseases actually linked to chromosomes? Do better understand this, we are going to look at a specific chromosome (such as the X chromosome shown above) and investigate some diseases attached. 

To begin with, we are going to research Rett Syndrome. This disease is attached to the X chromosome through the mutated gene MeCP2.  It is most common for this mutation to occur on X chromosomes transferred by sperm cells. Rett syndrome is a progressive neurodevelopmental disorder that affect females and very rarely affect males. A possible explanation as to why this disease is rare in males is that males do not inherit the X chromosome from their father, as girls do. As previously stated, most mutated MeCP2 genes are attached to the X chromosome inherited from the father. Rett syndrome occurs in about 1 of 10,000 births and is a common cause of mental impairment. This disease causes multiple symptoms in addition to mental impairment, such as the loss of hand usage ability, reduced muscle tone, seizures, intense eye gaze, and an "autistic-like" phase.    

Another disease we are going to look at is Alport Syndrome, which is also attached to the X chromosome. As MeCP2 was effected in Rett Syndrome, COL4A5 is mutated in order to develop Alport Syndrome. Alport Syndrome is a collagen-related disease affecting the kidneys, ears, and eyes.  Due the fact that the mutated gene associated with Alport Syndrome is passed along with the X chromosome, and the fact that women have two X chromosomes, affected women usually have one normal X chromosome, causing the effects to be much less drastic. This also means that since males only have on X chromosome, affected males will have more severe effects. Symptoms of Alport Syndrome include the breakdown of filters associated with glomerular basement membranes, causing the loss of proteins and red blood cells into the urine. Alport Syndrome also causes progressive deafness and issues regarding the lens of the eyes. 

Resources used for this post are listed below:

http://www.ncbi.nlm.nih.gov/books/NBK22188/

http://www.ncbi.nlm.nih.gov/books/NBK22265/

Investigating Independent Assortment

In this post, we are going to discuss independent assortment. To give you a basic understanding, independent assortment is where individual heredity factors assort themselves independently during gamete production. This gives different traits equal opportunity of occurring together.

To begin, we are going to determine which allele combinations are possible in two or three trait crosses. To accomplish this, I used http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html as a source. Based on the animation, we are able to determine that if the dipoid cell aligns its chromosomes in all ways possible,  the result will consist of equal amounts of sY, sy, Sy, and SY genotypes. We are also able to determine that the alleles of the S and Y genes assort themselves independently.

Next, we are going to answer some questions surrounding the topic of independent assortment.  To do this, we are going to use http://www.biology.arizona.edu/mendelian_genetics/problem_sets/dihybrid_cross/dihybrid_cross.html as a source for information.

1. What type of gametes will be produced by a plant of genotype AaBb?

    A plant with the genotype AaBb will produce AB, Ab, aB, and ab gametes, due to the fact that if you go through all possibilities this is the only combination that can be achieved.

2. What type of gametes will be produced by a plant of genotype aabb?

A plant with the genotype aabb will produce ab gametes, due to the fact that there are no dominant genes present.

3. List all the genotypes you would find among the offspring of an AaBb x aabb test cross.

    A cross between the genotype AaBb and the genotype aabb will produce AaBb, Aabb, aaBb, and aabb genotypes. These are all the combinations possible in a cross between AaBb and aabb types.

4. What is the expected phenotypic ratio of the offspring of an AaBb x aabb test cross? Show the punnett square you would use for predicting the outcomes.

   A cross between the genotype AaBb and the genotype aabb will produce AaBb, Aabb, aaBb, and aabb genotypes. The punnett square used to determine these genotypes is displayed below.

The phenotype created would be shown through the dominant alleles in the combination.

5.What are the genotypic and phenotypic ratios from a AaBb x Aabb mating?

   A cross between the genotype AaBb and the genotype aabb will produce a phenotypic ratio of 1:1:1:1 because there is only one occurrence of each genotype. The ratio is also 1:1:1:1 for the genotypic ratio.

6. List all possible gametes from a trihybrid individual whose genotype is RrSsTt.
   A trihybrid individual with the genotype RrSsTt will produce the following gametes: RST, RSt, RsT, Rst, rST, rsT, rSt,  and rst.