In 2003, Researchers Completed the Entire Genome (Genetic Sequence) of Dog DNA. This Project

MITOSIS

The puppies of purebred dogs will grow up to look very much like their parents. However when the parents are of a mixed breed, the final sizes, markings, and builds of these puppies, as well in their behavioral traits, will be more varied. But whether dogs are purebred or mutts, we can ultimately explain their innate traits by using genetics, the science of heredity. Because of inbreeding, purebred dogs often suffer from serious genetic defects. For instance, a type of hereditary blindness called progressive retinal atrophy is common among Labrador retrievers, spaniels, and several other breeds. About 60% of the genetic disorders in dogs are similar to human genetic diseases, so genetic researchers often examine the genetic problems of dogs.

In 2003, researchers completed the entire genome (genetic sequence) of dog DNA. This project is yielding a wealth of data that can be used to compare the DNA of humans and dogs. Biologists are also studying certain dog breeds to shed light on the relationship between genetic makeup and behavior. Dogs of different breeds tend to have different temperaments. Pit bulls and Rottweilers, for instance, are more likely to be aggressive than some other breeds because they were bred as guard dogs: in generation after generation, the most aggressive dogs were selected for mating with each other. In fact, selective breeding is the basis for many of the dog breeds that exist today.

It is estimated that until about 14,000 years ago, dogs were very much like wolves, the species from which dogs evolved. It was at this time that dogs began to move with people into more permanent settlements, which were often geographically isolated. As a result, different populations of dogs became isolated from one another and eventually became inbred. At the same time, different groups of people selected dogs for different traits, depending on their needs. Herders selected dogs that were good at controlling flocks of animals, producing breeds such as the Border collie. Hunters had developed breeds of dogs, such as the laboratory retriever, which are good at retrieving wounded prey. Such dogs were bred to be less aggressive than some other breeds because hunters don't want their dogs to eat the quarry!

Genetics isn't everything: a dog’s level of aggression and other behavioral characteristics are influenced not only by its genes but also by its environment and care. The same is turning out to be true of human behavioral characteristics. Traits such as shyness have some genetic basis but can also be amplified or reduced by upbringing. Before you can begin to understand the rules that govern how inherited characteristics are passed from parent to offspring, you need to understand the basic biological concept of how chromosomes behave during the fertilization of an embryo. Therefore, we will first cover mitosis, how cells divide.

Mary was a very attractive young woman who liked to play ice hockey. Her high school gym teacher took an interest in her ability and gave her extra coaching. She hoped that one day Mary would play on an Olympic team. But something was wrong. Mary was sixteen and still not menstruating. Her parents decided to have her undergo a series of medical tests. Much to the surprise of everyone, Mary had an X and Y chromosome in the nucleus of her cells. She was a chromosomal male.

The doctor explained to Mary and her parents that Mary had testicular feminization syndrome. She has internal testes that produce testosterone but her cells won’t respond to it. Her genitals are like those of a female and she has well-developed breasts. However, she will never be able to have children. Mary will be able to go on and play hockey in the Olympics but she has to always carry a letter explaining her condition. Otherwise she will be disqualified because of her sex chromosomes.

This is an example of various syndromes that occur when people have chromosomal abnormalities.

Chromosomal Inheritance

In the nucleus of a cell there are chromosomes, arranged in pairs which are joined in the center. Each of these pairs are called a called a karyotype (type of chromosome pair). Humans have 23 pairs of chromosomes. When they are taken out of a cell for genetic testing, scientists arrange the pairs in order of their assigned name (which is a number from 1-23). This procedure is called karyotyping.

Males have one pair of chromosomes that are not the same length. The larger chromosome in this pair is the X chromosome and the smaller is the Y chromosome. Females have two X chromosomes. This pair is called the sex chromosomes because they contain the genes that determine the sex. Therefore, males are called XY and females are called XX. The male is the parent that determines the sex of the child.

GENES are the structures constructed of DNA that control the characteristics of the individual. Genes are like small dots on each chromosome. Each dot contains the blueprint for some aspect of the body. One gene is for eye color, one for liver function, one for right or left handedness, etc. Every aspect of the body corresponds to a gene on one of the chromosomes.

Human Cell Division

All cells in our body divide by a process called mitosis, which ensures that each and every cell has a complete number of chromosomes. Mitosis produces two daughter cells with the same number and kind of chromosomes as the parent cell.

Here’s a TQ hint: If a parent cell has 7 chromosomes prior to mitosis, how many chromosomes will the daughter cells have? Answer = 7.

In addition to mitosis, the sex cells (and only the sex cells) undergo a second process called meiosis, in which each daughter cell has only half of the chromosomes. In males, it produces the cells that become sperm; in females, it produces the cells that become eggs. The sperm and the egg are the sex cells, or gametes. GAMETES contain half the number of chromosomes compared to the rest of the body cells.

