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This question is from "Concepts of Genetics," Klug & Cummings, 10e.
"Thalassemia is an inherited anemic disorder in humans. Affected individuals exhibit either a minor anemia or a major anemia. Assuming that only a single gene pair and two alleles are involved in the inheritance of these conditions, is thalassemia a dominant or recessive disorder?"
I know from a quick Google search that the disorder is recessive, but how would I come to this conclusion based on the question alone?
Edit: I've spoken to my professor, and he shared his opinion that is essentially the same as canadiener's answer. Thanks to everyone that replied!
Given that this is a text book question about classical genetics, you can safely disregard any reality about the disorder.
I'd argue that the two alleles show incomplete dominance. The mild anemia, which is intermediate between healthy individuals and those with severe anemia, can be attributed to heterozygous individuals. In this case, the expression of the functional allele is not enough to compensate for the nonfunctional allele, producing the mutant phenotype.
Protein S deficiency
Protein S deficiency is usually hereditary, but may be acquired. The hereditary form of protein S deficiency is caused by a mutation in a gene called PROS1. This condition is inherited in an autosomal dominant manner, which means that an individual who inherits only one mutated copy of PROS1 has an increased chance of developing symptoms of this disease. Individuals who inherit one mutated copy of the PROS1 gene are said to be heterozygotes while those that inherit two mutated copies of the PROS1 gene are called homozygotes. Blood clots (thrombosis) may occur in both heterozygotes and homozygotes however, homozygotes may develop a severe form of thrombosis called purpura fulminans. Purpura fulminans involves severe clotting throughout much of the body and is a life-threatening condition.
Rarely, protein S deficiency is acquired, meaning it develops as a result of another condition such as liver disease, kidney disease or vitamin K deficiency. 
The first symptoms of Tay-Sachs disease may appear from infancy to adulthood, depending on how much beta-hexosaminidase A enzyme activity a person has (if any).
In the most common form, the infantile form, infants have no enzyme activity, or an extremely low level (less than 0.1%). They typically appear healthy in the newborn period, but develop symptoms within 3 to 6 months of age. The first symptom may be an exaggerated startle response to noise. Infants with this form begin to lose milestones such as rolling and sitting (regression) and develop muscle weakness, which gradually leads to paralysis. They also lose mental functions and become increasingly unresponsive to their surroundings. By 12 months of age, they begin to deteriorate more rapidly, developing blindness, seizures that are hard to treat, and difficulty swallowing. Infants with this form of Tay-Sachs disease typically do not survive past 4 years of age. The most common cause of death is complications from lung inflammation (bronchopneumonia).   
The juvenile form is less common and is characterized by having very little enzyme activity, typically less than 1% of normal activity. Depending on exactly how much activity there is, symptoms may begin any time during childhood, most commonly between ages 2 and 5. Children with this form often develop frequent infections, behavioral problems, and have more slowly progressive loss of movement control, speech, and mental function. They may also begin to have seizures and lose their vision. Children with the juvenile form often spend several years having no responsiveness or awareness before passing away in late childhood or adolescence. Infection is a common cause of death.   
The late onset form, sometimes called the adult or chronic form, is also less common and is characterized by having less than 10% of normal enzyme activity. Symptoms and severity vary more among people with this form. Symptoms may begin in childhood to adulthood, but the disease is often not diagnosed until adolescence or adulthood. Neurological impairment is slowly progressive and may lead to clumsiness and loss of coordination, muscle weakness, tremors, difficulty speaking or swallowing, and uncontrollable muscle spasms and movements. Many people eventually need mobility assistance. In some people with this form, the first obvious symptom is a severe psychiatric disorder such as schizophrenia. Impaired intellect or dementia may or may not develop. Some people with the late onset form have a shortened lifespan due to the disease, while others do not.   
This table lists symptoms that people with this disease may have. For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed. This information comes from a database called the Human Phenotype Ontology (HPO) . The HPO collects information on symptoms that have been described in medical resources. The HPO is updated regularly. Use the HPO ID to access more in-depth information about a symptom.
Part 4: Pedigree Analysis
We will trace the inheritance pattern of the autosomal recessive trait albinism through four generations. The legend is as follows:
In the pedigree chart below determine the genotypes of each individual. Use a Punnet Square analysis to help you. Remember that the genotype of affected individuals is nn. If you cannot determine both gene pairs of a normal individual, indicate the genotype as N_. Put the genotype next to each symbol.
The Paw Print Genetics Blog
The field of genetics has progressed rapidly in recent years. Perhaps you&rsquove seen headlines about these top genetic topics in 2013. These stories show the importance of genetics and how it affects us as individuals and as a society. To understand the impact, though, one may need a review of Genetics 101: dominant vs. recessive disease traits.
In order for our bodies to work properly, our DNA must be coded in specific sequences. DNA sequences are grouped into units called genes, which tell our bodies what to make to build cells and metabolize nutrients. We are all a unique combination of re-shuffled genes from previous generations. Everything from eye, hair and skin color, muscle, bone, etc. is coded by genes. A mutation in a gene usually causes something to change and many of these changes can lead to disease. There are thousands of genes, and in humans, thousands of genetic disorders that result from mutations.
