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Genetics is the study of inheritance.  Most, if not virtually all, human characteristics are regulated at least partially by genes.  Each of the cells of an individual (except mature RBC's and egg and sperm) contains 46 chromosomes.  These chromosomes are in 23 pairs called homologous pairs.  One member of each pair has come from the egg and is called maternal, the other member has come from the sperm and is called paternal.

The autosomes are the chromosome pairs designated 1 to 22.  The sex chromosomes form the remaining pair.  In women these are designated XX and in men XY.

Chromosomes are made of DNA and protein; the DNA is the hereditary material.  The DNA code for one protein is called a gene.

For example, a specific region of the DNA of chromosome 11 is the code for the beta chain of hemoglobin.  Because an individual has two of chromosome 11, he or she will have two genes for this protein, a maternal gene inherited from the mother and a paternal gene inherited from the father.  This is true for virtually all of the 70,00 genes estimated to be found in our chromosomes.  In our genetic make up, each of us has two genes for each protein.  

For each gene of a pair, there may be two or more possibilities for its “expression”, that is, its precise nature or how it will affect the individual.  These possibilities are called alleles.  A person, therefore, may be said to have two alleles for each protein or trait; the alleles may be the same or different.  If the two alleles are the same, the person is said to be homozygous for the trait.  If the two alleles are different, the person is said to be heterozygous for the trait.

The genotype is the actual genetic make up, that is the alleles are present. The phenotype is the appearance or how the alleles are expressed.  When a gene has two or more alleles, one allele may be dominant over the other, which is called recessive.  For a person who is heterozygous for a trait, the dominant allele (or gene) is hidden but may be expressed in children.

The determination of eye color is an example of a trait determined by a pair of alleles, one which may dominate another.  Another example is sickle cell anemia.  The gene for the beta chain of hemoglobin is on chromosome 11; an allele for normal hemoglobin is dominant, and an allele for sickle cell hemoglobin is recessive.

An individual who is heterozygous is said to have the sickle cell trait; an individual who is homozygous recessive will have sickle cell anemia because the characteristic of sickle cell anemia will be expressed of two genes are present.

See the Punnet square example to look at the percentages of a trait or disease.

Sex linked traits may also be called X-linked traits because the genes for them are located only on the X chromosome.  There are no corresponding genes on the Y chromosome.  Genes are recessive is sex linked traits; meaning that a characteristic will only be expressed if two genes for it are present in the homologous pair.

Women with one gene (and one gene for normal functioning) are called carriers.  Men cannot be carriers, they either have the trait or they do not have it.

Congenital anomalies or diseases have a wide range of severity, from the trivial to the fatal.  They are present at birth or are detected later in infancy or childhood.  The cause of congenital defects may be genetic, nongenetic, or a combination or both.  The cause is usually unknown; thus measures of prevention have usually not been effective.


Cerebral Palsy-  congential non-progressive paralysis resulting from damage to the central nervous system.  This primarily affects motor performance.  The muscles may be floppy or stiff, with diminished voluntary movement.  This syndrome usually stems from inadequate blood or oxygen supply to the brain during fetal development, during the birthing process or in early childhood until approximately 9 years of age.

There is no treatment for this disorder.  The goal of treatment is to minimize the handicap by providing every possible therapeutic measure.

Muscular Dystrophy-  is a progressive degeneration and weakening of the skeletal muscles.  There are several types of the disease, but all are rare; the most common and well known type is Duchenne's MD.  This begins soon after birth or during childhood, usually before the age of 5.  Duchenne's is a result of a genetic defect.  This disease affects only males and is genetically inherited through female carriers.  In one third to one half of cases, there is no family history.  This means that the disease may be caused by a newly acquired mutation.

Spina Bifida-  is a malformation of the spine in which the posterior portion of the spinal column (usually lumbar area) is completely or partially lacking.  The cause of this congenital anomaly is unknown, but it has been associated with exposure to ionizing radiation during early uterine life.  This condition occurs when the neural tube fails to close in the early stages of fetal development.  Usually there is no spinal cord or spinal nerve involvement below the area of the defect.

Hydrocephalus-  a large amount of CSF accumulates in the skull, causing increase intracranial pressure.  An impairment of the circulation of the CSF which does not allow for reabsorption.  This may be the result of intracranial hemorrhage from head trauma, a blood clot, prematurity, or infection such as meningitis.

Cri du Chat syndrome (Cat's Cry Syndrome)-  is a hereditary condition that is a result of a chromosomal aberration due to deletion of part of chromosome 4 and 5.  Infants with this syndrome have microcephaly with a deficiency of brain tissue, and mental retardation.  Those who are born alive have a weak, cat like cry and the orbits of the eyes are spaced far apart.   

Down Syndrome-  is a congenital form of mild to severe mental retardation.  Infants with Down syndrome have an extra chromosome on number 21 (also called trisomy 21)  It occurs in 1 in 650 live births and more often to women who bear children over the age of 35 years.

Cleft lip and palate-  is a congenital birth defect consisting of one or more clefts in the lip.  Cleft palate is when there is a hole in the roof of the mouth which may extend into the nasal area.  The cause is a failure in the embryonic development of the fetus.  It is considered a multifactoral genetic disorder and occurs in approximately 1 in 10,000 births.
Cystic Fibrosis-  is a chronic dysfunction of the exocrine glands affecting multiple body systems, it is the most common fatal genetic disease.  This is an inherited disorder and is transmitted as an autosomal recessive trait.

Phenylketonuria (PKU)-  is an inborn error of metabolism of amino acids that causes brain damage and mental retardation when not corrected.  This is an autosomal recessive trait and causes defective protein metabolism, resulting in an accumulation of phenylalanine in the blood.


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