Therefore, meiosis requires two nuclear divisions, resulting in four daughter cells, each with half of the parent chromosomal number

When a sperm cell fertilizes an egg, the resulting zygote has the all of the chromosomes from each of the sex cells, for a total of 46 (or 23 pairs). Thereafter, when the cells of the zygote/embryo/fetus/infant/child/adult divide by mitosis, each cell has 23 pairs of chromosomes.

MITOSIS

Mitosis is cell division that produces two daughter cells, each with the same number and kinds of chromosomes as the parent cell (the cell that divides). Therefore, following mitotic cell division, the parent cell and the daughter cells are genetically identical.

Stages of Mitosis

The process is divided into five phases: interphase, prophase, metaphase, anaphase, and telophase.You need to know the order of these phases.

MEIOSIS

Meiosis only occurs in the testes and ovaries when they are ready to make an egg cell or a sperm cell. First, mitosis occurs as normal. But right after that, the two daughter cells divide again, but this time there is no prophase, so the chromosomes have not made a clone of themselves.

When the second cell division is at the metaphase stage, the chromosomes touch each other and exchange a few genes. The exchange of genetic material between chromatids is calledcrossing-over.That is what allows for genetic variation.

Meiosis results in four daughter cells, each having half the number of chromosomes as the parent cell. The daughter cells are not genetically identical, and neither is identical to the parent cell. For example, in MEIOSIS, if the parent cell has 14 chromosomes, the GAMETE will have 7.

The daughter cells that are formed are sperm cells in males and egg cells in females (these are gametes because each one has half the number of chromosomes). When a sperm and egg combine and contribute their chromosomes, the fertilized egg (called a zygote) will now have 23 pairs of chromosomes.

Nondisjunction

This is when chromosomes can become abnormal if the sister chromosomes do not separate properly during meiosis in the formation of a sex cell.This is what causes a sex cell to wind up with XXY or other chromosome abnormalities.

SUMMARY:

Preventing Birth DefectsHuman Biology by Sylvia Mader (page 322)

Although some congenital (Birth) defects are not preventable, certain ones are, and therefore all females of childbearing age are advised to take everyday precautions to protect any future or presently developing embryos and fetuses from these defects. For example, it is best if a woman has a physical exam even before she becomes pregnant. At that time, it can be determined if she has been immunized against rubella (German measles). Depending on exactly when a pregnant woman has the disease, rubella can cause blindness, deafness, mental retardation, heart malformations, and other serious problems in an unborn child. A vaccine to prevent the disease can be given to a woman before she gets pregnant, they can not be given to a woman who is already pregnant because it contains live viruses. Also, the presence of HIV virus should be tested for because preventative therapies are available to improve maternal and infant health.

Good habits are most during pregnancy, including proper nutrition, adequate rest, and exercise. Moderate exercise can usually continue throughout pregnancy and hopefully will contribute to the ease of delivery. Basic nutrients are required in adequate amounts to meet the demands of both fetus and mother. A growing number of studies confirm that the small, thin newborns are more likely to develop certain chronic diseases such as diabetes and high blood pressure, when they become adults than our babies who are born heavier.

An increased amount of minerals, such as calcium for bone growth and iron for red blood cell formation, and certain vitamins, such as vitamin E6 for proper metabolism and folate, are required pregnant women need more folate and day to meet an increased rate of cell division and DNA synthesis in their own bodies and that of the developing child. A maternal deficiency of folate has been linked to development of neural tube defects in the fetus. These defects include spina bifida and anencephaly. Perhaps as many as 75% of these defects could be prevented by adequate folate intake even before pregnancy occurs. Consuming fortified breakfast cereals is a good way to meet folate needs, because they contain a more ups or mobile form of folate.

Good health habits include avoiding substances that can cross the placenta and harm the fetus. Cigarette smoke poses a serious threat to the health of the fetus because it contains not only carbon monoxide and other toxic chemicals. Children born to smoking mothers have a greater chance of a cleft lip or palate, increased incidence of respiratory diseases, and later on, more reading disorders than those born to mothers who did not smoke during their pregnancy.

Alcohol easily crosses the placenta, and even one drink a day appears to increase the chance of a miscarriage. The more alcohol consumed, the greater the chances of physical abnormalities that the pregnancy continues. Heavy consumption of alcohol puts a fetus at risk for a mental defect because alcohol enters the brain of the fetus. Babies born to heavy drinkers are apt to undergo delirium tremors after birth-- shaking, vomiting, and extreme irritability-- and to have fetal alcohol syndrome. Babies with fetal alcohol syndrome have decreased weight, height, and head size, with malformation of the head and face. Later, mental retardation is common, as are numerous other physical malformations.