One way to classify genetic disorders is to group them by how they are inherited. With the exception of the sex chromosomes, X and Y, each of us has two copies of our genes. One comes from our mother, the other from our father. Recessive conditions need both copies of the same gene to have a mutation in order to show symptoms. Those who have only one copy of a mutation of a recessive condition are called carriers. Carriers have no symptoms, and the disease does not affect their health, but, it can affect the health of future generations. If a breeding pair are both carriers of the same genetic disorder, they have a 25% chance of having offspring with that disease, 50% chance of having carrier offspring (no symptoms), and 25% of having offspring that are not carriers or affected.
Dominant disorders work differently than recessive conditions. It only takes one copy of the gene with the mutation to have the disease. With dominant disorders, there is a 50% chance for the offspring to be affected like the parent. Sometimes, there is a new dominant mutation in which the mutation was not inherited from a parent, but rather, new in that individual. Both dominant and recessive disorders may show symptoms at birth, or they may not develop until later in life these are called &ldquoadult onset&rdquo.
The concepts of dominant and recessive can easily be applied to dog breeding. For example, polycystic kidney disease in Bull Terriers is caused by a dominant mutation in the PKD1 gene. Symptoms may be variable, and may not be diagnosed before breeding. Kidney cysts do not usually cause pain but may lead to kidney infections, frequent urination, poor eating and weight loss. In some dogs, the disease will progress and lead to kidney failure. A breeder may choose not to breed a dog at all with this dominant mutation in PKD1, knowing there is a 50% risk of passing this on, which may affect many pups in a single litter. Remember, it only takes one copy of this gene from either parental line for this disease to potentially show up in the next generation.
Degenerative myelopathy is a progressive neurological disorder found in dozens of breeds of dogs that is inherited in a recessive manner. It&rsquos very similar to Lou Gehrig&rsquos disease in people. Degenerative myelopathy may be misdiagnosed as arthritis in the early stages. Dogs eventually become unable to walk. An affected dog may not be diagnosed before breeding because of the later onset of symptoms. Because this is a recessive condition, it requires two copies of the mutated gene to be affected. Dogs with one copy of the mutation have no symptoms. It is important to identify carriers when breeding. Owners may not want to breed two carriers of degenerative myelopathy together, because there is a 25% chance of pups being affected. It is possible that several pups in a single litter could be affected. Although they may choose to breed a carrier with a non-carrier, because there is not an increased risk of having affected pups, this will keep the disease mutation in the breeding lines, which may be undesirable.
Genetic testing technology is increasing at an exponential rate. Understanding some basic genetic concepts may help owners breed &ldquosmarter&rdquo with the potential of healthier dogs. A healthier dog means a better quality of life for the dog and the owner.
How to determine if this blood disorder is recessive or dominant? - Biology
(2017 October 17). What kinds of gene mutations are possible? Genetics Home Reference. Available online at https://ghr.nlm.nih.gov/primer#mutationsanddisorders. Accessed October 2018.
(2017 October 17). What are the different ways in which a genetic condition can be inherited? Genetics Home Reference. Available online at https://ghr.nlm.nih.gov/primer/inheritance/inheritancepatterns. Accessed October 2018.
(2017 October 17). How do genes control the growth and division of cells? Genetics Home Reference. Available online at https://ghr.nlm.nih.gov/primer/howgeneswork/genesanddivision. Accessed October 2018.
(2017 October 17). What are single nucleotide polymorphisms (SNPs)? Genetics Home Reference. Available online at https://ghr.nlm.nih.gov/primer/genomicresearch/snp. Accessed October 2018.
Wang QJ, Rao SQ, Zhao YL, Liu QJ, Zong L, Han MK, Han DY, Yang WY. The large Chinese family with Y-linked hearing loss revisited: clinical investigation. Acta Otolaryngol. 2009 Jun129(6):638-43.
Illustration demonstrating how genes are passed down from parents to children. In this example, two unaffected parents each carry one copy of a gene mutation for an autosomal recessive disorder. They have one affected child and three unaffected children, two of which carry one copy of the gene mutation.
Two copies of an abnormal gene—one from each parent—must be present for an autosomal recessive disease to develop. Typically, both parents of an affected child carry one abnormal gene and are unaffected by the disease because the normal gene on the other chromosome continues to function. In this case, each child has a 25 percent, or one in four, chance of being affected by the disease. Each child also has a 50 percent chance of inheriting one copy of the mutated gene. People who inherit one abnormal gene copy will not develop the disease, but they can pass the mutation on to their children.
In the example, two unaffected parents each carry one copy of a gene mutation for an autosomal recessive disorder. They have one affected child and three unaffected children, two of which carry one copy of the gene mutation.
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When we were having our first child, we went to a person called a genetic counselor to make sure our child wasn't going to have any problems. Basically, she just took a sample of the blood and tissue from the child and tested it for different possible disorders. I don't know everything they check for, but I know they look for things like Down Syndrome. Fortunately, we had no problems.
