Other topics in this lecture: Linkage, Pedigree Analysis, Autosomal Dominant Inheritance, Genetic Counseling
Other topics in this lecture: Linkage, Pedigree Analysis, Autosomal Dominant Inheritance, Genetic Counseling
Gregor Mendel's research with genes and the way they work was the main starting point towards future research in the field of genetics. Mendel first entered a monastery in order to "free [himself] from the bitter struggle for existence." The Abbot of this monastery had an interest in plant breeding- particularly pea plants - and organized a course in crossbreeding during Mendel's first year at the monastery. Mendel was intrigued by the results of crossbreeding. After his participation in the Abbot's course, he left the monastery to study math and physics at the University of Vienna. Two years later he returned to the monastery, where he began his own experiments with pea plants.
In 1865, Mendel published the paper "Experiments in Plant Hybridization", which appeared in the journal of a local natural history society. This paper showed that each organism has physical traits that correspond to invisible elements within the cell. These invisible elements, which we now call genes, exist in pairs. Mendel showed that only one member of this genetic pair is passed on to each progeny. The gene is incorporated into a sperm or egg cell. At the time that Mendel did his research, there was no knowledge of chromosomes, cell structure, fertilization, mitosis and meiosis, which is now taught as common knowledge to most school children. Mendel's findings, therefore, show a remarkable use of observation and deduction which was quite ahead of his time. In 1868, the Abbot died. Mendel then ran the monastery, thus stopping all of his scientific work. However, he had laid the groundwork for the field of genetics.
Geneticists distinguish between genes and their expression, as it is then easier to discuss experimental results. A genotype is the actual set of genes that an organism has; it is the blueprint of genetic material. A phenotype is a measurable characteristic of an organism, such as eye color, hair color, the shape of one's nose, number of fingers, behavioral traits such as smiles per hour, physiological traits such as heart rate, or biochemical traits such as cholesterol levels and blood type.
In Mendel's studies, he used peas because they:
Mendel was extremely careful in gathering his data and scrutinizing the results of every experiment. This attention to detail, along with his mathematical insight, allowed Mendel to keep meticulous data.
When doing his work, Mendel posed questions dealing with how the physical characteristics of peas were transmitted from generation to generation, and whether these transmissions were unchanged or altered when passed on. He also questioned whether hereditary particles existed. His studies were based on seven traits of peas. The seven traits were qualitative (they could be measured and a value assigned); therefore, specified qualities could be assigned to each plant. These characteristics were visible and it was through them that he could study the effects of reproduction.
The seven traits were:
Mendel began his experiments using a set of pure-breeding pea plants, meaning that the second generation of plants had consistent traits with those of the first. He performed monohybrid crosses, meaning that the experiment was carried out between two strains of plants that differed only in one characteristic. He crossed parents of different phenotypes to see what resulted. The parents were denoted by a P, while the offspring - the filial generation - was denoted by F1, the next generation F2, etc.
Mendel proceeded to conduct a series of monohybrid crosses (testing one trait at a time), meaning that for each of the seven phenotypes, he crossed two plants of opposite phenotypes. Consistently, Mendel found that in the first generation of these crosses, all of the F1s were identical to one of the parents. For example, when testing the shape of the seed, crossing one pure-bred round seed with a pure-bred wrinkled seed, all of the offspring were round. The one trait - in this case a wrinkled seed - not expressed in the offspring he called a recessive trait. In each case of this crosses, the round trait was dominant over the wrinkled trait and is said to be the dominant trait. This conclusion is now referred to as Mendel's Law of Dominance. During the experiments, Mendel also observed that the sex of the parent was irrelevant for the dominant or recessive trait exhibited in the offspring. A cross between a male round seed with a female wrinkled seed would offer identical results to a female round seed crossed with a male wrinkled seed. This is known as Mendel's Law of Parental Equivalence.
Mendel found new principles in that the phenotypes absent in the F1 generation reappeared in approximately 1/4 of the F2 offspring. Mendel could not predict what traits would be present in any one individual, but he did deduce that there was a 3:1 ratio in the F2 generation for dominant/recessive phenotypes.
In describing his results, Mendel used the term elementen, which he postulated to be hereditary particles transmitted unchanged between generations. Even if the traits are not expressed, he surmised that they are still held intact. For example, even if a trait is not expressed (such as the wrinkled seed in the first filial generation of Mendel's crossing), that plant still has a wrinkled seed allele; the elementen are still passed on. We now call these `particles' alleles. An allele that can be suppressed during a generation is called a recessive allele, while one that is consistently expressed is a dominant allele.
We have developed terms to describe the existence of recessive and dominant alleles in any given genotype. Homozygous plants or individuals are those who have two copies of the same gene, e.g. Round/Round or Wrinkled/Wrinkled. Heterozygous individuals received a different type of gene from each zygote, eg. Round/Wrinkled or Wrinkled/Round. In this case, Round being the dominant trait, the phenotype of the plant would be Round, though the genotype Round/Wrinkled.
To help clarify the difference between a gene and an allele, think of it this way. A gene is a location on a chromosome. Alleles are different options for the same gene. For example, there may be a specific gene for eye color - meaning a location on the chromosome at which eye color is specified. Whichever allele (for green eyes, blue eyes, brown eyes) gets placed in that location will determine the specific color of the eyes.
Mendel observed that although the dominant trait was the one expressed in the F1 generation, the recessive trait still had an effect on the genotype of a heterozygote's offspring. Mendel developed a kind of shorthand for distinguishing alleles. They are designated by one or more letters. The first letter of abbreviation of a dominant allele is uppercase, for instance
Rr have the same phenotype, however, because the dominant trait is the one expressed.
Sometimes mating results can be expressed in the form of a Punnett Square.