Certainly, illegal drugs, such as marijuana, cocaine, and heroine, should be completely avoided during pregnancy. Crack babies now make up 60% of drug affected babies. Cocaine use causes severe fluctuations in a mother's blood pressure that can temporarily deprive the developing fetus's brain of oxygen. Cocaine babies have visual problems, lack coordination, and are mentally retarded.

Children born to women who received x-ray treatment (for cancer) during pregnancy are apt to have birth defects or develop leukemia later. It takes a lower amount of x-rays to cause mutations in a developing embryo or fetus then in an adult. Dental and other diagnostic x-rays that result in only a small amount of radiation are probably safe. Still, a woman should be sure a physician knows that she is or may be pregnant. Similarly, toxic chemicals such as pesticides, industrial chemicals, formaldehyde, asbestos, and benzene, are mutagenic and can cross the placenta, resulting in abnormalities. Lead circulating in a pregnant woman's blood can cause a child to be mentally retarded. Agents that produce abnormalities during development are called to rat engines.

A woman has to be very careful about taking medications and supplements while pregnant. Excessive vitamin A., sometimes used to treat acne, may damage in embryo. In the 1950s and 60s, a synthetic hormone was given to pregnant women to prevent cramps, bleeding, and miscarriage. But in the 1970s and 80s, some adolescent girls and young women whose mothers had been treated with this hormone showed various abnormalities of the reproductive organs in an increased tendency towards cervical cancer. Other sex hormones, including birth control pills, can possibly cause abnormal fetal development, including abnormalities of the sex organs.

The drug thalidomide was a popular tranquilizer during the 1950s and 1960s in many European countries and to a degree in the United States. The drug, which was taken to prevent nausea and pregnant women, arrested the development of arms and legs in some children and also damaged heart and blood vessels, years, and the digestive tract. Some mothers of the affected children report that they took the drug for only a few days. Because of such experiences, physicians are very cautious about prescribing drugs during pregnancy, and no pregnant woman should take any drugs, even ordinary cold remedies or aspirin, without checking first with her physician.

Unfortunately, immunization for sexually transmitted diseases is not possible. The HIV virus can cross the placenta and cause mental retardation. As mentioned, proper medication can greatly reduce the chance of this happening. When a mother has herpes, gonorrhea, or Chlamydia, newborns can become infected is a pass through the birth canal. Blindness and other physical and mental defects may develop. Birth by cesarean section could prevent these occurrences.

In Rh negative woman who is given birth to an Rh positive child should receive an Rh immunoglobulin injection within 72 hours to prevent her body from producing Rh antibodies. She will start producing these antibodies when some of the child's Rh positive red blood cells enter her bloodstream, possibly before but particularly at birth. Rh antibodies can cause nervous system and heart defects in the fetus. The first Rh positive baby is usually not affected. But in subsequent pregnancies, antibodies created at the time of the first birth cross the placenta and begin to destroy the blood cells of the fetus, thereby causing anemia and other complications.

The birth defects we have been discussing our particularly preventable because they are not due to inheritance of an abnormal number of chromosomes or any other genetic abnormality. More women are having babies after the age of 35, and first births among women older than 40 had increased by 50% since 1980. The chance of an older woman bearing a child with a birth defect unrelated to genetic inheritance is no greater than that of a younger woman. However, there is a greater risk of an older woman having a child with a chromosomal abnormality leading to a premature delivery, cesarean section, low birth weight, or certain syndromes. Some chromosomal and other genetic defects can be detected in utero so that therapy for these disorders can begin as soon as possible.

Now that physicians and laypeople are what way are of the various ways birth defects can be prevented, it is hoped that the incidence of birth defects will decrease in the future.

Maternal Health Habits Human Biology by Sylvia Mader (p. 329)

and the fetus is subject to harm if the mother uses medicines and drugs of abuse, including nicotine and alcohol. Also, various sexually transmitted diseases, especially HIV infection, can be passed on to the fetus by way of the placenta. Women need to know how to protect their unborn children from harm. Indeed, if they are sexually active, their behavior should be protective, even if they are using a recognized form of birth control. Harm can occur before a woman realizes she is pregnant!

Because maternal health habits can affect the child before it is born, there is been a growing acceptance of prosecuting women when a newborn has a condition, such as fetal alcohol syndrome, that could only have been caused by the drinking habits of the mother. Employers have also become aware that they might be subject to prosecution if the workplace exposes pregnant employees to toxins. To protect themselves, Johnson Controls, a US battery manufacturer, developed a fetal protection policy. No woman who could bear a child was offered a job that might expose her to toxins that could negatively affect the development of her baby. To get such a job, a woman had to show that she had been sterilized or was otherwise incapable of having children. In 1991, the US Supreme Court declared this policy unconstitutional on the basis of sexual discrimination. The decision was hailed as a victory for women, but was it? The decision was written in such a way that women alone, and not an employer, are responsible for any harm done to the fetus by workplace toxins.