Depending on how you look at the situation, it can be very helpful or stressful. If you find out that your child has no disorders and should be healthy, it is a relief. I can only imagine if the doctor came back and said that there was something wrong. It would put the parents in a tight spot about how they wanted to handle everything.
We didn't know about it at the time, but you can ever get testing before you plan to have a child. We did this for the second child. The doctor can take samples from both parents and give the probability of different situations. She described Punnett squares and how the different traits worked. Genetics are even advanced enough that once the child is developing they can tell you what its different features will look like. We chose to be surprised, though. matthewc23 October 17, 2011
@kentuckycat - You stumbled onto the same problems that it took scientists a long time to figure out. We still don't have all the answers.
As for the big toe issue, the answer is not necessarily. If a parent had a large toe, it means they either have two dominant alleles or a mix. If the other parent has a shorter toe, they have two recessive. Each child will have a 50% chance of having either kind of toe. Figuring it out is fairly simple. Look up Punnett tables for the explanation.
It is possible for different traits to be expressed using more than one gene. I believe eye and hair color are actually examples of this. You can even have situations where neither gene is dominant. The classic example is flower color. The other option is blood type where two alleles are dominant (A and B blood) and one is recessive (O). Genetics can be really interesting but really complicated depending how in depth you get.
@matthewc23 - Very interesting. I never really noticed it, but I guess I would have to have the recessive alleles. That got me thinking, though, if the dominant allele outcompetes the recessive, then if even one of your parents had the dominant genes where their big toe was larger, you would have to have it too, right?
I was wondering, too, is it ever possible for more than one gene to control a certain trait? How would you be able to predict what someone's children would look like in a situation like that if it is possible? And a final thing, what about something like eye or hair color where there is more than two different outcomes? matthewc23 October 15, 2011
I don't know if it is necessarily fair to say that dominant genes are expressed and recessive ones are not. In situations where an organism has two recessive alleles, the recessive trait is expressed in the phenotype.
As a matter of fact, there are quite a few traits in humans and other plants and animals where the recessive alleles are much more common, and that is the "normal" phenotype.
The one I specifically remember from my biology class for some reason is the length of your big toe. If you have one of the dominant alleles, your big toe is noticeably longer than your next toe. If you have two of the recessive alleles, your big toe is either about the same size or smaller than the one next to it. The recessive alleles are much more common, so most people have a smaller big toe.
Difference Between Dominant and Recessive
Genetics is the science of heredity, genes, and the differences in living organisms. It is a biological discipline that deals with the structure and function of genes, their behavior and patterns of inheritance from the parents to the offspring.
In the case of humans, as each individual is formed by the union of the egg and sperm cells of both parents, a diploid cell containing the necessary genetic materials to create him is developed. This genetic material is composed of chromosomes with individual genes or alleles containing specific traits. For each trait, an individual acquires two copies of genes or alleles, one from the mother and the other from the father.
If the two alleles from both parents are similar, the offspring is homozygous, and if they are different, the offspring is heterozygous in which case the stronger of the two will show up in the offspring while the weaker one is masked.
The allele or gene that shows up is called “dominant,” and the allele that is masked is called “recessive.” Recessive alleles or genes will only show up if the offspring inherits recessive copies of the trait from both parents.
Dominant genes are usually the ones that are oftentimes observed in an offspring and passed down to subsequent generations while recessive genes will only show for a few generations and eventually disappear. Dominant genes are represented by capital letters and recessive genes are represented by small letters. There are three combinations of genotypes or alleles: AA (receives dominant traits from both parents), Aa (receives a dominant trait from one parent and a recessive trait from the other), and aa (receives recessive traits from both parents).
Eye color is one example wherein dominant genes mask recessive genes. If one parent has brown eyes and the other blue, brown is the dominant color and blue is the recessive.
“AA” would mean that the offspring will have brown eyes receiving dominant genes from both parents “Aa,” that he will have brown eyes with the recessive gene being masked by the dominant and “aa,” that he will have blue eyes since he gets recessive genes from both parents.
In the case of an individual’s hair type, if both parents have straight hair, the offspring will surely have straight hair. If one parent has curly hair and the other straight hair, either the offspring gets a straight, curly, or wavy hair depending on the genetic makeup of the alleles of both parents.
1.An individual receives two copies of each trait that he inherits from his parents, one from the mother and one from the father, with one of them dominant and the other recessive.
2.A dominant gene is one which is strong while a recessive gene is one which is weak.
3.A dominant gene will show up in the trait while a recessive gene, although still present, is masked or hidden by the dominant gene.
4.A recessive gene will only show up if the offspring inherits recessive genes from both parents.
5.Dominant genes are most likely to be passed down to future generations while recessive genes will slowly disappear.
Genes are sequences of DNA that code for a particular trait. Different versions of a gene are called alleles—sometimes alleles can be classified as dominant or recessive. A dominant allele always results in the dominant phenotype. In order to exhibit a recessive phenotype, an individual must be homozygous for the recessive allele. Genes affect both physical and psychological characteristics. Ultimately, how and when a gene is expressed, and what the outcome will be—in terms of both physical and psychological characteristics—is a function of the interaction between our genes and our environments.