Not saying I am for against the original post, but I do have some background for genetics. I had to take a course in genetics to finish my doctorate. When you think about human genetics the number of offspring is kept to a minimum but when you try to start analyzing phenotype and genotype in dog offspring the numbers get way out of hand since the dames may have several litters from different sires. So good luck on trying to figure out the actual way to determine the result of a chocolate. Here is a basic description on inheritance patterns in humans:
Unit 1-Inheritance Patterns
Autosomal Dominant
The autosomal dominant inheritance pattern will reside on one of the 22 pairs of chromosomes known as autosomes. It may manifest itself as a trait, abnormality, or genetic disease/syndrome. In this pattern the dominant gene will express itself regardless of the other parent’s contributed gene. If the dominant trait resided in the father and a non-dominate trait in the mother, then each child would have a 50 percent chance of inheriting the dominant trait or characteristic. For example, if the parents had four offspring, they statistically would produce two children with the dominant trait and two children without the trait. However, it should be kept in mind that the dominant gene may not express itself and this is known as nonpenetrance. This can be seen when the dominant trait is present in the grandparent and grandchild, but the dominant trait has passed over the parent’s generation.
Autosomal Recessive
The autosomal recessive inheritance pattern also resides on one of the 22 pairs of chromosomes known as autosomes. It may manifest itself as a trait, abnormality, or genetic disease/syndrome. However, with a recessive pattern the trait will only manifest itself if two recessive genes are contributed from the parents; otherwise, the child may become a carrier of the recessive trait but not manifest the trait. If the recessive trait resided both in the father and mother, then there would be a 25 percent chance of a child inheriting and manifesting the recessive trait or characteristic. For example, if the parents produce four offspring, they statistically would produce one child who will manifest the recessive trait, one child who does not manifest the trait nor carry the trait, and two that do not manifest the trait but are carriers. It should be kept in mind that the autosomal recessive inheritance pattern is purely random and it is difficult to determine what traits may or may not appear in future offspring in reference to existing offspring.
X-Linked Dominant
Unlike autosomal inheritance, X-linked inheritance patterns are found on the sex chromosomes. They are known as sex chromosomes because they will determine the sex of the offspring. Each person receives one pair of sex chromosomes: a male will exhibit one X and one Y and a female will exhibit two X chromosomes. The Y chromosome will have very few genes while the X chromosome will have many genes. A male will most likely present all the traits on the X chromosome, whether recessive or dominant, since there are no similar genes on the Y chromosome. This is because the male presents only one X chromosome. A female has two X chromosomes which results in dominance and recessive structure coming into play. If father exhibits the X-linked gene trait, then all of the daughters will exhibit the trait but none of the sons since they receive the Y chromosome from their father. If the mother carries the X-linked gene trait, then half of all the children will exhibit the X-linked gene trait. For example, if the parents produce four offspring with the mother as the carrier of the X-linked gene trait, they statistically would produce one girl and one boy with the trait and one girl and one boy with out the trait. However, if the father is the carrier of the X-linked gene trait, they statistically would produce two girls with the trait and two boys without the trait. It should be kept in mind that since the female presents two X chromosome the degree that the trait is exhibited may be reduced or counteracted by the other X chromosome.
X-Linked Recessive
X-Linked recessive inheritance patterns are also found on the sex chromosome. As discussed above, the X chromosome will have many genes while the Y has very few which results in the male exhibiting all the traits on the X chromosome, regardless of recessive or dominant, because there are no corresponding traits on the Y chromosome. If the mother is the carrier of the recessive X-lined trait, there is a 25 percent chance of a boy who exhibits the trait, 25 percent chance of a boy without the trait, 25 percent chance of a girl without the trait, and a 25 percent chance of a girl carrying the trait. If the father exhibits the recessive X-link trait and the mother is not a carrier, then there is a 50 percent chance of a boy without the trait and a 50 percent chance of girl carrying the trait. The recessive X-linked inheritance pattern is most often thought that the trait will only occur in males; however, this is not true. It is extremely rare but a recessive X-linked trait would have to be passed on the X chromosome from both parents, since the female has two X chromosome. This might happen in the following ways: the mother is a carrier and the father exhibits the trait, then there is a 25 percent chance of a boy without the trait, 25 percent chance of a boy who exhibits the trait, 25 percent chance of a girl carrying the trait, and a 25 percent chance the girl exhibits the trait. Another extremely rare circumstance would be if the mother exhibits the trait as well as the father, then there would be a 100 percent chance of the child exhibiting the trait regardless of sex.
Mitochondrial Inheritance
Mitochondrial inheritance pattern are traits that passed through the maternal inheritance because mitochondria come from the female’s egg at conception. It is believed that there is very little contribution from paternal inheritance. The Mitochondria are found in the majority of the body’s cells and are responsible for turning chemicals in foods into an energy source for the cells known as adenosine triphophosphate (ATP). Mitochondria contain their own form of DNA and due to mutations in the mitochondrial DNA this will result in a hereditary disorder that can be passed to either sex. A disease may result from mitochondrial mutations in the presence of certain environmental factors or a specific nuclear genotype. Mitochondrial DNA pathologies such as mutations, deletions, or depletions may result from autosomal dominant or recessive traits or from the use of the medications like AZT. Mitochondrial mutations that are essentially identical do not necessarily results in the same phenotypes. A mitochondrial disorder may not necessarily be evident at birth, but can manifest itself at any age and also may present a wide diversity of symptoms and traits. The different disorders may be exhibited as: metabolic disturbances, developmental delay, blindness, hearing loss, short stature, and gastrointestinal dysfunction.
REFERENCES
Information on Hearing Loss: Genetics and Deafness. Boys Town National Research Hospital. 6 May 2005.
Welcome to the Boys Town National Research Hospital.
Kathleen’s World. Mitochondrial Diseases: What They Are, How to Learn More. 11 June 2004. Kathleen’s World. 6 May 2005.
Mitochondrial Disease & Disorders in children.
MedlinePlus. Medical Encyclopedia: Genetics. 19 August 2003. U.S. National Library of Medicine and the National Institutes of Health. 6 May 2005.
MedlinePlus Medical Encyclopedia: Genetics.
Toriello, H.V., Reardon, R., & Gorlin, R.J. (2004). Hereditary Hearing Loss and Its Syndromes (Second Edition). New York, NY: Oxford University Press.
Ok, now wakeup everyone!!!
