In human genetics, heredity refers to those human characteristics transmitted to a child by their parents during conception. Human life begins as a single cell called an egg or ovum. This egg is 1/165th of an inch in diameter. It is the largest cell in the human body. Once this egg is fertilized by the male sperm cell, it becomes a zygote. The single-celled zygote has a nucleus which contains 46 tiny structures called chromosomes. Twenty-three are contributed by the female egg, and 23 are contributed by the male sperm cell. This makes 23 pair or 46 total chromosomes. Pairs are formed by homologous genes from each parent. Each homolog carries the same trait as its partner.
These 23 pairs of chromosomes contain our genetic code. These chromosomes are
composed of small molecules called genes, and genes are composed of a chemical
called deoxyribonucleic acid or DNA. Although our genetic code codes only for
protein formation, it is the master plan or blueprint for a developing person.
Our genetic code is the blueprint for our physical and mental development.
Our genetic code tells the single-celled zygote to divide again and again. Each time it divides, the genetic code is duplicated so that each cell gets a complete set of genetic instructions. Under the direction of our genetic code, these cells divide and specialize into every component of the human body. Some cells are told by our genes to become muscle cells, others are directed to become bone cells, other blood cells, skin cells, nerve cells, heart, liver, kidney, eyes, lungs, etc. until you have every component of the human body.
Our genetic code gives us our potentials and limitations. It is the blueprint for our physical structure--determining whether we will be tall or short, large or small, male or female, black, white, red, or yellow. It is the blueprint for our brain, and therefore predisposes us to be smart or dumb, active or passive, shy or out-going, and predisposes us to develop certain personality traits. Our genetic code gives us susceptibilities and predispositions toward certain physical and mental illness. Our genetic code helps determine such things as how long we will live, age of reaching puberty, baldness in males, graying of the hair, color blindness, albinism, double jointedness, etc.
Chromosomes can be lined up in homologous pairs and numbered according to size. Chromosome pair number one is the largest and chromosome pair number 22 is the smallest. However, an exception is the 23rd pair. The 23rd pair is referred to as being the sex chromosomes because their combination determines the sex of the child. The only possible contribution that a female can make to this 23rd pair of chromosomes is an X sex chromosome; called an X because of its general appearance when viewed under a microscope. However, the human male can contribute either an X or a Y. The Y chromosome is called a Y because it lacks one arm of the X.
If during conception the child gets an XX sex chromosome combination, they develop into a girl. If during conception the child gets and XY sex chromosome combination, they develop into a boy. Since only the father can contribute a Y sex chromosome, it is the father's sperm which determines the sex of a child. From a probability point of view half of all children should be male and half should be female. However, in real life, there are about 103-105 males born for every 100 females. No one knows why. However, the mortality rate is higher for males than females, so that in time females will out number males. It is believed that the extra chromosomal arm that females receives give them biological strengths that males do not have. These strengths probably involve a group of hormones called estrogens, which give the female body a female appearance. Because of the Y chromosome, the body of a male is dominated by hormones called androgens. Testosterone is the main androgen. While testosterone does cause males to be physically stronger than females, it is hard on the body, especially the heart. The lack of this chromosomal arm probably causes males to have higher rates of mental retardation, addiction, accidents, criminal behavior, violence, as well as heart disease.
On our chromosomes are a total of about 40,000 genes. This would mean that there are about 40,000! combinations of human beings. If humanity could exist infinitely, all people would live again and again. This is a doubtful occurrence. On these 40,000 genes, each person has 5-8 bad genes. However, these seldom have any effect at all.
But sometimes there are serious errors in our genetic code and these can give us the blueprint for genetic disease. When we have a serious genetic error, there are three primary ways that the error can be inherited (passed from generation to generation). These three patterns of inheritance are: sex-linked, dominant and recessive.
