The most common chromosome abnormality, occuring in about one in 600 boys, is Klinefelter syndrome. Normaly people have 46 chromosomes with females having XX chromosomes and males have an XY. Guys with Klinefelter syndrome have a chromosome count of 47 XXY.

The features of Klinefelter are relatively mild. Boys with this disorder look normal, but as they reach puberty can have less facial hair, more breast development, and smaller testicles. They are almost always infertile. Boys with XXY are also more likely to have learning problems

The Androgen Insensitivity Syndrome (AIS), formerly testicular feminization syndrome, is due to an error in fetal genital development. An insensitivity to adrogens in genetic males leads to feminization or undermasculinization of the external genitalia, abnormal secondary sexual development in puberty, and infertility. Mostly, these individuals look and act like normal females, but on the inside have testicles, not a uterus with ovaries.

AIS is inherited as an X-linked recessive condition. So it is carrier by the mother and passed to half of her sons (who then would be her daughters?). It is rumored that Jamie Lee Curtis has this syndrome.

Mobius syndrome is a very rare disorder invovling lack of development of the 6th and 7th cranial nerves. This means people with mobius syndrome have facial paralysis. Symptoms include inability to squint or move the eyes (they have to move their head to see something), difficulty sucking on a bottle or eating, and no facial expressions (they can't even smile).

The cause of Mobius syndrome is unknown. It is not inherited. Although it occurs sporadically, there may be an underlying genetic cause.

One of the most common types of cancer in both men and women, and second largest cause of cancer deaths in the United States is Colorectal Cancer. Like most other typs of cancer, the majority of colorectal cancer is sporadic. But about 5-10% of colorectal cancer is hereditary.

One type of hereditary colorectal cancer is hereditary nonpolyposis colorectal cancer (HNPCC). There are two main DNA mismatch repair genes that can cause this syndrome - MSH2 and MLH1. If a person, male or female, inherits a mutation in one of these genes that individual has an 80% chance of getting colorectal cancer. There is also an increased risk for other cancers like, uterine and stomach.

Another very common type of genetic disease is Glucose-6-Phosphate Dehydropenase deficiency, or G6PD deficiency. It is a type of hemolytic anemia (blood cells dying) due to sensitivity to certain substances. Most people with G6PD deficiency live their lives unaware of thier condition, unless they are exposed to certain foods or medicines. G6PD deficiency is also called Favism because eating fava beans can trigger a reaction.

G6PD deficiency is an X-linked recessive trait. It is more common in people of African or Mediterranean descent (possibly due to a protective effect against malaria). Approximately 10 to 14% of the black American male population is affected

Turner syndrome is a genetic condition seen exclusively in females caused by complete or partial absence of one of her X chromosomes, also called 45 X .

Although almost all pregnancies with 45 X miscarry, about one in 3000 girls is born with Turner syndrome and have specific features. They usually have short stature, infertility and different physical features. Intelligence is not usually afected although they can have learning disabilities. They often have birth defects like heart or kidney problems. in carriers.

The most common genetic disease on the planet, for which about 5% of the world's population is a carrier, is Thalassemia. Thalassemia, anemia where not enough blood cells are produced, is inherited in a recessive fashion. There are different types, caused by either a defect in the alpha or beta hemoglobin genes.

Thalassemia is most common in people of Meditarranean, African, Asian, and SE Asian descent. As is the case with sickle cell anemia, thalassemia is thought to be more common in these ethinic groups because of the protection against malaria in carriers.

A genetic disorder that is near and dear to my heart, and my genes, is Red-Green Colorblindess. A defect in the cone pigments causes affected individuals to see red and/or green colors like grey or brown. Test out your color vision.

This type of colorblindness is inherited in an X-linked manner. The reason why I identify with this disorder is that I am an obligate carrier. That is because my father and cousin are both colorblind. Since I had to inherit my father's X chromosome (otherwise I'd be a boy if I got his Y), I must be a carrier. So statistically speaking, my future sons have a 50% of inheriting this disorder.

One in ten couples are unable to achieve pregnancy after a year of trying. Sometimes there is a genetic reason for infertility. One example is Kallman syndrome. The symptoms are relatively minor- the inability to smell (anosmia) and underdeveloped gonads (hypogonadism). Oddly enough, the part of the brain which processes smell, the olfactory bulb, sends out hormones in fetal life necessary for normal genetalia development.

Kallman syndrome is an X-linked genetic disorder. A male inherits this disease from his mother. Carrier females are generally normal, but may have some degree of anosmia.

The most common recessive genetic disease in the Caucasian population is Cystic Fibrosis (CF). It has also earned the nick name "65 roses" from many affected children. The disorder is mainly due to a build up of mucus in the lungs leading to chronic infections. There are other symptoms including difficulty digesting, infertility in males, and chronic repiratory problems. The average life span is about 30.

