Objectives Students will have opportunities to
1. Define dominance, recessive, trait, sex-linked trait.
2. Give an example of how multiple aLleles determine a human trait.
3. List the different types of genetic disorders. Give an explanation for each type of disorder.
4. Describe and give an example of each disorder.
Approximate time one-half week.
Any trait is the result of the combined action of genetic and environmental factors. Three main types of disorders are recognized.
This is caused by mutant genes. The mutation may be present on only one chromosome of a pair or on both chromosomes of the pair. In either case, the cause of the defect is a single major error in the genetic information. Single-gene disorders usually exhibit obvious and characteristic pedigree patterns.
In regards to traits such as tasting certain chemicals, the pattern of inheritance are as follows: (T) those traits or genes for them that are fully or almost fully expressed, phenotypically that is, are known as
—taster (T). The dominant gene has the same expression in the heterozygous or homozygous state. Those genes carried on the autosomal chromosomes are classed with autosomal dominant inheritance.
A trait that is transmitted as autosomal
is expressed only in persons who have the gene in a homozygous state; that is, a non-taster (tt).
Individuals with X-chromosome and no Y-chromosome, i.e. XO genotype . . . they are usually short, underdeveloped sterile females. Females with an XXX genotype often have limited fertility and may have slight intellectual impairment. An XXY chromosome combination results in KlinefeLter’s disease or syndrome. These males often are tall, sexually underdeveloped and may have sLight intellectual impairment.
This disease is caused by an autosomal dominant gene. This disease is characterized spasmodic movements and incoordination and progressive mental deterioration.
The most common genetic disorder among Caucasians is cystic fibrosis. Approximately, one in every twenty Americans of Northern European ancestry is heterozygous for this trait. This disorder is characterized by the excessive secretion of mucus, which accumulates and damages the lungs, digestive tract and other organs. With treatments such as the administration of pancreatic enzymes and the control of infections, the life span of people with such disease has been ex-tended into early adulthood.
Most recessive disorders can be traced to an allele for a recessive trait that codes for an ineffective or nonfunctional protein product. An allele is a gene which codes for a protein, a mutant allele codes for an abnormal protein. If half of the normal protein is all right, the disease is recessive.
This is an inherited disease that causes the blood to clot slowly or not at all. This is an example of a sex-linked recessive trait. Blood fails to clot normally because of abnormally low antihemophilic globulin factor VIII. The incidence of the trait is much higher in males than in females.
Due not to a single mistake in the genetic blueprint but to an excess or deficiency of the whole chromosome or chromosome segments, which upsets the normal balance of the genome. For example, the presence of a specific extra chromosome, say chromosome #21 produces a characteristic disorder, Down syndrome, even though all the genes on the extra chromosome may be quite normal. People with this disease may have various physical problems and some degree of mental retardation.
During Meiosis, chromosomes pairs usually separate. But in rare cases, a pair may remain joined. This failure of chromosomes to separate is known as non-disfunction. When this happens, body cells inherit either extra or fewer chromosomes than normal. It is important to note that the extra chromosome on the twenty-first pair producing the condition, is a perfectly normal chromosome and can only come from either the mother or the father.
This kind of inheritance is seen in a number of common disorders especially in developmental disorders resulting in congenital malformation. There is no major error in the genital
information but rather a combination of small variations that together can produce a serious defect. (Environmental factors may also be involved.) Multifactorial disorders tend to recur in families but do not show the characteristic pedigree patterns of single gene traits. These disorders arise from the action of at least several genes and their interaction with their environment. Heart disease, which is the leading cause of death in the United States today, has been linked with smoking, diets high in cholesterol and saturated animal fats, stress and lack of exercise. These environmental factors can result in elevated blood cholesterol levels in individuals with and without the trait, that results in the accumulation of very high levels of choLesterol in the blood.
This is a birth defect of the backbone often called ‘open spine’. Scientists believe that genetic factors and some environmental factors act together to cause spinal and reLated central nervous system defects.
1. Students will design a survey form and colLect data by observing about six students in the class demonstrating dominant or recessive traits for example, hair color, tongueroller, tasting.
In this experiment, the students will practice their skills in observing, comparing, relating, applying and recording by designing and drawing bar graphs of their data.
2. To predict the results of genetic crosses using probability by flipping a coin six times. Each flip represents one child; the six fLips represent the six children in a family. This activity will reflect the changes of a girl rather than a boy being born into a famiLy is 50:50.
Statistics will also come into play when students are allowed to manipulate the laboratory kit on Mendelian experiments—‘Chances and Choices’.
3. Invite speaker or speakers on genetic conditions relative to the genetic disorders and other abnormalities covered in class.
4. Show to students the video on ‘Anne’ from the Yale Gene Department. Also students can Listen to Tel-Med tapes of certain genetic disease from the hospital of St. Raphael.
Another video of interest for students to view is, ‘The book of Life’ from Tommy-K Video Store.
5. Call or write to your local March of Dimes for literature on hereditary diseases.