Sex-linked genetic disease occurs due to errors in the 23 pair of chromosomes (the
sex chromosomes). These genetic diseases occur only in males. They do not occur
in females because the homolog carries the same trait as its partner. Since males
have an XY sex chromosome combination, they are vulnerable to defects on the arm
of the X which lacks a homolog. In females, any defect here will be compensated
by the other X. Since males get a Y, there is no compensation. Although the
defective gene is carried by the female, females do not develop the disease, but
are carriers for the disease. Some sex-linked genetic diseases are:
a. hemophilia--a disease in which the blood has less than 1% of the normal clotting factor.
b. color-blindness--in red-green color-blindness, the most common form of color-blindness, reds and greens are seen as gray.
c. Lesch-Nyhan syndrome--occurs once in every 10,000 live births. The male child appears normal at first, but after about two months they become abnormally irritable. By two years of age, self-mutilation begins, such as lip biting, finger chewing, teeth grinding, and head-banging. Death occurs in early childhood due to kidney failure and brain deterioration.
In the dominant pattern of inheritance one bad gene from either parent will cause the genetic disease. Since only one bad gene causes the disease, if either parent has the disease the chance of the child inheriting the bad gene and developing the disease is 50% with each conception. An example of a dominant genetic disease is Huntington's Disease. Huntington's is sometimes called Huntington's Chorea or St. Vitus Dance. "Chorea," as in choreograph, is the Greek word for dance. It is called this because the disease produces involuntary twitching and other involuntary movements. Huntington's usually has its onset when a person is in their forties. It is characterized by a deterioration of parts of the central nervous system which affect both muscle control and behavior. People with Huntington's often must be cared for in a mental institution.
To develop a recessive genetic disease both parents must contribute the same defective gene. The parents usually don't have the disease, but carry one defective gene and are carriers for the disease.
For a child born of parents who are carriers for a recessive genetic disease, the
child has these genetic probabilities:
a. a 1/4 chance of inheriting neither defective gene. If they inherit neither defective gene they will never develop the disease and they will not be carriers of the disease.
b. chance of inheriting only one bad gene. If they inherit only one bad gene, they will never develop the disease, but they will be carriers for the disease.
c. 1/4 chance, the losing chance, of inheriting both bad genes and developing the disease.
Some recessive genetic diseases are :
a. sickle-cell disease--a disease which effects people whose ancestors once inhabited any part of the earth where malaria is a high risk. In sickle-cell disease some of their hemoglobin is abnormal and these red blood cells take on a sickle-like shape. The sickle-shaped red blood cells clog the capillaries so that cells cannot get needed oxygen and nutrition. Some people with sickle-cell disease suffer year round. However, others have only crisis periods which are more common in spring and fall than other times of the year. Sickle-cell disease is treated by blood transfusions, pain killers and chemicals which increase the oxygen carrying capacity of the blood. Many people with the trait for sickle-cell appear to be immune to certain types of malaria. The sickle-cell trait can be detected by a blood test.
b. cystic fibrosis--a disease that affects various glands, such as mucus, salivary and sweat glands. It is diagnosed by the sweat test. Over-active mucus glands give these people chronic respiratory problems.
c. phenylketonuria (PKU)--a disease of metabolism in which the child cannot metabolize phenylalanine--an amino acid. To control the effects of the disease they must avoid this amino acid. Since phenylalanine is in aspartame (NutraSweet), warning labels appear on soft drinks which are sweetened with this product.
d. Tay-Sachs disease--a recessive genetic disease in which the child cannot metabolize a certain lipid (fat). These lipids encase the CNS and prevent the brain from expanding. Death occurs in early childhood.
Other genetic disease does not fall neatly into one of these three patterns.
Examples are:
a. Klinefelter's Syndrome--occurs only in males when the male child receives an XXY sex chromosome combination instead of the normal XY. For this reason, it called a form of trisomy 23, meaning that there are three chromosomes where the 23rd pair should be. Klinefelter's results in retarded sexual development due to the fact that their testicles are small and do not produce normal amounts of testosterone. Klinefelter's boys have an incomplete puberty and a child-like appearance. Klinefelter's is often accompanied by mental deficiency or mental retardation.
b. Turner's Syndrome--occurs when the female child receives only a single X sex chromosome instead of the normal XX combination (monosomy 23). Turner's girls have retarded sexual development, are sterile, have a webbed neck, and have very specific mental deficiencies involving visual recognition and spacial arrangements.
