¶ … roles do genes play in determining cell structure and function? How is gene expression regulated? Genes are composed of sequences of DNA which pass on the organism's genetic blueprint through the process of replication. "By serving as the blueprints of proteins in the body, genes ultimately influence all aspects of body structure and function... An error in one of these genes can lead to a recognizable genetic disease" (McCance & Huether 2012: 126).

What is the role of the environment in development of congenital disorders?

Genes have a considerable influence upon the probability of individuals developing particular disorders. For example, not every smoker develops lung cancer; not every overweight person develops type II diabetes. However, genes determine the likelihood that individuals will develop such disorders. The environment can trigger the expression of certain genes or support the conditions that make the manifestation of such conditions more or less likely to occur. In the case of lung cancer "most smokers do not develop lung cancer" but smoking can significantly increase the chances of developing the disorder, as can exposure to certain environmental toxins. "Some studies have suggested that mutations in a gene called FHIT may make some individuals more sensitive to the carcinogenic effects of tobacco smoke" (McCance & Huether 2012: 165). There is also a correlation between parent's blood pressure and the blood pressure of their children, yet scientists believe this is due to a combination of lifestyle factors, not genetics alone (McCance & Huether 2012: 165).

Q10. How might 1) over-expression of proto-oncogenes and 2) under-expression of tumor suppressor genes lead to abnormal cellular proliferation?

Proto-oncogenes regulate cell growth and differentiation, while tumor suppressor genes suppress the growth of tumors. If a proto-oncogene is damaged or has a chromosomal abnormality this could result in its transformation to an oncogene, or cancer cell. With under-expressed tumor suppressor genes, the likelihood of uncontrolled tumor growth is high.

Q11. Describe a teaching strategy to explain DNA replication and protein synthesis

Think of the double helix DNA molecule as a series of different jigsaw pieces stuck together -- the DNA is detached from its corresponding, opposing sides during the process of replication. Each half of the molecule binds to another 'loose' complementary parental half to make two new DNA molecules. This means that each parent contributes half of the offspring's genetic material to make a new organism (DNA replication, 2002, VBS homepage).

12. You have been asked to counsel a family in which the father has Neurofibromatosis. Explain why autosomal dominant disease penetrance can vary from one family to another.

"Neurofibromatosis can either be an inherited disorder or the product of a gene mutation" (Learning about Neurofibromatosis, 2010, National Human Genome Research Institute).If one parent has the affected NF1 gene on chromosome 17 or the affected NF2 gene on chromosome 22 there is a 50/50 chance that the child will develop NF. "The only difference between the child and the parent in these circumstances is the severity of NF and the appearance of symptoms. The presence of only one changed or affected gene can cause the disorder to appear. However, the action of the unaffected gene that is paired with the dominant gene does not prevent the disorder from appearing" (Learning about Neurofibromatosis, 2010, National Human Genome Research Institute).

This means that although only one parent with the disorder is necessary for the full expression of the disease in the offspring, "people with NF can make two different kinds of reproductive cells: one that can cause a child to have NF and the other that will produce an unaffected child, if that is the gene that happens to be used. When an unaffected individual conceives a child with a person with NF, there are four possible cell combinations - two combinations that will yield a child with NF and the other two that will yield an unaffected child" (Learning about Neurofibromatosis, 2010, National Human Genome Research Institute). Thus,...

Cri du chat syndrome
Cri du chat syndrome is caused by the deletion of genetic material on the p-arm of a chromosome. Affected infants make a "high-pitched cat-like cry" which gives the syndrome its name (Cri du chat syndrome, 2010, National Human Genome Research Institute). Other symptoms include "mental retardation, delayed development, distinctive facial features, small head size (microcephaly), widely-spaced eyes (hypertelorism), low birth weight and weak muscle tone (hypotonia) in infancy" (Cri du chat syndrome, 2010, National Human Genome Research Institute).

The reason for the chromosomal deletion is unknown. "The chromosomal deletion usually occurs as a random event during the formation of reproductive cells (eggs or sperm) or in early fetal development. People with cri du chat typically have no history of the condition in their family" (Cri du chat syndrome, 2010, National Human Genome Research Institute). However, in 10% of all cases, genetics may play a factor, when the developing fetus inherits a segment of chromosome with the deletion from an asymptomatic parent. "In these cases, the parent carries a chromosomal rearrangement called a balanced translocation, in which no genetic material is gained or lost. Balanced translocations usually do not cause any medical problems; however, they can become unbalanced as they are passed to the next generation. A deletion in the short arm of chromosome 5 is an example of an unbalanced translocation, which is a chromosomal rearrangement with extra or missing genetic material" Cri du chat syndrome, 2010, National Human Genome Research Institute).

Learning verbal skills at a normal rate is a particular challenge for children with the syndrome. Only half of all individuals with the syndrome learn to communicate through speech. "Individuals learn to use short sentences, while others express themselves with a few basic words, gestures, or sign language (Cri du chat syndrome, 2010, National Human Genome Research Institute). Although some children have physical problems related to the disorder, including feeding difficulties, scoliosis and organ defects, the overall life expectancy of individuals with the syndrome is normal. The 'catlike' cry also tends to become less pronounced with age (Cri du chat syndrome, 2010, National Human Genome Research Institute).

Children with the syndrome require additional physical and academic support. Diagnosis is usually made based upon the 'cry' and can also be determined through chromosomal analysis. A recent study on of 91 patients from the Italian Registry fount that "half of the patients walk by themselves at three years old and that all learn to walk later; with regard to the language, 25% of the children are able to make short sentences at 4.5 years old, 50% at 5.5 and almost all the children make short sentences before the age of 10; 50% of the patients feed themselves with a spoon at 3.5 years old and dress at 5" (Mainardi 2006). Hyperactivity, "excessive attachment to objects" and self-injury were problems in approximately 50% of all patients in another study (Mainardi 2006).

Q14. Multifactorial inheritance

Twin studies are often used to examine the extent to which nature and nurture can contribute to the expression of diseases. On one hand, there is a clear link between genetics and the likelihood of developing certain conditions, such as schizophrenia. On the other hand, not every twin that develops a disease such as schizophrenia (or breast cancer, colon cancer, or diabetes) has a twin that develops the disorder. This indicates that environmental factors can also have an impact upon the expression of the disorder.

Because of the impact of the environment on gene expression, however, multifactorial inheritance can be very difficult to determine unless one uses twin studies, or individuals with the same genetic material. Multifactorial inheritance means that several, rather than simply one gene causes a particular trait to be expressed…