Because CF is an autosomal recessive manner, both parents have to be carriers in order for a child to inherit CF. About one in 25 white individuals are carriers. A simple blood test is available to check a person's carfier status and is offered in all pregnancies.

Another example of mitochondrial disease is the disorder called Mitochondrial Encephalopathy with Lactic Acidosis and Stroke (MELAS). Because mitochondria are used to make energy for the cell, when they stop functioning properly the most energy hungry systems in the body are affected worst. So places like, the brain, muscle, heart, and eyes. The most characteristic symptoms of MELAS syndrome are recurring, stroke-like episodes in which sudden headaches are followed by vomiting and seizures. Muscular weakness on one side of the body (hemiparesis) is typically present. Visual symptoms may include impaired vision or blindness. Like other mitochondial disorders, this disease is passed through the female line to all her children in a pattern called maternal inheritance.

Moving on to a new type of genetic disease, Leber's Hereditary Optic Neuropathy (LHON) is an example of mitochondrial inheritance. As you may remember from high school biology, mitochondria are structures in cells that provide energy and ATP. It is believed that way back, in the primordial ooze, mitochondria were seperate organisms that integrated in symbiosis with other multicellular organisms. This means that a mitochrondria has its own genes, seperate from the rest of the cell. Also, because no mitochondria from sperm end up joining an egg, a person gets all their mitochondrial genes only from mom. This is neither dominant, recessive, nor X-linked; this is called maternal inheritance.

LHON is mainly a disease of rapid vision loss that hits in the 20's to 30's. People with LHON have normal vision until suddenly, over a period of hours to weeks, they become nearly blind. It occurs more often in males. Although many people with a family history don't get LHON, no one really knows why the vision loss occurs, nor why it hits males more than females.

Tay-Sachs Disease. is devastating neurodegenerative disorder that is more common in people of Jewish, French Canadian, and Cajun descent. Babies with Tay-Sachs are completer normal at birth. Gradually, children lose motor skills and mental functions. Over time, the child becomes blind, deaf, mentally retarded, paralyzed and non responsive to the environment. Tay-Sachs children usually die by age five.

Tay Sachs, also called GM2 Gangliosidosis, is a lysosomal storage disease due to a deficiency in the Hexosamindase A enzyme. Basically, this means toxic byproducts build up in the lysosomes of cells, mainly in the brain and nerves. It is an autosomal recessive disease, and the carrier frequency among Ashkenazi (Eastern European) Jews is about one in 27.

There are more than 150 different types of Ectodermal Dysplasia. People with ED have abnormalities in ectodermal tissue such as the hair, teeth, nails, sweat glands, facial structure, digits and other parts of the body. They have unique facial features and sparse thin hair. They can have hearing loss.

The worst part about ED is defective sweat glands. Parents may first find out their child has ED when the child appears to develop a high fever in the summer - but they aren't sick. People with ED have to live in cold climates because they can't sweat.


The most common type is X-linked, so it runs through the maternal line and mostly males are affected. It can also be inherited in recessive and dominant forms.

About one in 2500 Caucasians suffers from alpha-1-antitrypsin deficiency. Alpha-1-antitrypsin is an enzyme that is made in the liver and helps protect the lungs from inflamation and infection. It is an autosomal recessive disease, so if both parents carry the Z allele their is a 1/4 chance for an affected child.

The symptoms in alpha 1 antitrypsin mimic those in people who smoke and abuse alcohol. People with this disease can develop emphysema due to lung inflamation. Smoking makes it much worse. They also get liver problems, like cirrhosis, because the defective protien builds up and damages the liver.

The most common genetic type of mental retardation in males is Fragile X. It mostly causes mental retardation, autism, and/or learning disabilities in boys. Males with Fragile X also have distinct physical features; they often have facial features like larger ears, long facies, high foreheads, and they also have large testicles

Fragile X mental retardation is caused by a triplet CGG expansion which leads to deactivation by methlyation of the FRMR1 gene on the X chromosome. Since it is a X-linked disease, it runs through the maternal line and unaffected women can pass it to their sons. Since males only have one X chromosome, males are affected, while women have a second X to compensate and are usually okay. About 1 in 250 women is a carrier of Fragile X, and a blood test is available to check a person's Fragile X gene. There is no treatment.

About 5-10% of breast cancer is hereditary. Two genes acount for the majority of hereditary breast cancer BRCA1 and BRCA2. These are dominant disorders that can be passed through men and women. If a woman inherits either of these genes, she has a 50-85% chance of developing breast cancer; there is also a higher chance for other cancers like ovarian and colon cancer.

Although it is expensive ( about $3000) genetic testing is available to check if these genes are responsible for breast cancer in a family. There are also many risk assessment programs to see if genetic testing is right for a family.