c. XYY condition--a form of trisomy 23 that occurs only in males. In the past, XYY males were called "super males," because they are abnormally tall, abnormally strong and abnormally aggressive. It is a genetic condition that is believed to predispose criminal behavior. XYY males often have sub-normal intellect.
d. Down's Syndrome--a form of trisomy 21 in which there is an extra member in what should have been the 21st pair of chromosomes. Down's Syndrome is sometimes called Mongolism, because of the oriental roundness of the face. Down's effects either sex, and is usually accompanied by moderate to severe mental retardation. They have a large head, a large nose, their eyes are spaced further apart than normal, their lips are configured different from normal, their tongue is protruding, they have white specks in the iris of the eye, their middle finger lacks a joint, and they usually breath through their noses.
Many genetic defects can be detected by a prenatal test for birth defects called amniocentesis, or by another called Chorionic villi Sampling.
Many physical and mental diseases are suspected to be of genetic origin or
predisposition, but as yet there is no absolute proof. The reason that they are
suspected to be of genetic origin is that they tend to run in families, and their
is a high rate of concordance in identical twins.
Fraternal twins are the result of a double ovulation and a double conception.
Therefore, their inheritance is no more alike than that of brothers and/or sisters
born of the same parents at different times. They develop in different amnions
(sacs). Fraternal twins can be the same sex, but they can also be of a different
sex. They may look similar to each other because they are brother and/or sister.
However, they do not have the near identical appearance of identical twins.
Identical twins are the result of a single ovulation and a single conception. In
the early stages of cell division, they separate. Therefore you have two people
with identical genetic codes. They develop in a single amnion. They will always
be the same sex. They will look almost identical to each other. What are called
Siamese twins (co-joined twins), are always identical twins. Since identical
twins have identical genetic codes, if a genetic disease develops in one, you can
expect it to also develop in the other.
1. Depression--an emotional mental illness characterized by sadness, loss of interest in life, and suicidal ideas. If you have identical twins and one twin develops a major depression, there is a 60-80% chance that the other twin will also develop the same major depression. However for fraternal twins, if one twin develops a major depression, the probability is only 15% that the other twin will develop the same depression.
2. Schizophrenia--a mental illness that is classified as a thought disorder. Schizophrenia is characterized by loss of contact with reality. Thinking become confused and disorganized. Many schizophrenics have delusions and hallucinations. In identical twins, if one develops schizophrenia, there is a 50% chance that the other twin will develop the same illness. However, for fraternal twins, if one develops schizophrenia, the probability is only 15% that the other twin will develop schizophrenia too.
3. Manic-depression--a mood disorder in which people alternate from deep
depression to manic euphoria. The manic euphoria generally lasts 1-3 months
followed by a depression which lasts 3-6 months. A defective gene has been found
on chromosome pair number 11. If this defect exists, 63% will suffer from manic
depression.
Studies of adopted children indicate that alcoholism, drug addiction and suicide
may have a genetic component. Children of alcoholic, drug addicted or suicidal
parents who were adopted by non-alcoholic, non-drug addicted or non-suicidal
families show that these children have a higher probability of being alcoholic,
drug addicted or suicidal than are the biological children of non-alcoholic, non-drug addicted or non-suicidal parents. In fact, the probability is about the same
as children who is raised by alcoholic, drug addicted or suicidal parents.
Since our genetic code directs the construction of our brain, intellectual
capacity is also genetically determined to a large degree. Adopted children will
always have an I.Q. that more closely resembles their biological parents than
their adoptive parents.
Aging is a genetic disease. The primary reason that some old people look young
and some young people look old is the stability of the genetic code. Some genetic
codes are so stable that they are not dramatically affected by the passage of
time. There is a genetic aging disease called Progeria, in which a child will die
of old age at about the age of ten.
Psychological tests of temperament and personality show that adopted children have
temperaments and personalities that more closely resemble their biological parents
that their adoptive parents.
All cancer is genetic. Cancer occurs when the genetic code gives bad instructions
to dividing cells to grow all out of proportion. It is believed that cancer
causing agents called carcinogens, alter our genes so that cancerous growth
occurs. Cancer may represent an errant attempt at immortality. Normal cells can
be artificially cultured for only a few days. Cancer cells can be artificially
cultured for an indefinite period of time.