Each box in the top row represents the gametes from one of the parents, while the left column shows the gametes from the other parent.
There is a 25% chance for each type of crossing (if there are only two gametes). However, though there are four different combinations, there are only two phenotypes (dominant or recessive), thus explaining Mendel's results. The ratio of dominant to recessive phenotypes is 3:1 - Mendel's results! Mendel's data was significant and compelling because he expressed it in ratios that always held true.
Mendel's hypothesis also allowed a prediction to be made that could be tested. He postulated that there were three different genotypes in F2 (
rr). Individuals with recessive (wrinkled) phenotype would produce only wrinkled individuals when self-fertilized in the F3 generation, since recessive homozygotes would breed true. He also assumed that round individuals would be either RR or Rr, in the ratio of 1:2. If these individuals were self-fertilized, then:
RR) should breed true and produce only round seeded plants
rR) should produce both round and wrinkled peas in the ratio of 3:1
This model explained the results of past experiments, but also rendered a means to predict future results.
Mendel's Law of Segregation states that each member of a pair of alleles maintains its own integrity, regardless of which one(s) is dominant. At reproduction, only one allele of a pair is transmitted to each gamete, and that choice is entirely random.
After his monohybrid crosses, Mendel did a series of dihybrid crosses. These were crosses between strains identical except for two characteristics. For example, Mendel did experiments with round/wrinkle (
R/r) and yellow/green (
Y/y). Mendel crossed a double dominant P (
RRYY) with a double recessive P (
rryy). All of the F1s that resulted from this were thus
RrYyand had the characteristics of round and yellow. These double heterozygotes were then used to make an F2 generation. The examination of one trait at a time demonstrates a 3:1 ratio:
Mendel thus observed that each of the traits he was following sorted themselves independently. Mendel's Law of Independent Assortment states that characteristics which are controlled by different genes will assort independent of all others. Whether or not a seed will be
RR has nothing to do with whether or not it will be
Although Mendel did not do experiments with humans, we know now that humans have approximately 30,000 genes. The reason that we are all so different and can even look so different from both of our biological parents is because there are so many different genetic combinations that can result. It is possible to generate about 8 million different types of gametes from a person who is heterozygous for only one gene on each pair of 23 chromosomes!
These laws Mendel discovered for the most part hold true. However, there are instances where Mendelian rules become complicated. Sometimes, within a population, more than two alleles can occur at a particular locus. This is multiple allelism and occurs in such instances as the ABO blood group which has at least three common alleles and some that are more rare. The HLA-A antigen group has at least 23 different alleles.
In some heterozygous organisms, both alleles of a given locus are expressed, showing codominance. For example, the ABO blood group shows codominance. If a red blood cell as an "A" allele it will express the "A" protein on its surface. If it has a "B" allele it will express the "B" protein on its surface. If both alleles are present, then both surface proteins will be present. In this case, neither allele is dominant over the other.
When one characteristic is reduced in a heterozygous organism, the alleles are said to exhibit partial or incomplete dominance because two copies of the gene will produce more or an effect than only a single copy of the gene. We see partial dominance in nature is some flower colors. When we see pink flowers, the allele for red is present only one time instead of two, resulting in the presence of only half the amount of red pigment.
Not all inheritance is not dependent solely on the copy of a single gene. Some characteristics, such as height, depend on the way in which many of the genes interact in an organism. It is, then, a polygenic trait.
The Law of Independent Assortment described above does not apply to all situations. On a given chromosome, there are sets of genes, and genes that are on the same chromosome are physically linked to one another. During reproduction, these linked genes tend to be transmitted as a unit instead of independently - this is referred to as linkage and these genes are said to be linked. Although these links may break during meiosis, this is not always the case. Therefore, the Law of Independent Assortment does not always hold true. Independent assortment sometimes occurs when genes are located on the chromosome but are relatively far apart from one another making the occurrence of crossover between the two locations likely.
Similarly, mutations sometimes occur in a gene. A mutation simply is a change in the DNA sequence of a gene. It can be due to a change in a base (letter) of the gene or due to an insertion or deletion of a piece of DNA. Although mutations occur in the normal course of DNA replication, the probability of one gene changing in one cell cycle is less than one in a million in humans. Because these occurrences are so rare, Mendel did not have any problems with his results.
Because humans reproduce so infrequently and have such a long lifetime, it is more difficult to study genetic disorders in humans. Instead of experimental biology (as in breeding pea plants), genetics is studied in humans through pedigree analysis.
In a pedigree diagram, every row represents a single generation, and these are labeled with Roman numerals. Couples within the generation are listed from left to right across the line, and horizontal lines connect the reproductive partners. Vertical lines that descend from these pairs are indicative of offspring from the two parents. Individuals demonstrating a specific phenotype are indicated with filled shapes.
Mating between blood relatives often causes genetic disorders. This is because the descendants of an individual who is a carrier for a specific disease have a much greater chance of inheriting the disease than the normal person. When close relatives mate, it increases the chances of recessive alleles (which might be rare in the general population) being passed on by both parents. Most humans, researchers say, carry within them four or five deleterious alleles in the heterozygous condition. Because relatives often carry the same deleterious alleles in this condition (they share common ancestors), there is greater likelihood that these alleles will pair up and be passed along to offspring. An example of this is the presence of hemophilia in the Royal Houses of Europe. Thus, the taboo subject of inbreeding does have validity.
Autosomal recessive traits are those that are expressed regardless of sex - both males and females are equally likely to be afflicted.
Albinism is an autosomal recessive human disorder. It occurs about once in 30,000 births, and the babies are born with extremely white skin and hair color. All groups of vertebrates, and plants and insects have instances of albinism. In some species, the eyes are red because of the visibilty of blood through the transparent skin (rabbits). These organisms lack the presence of the pigment melanin which gives our skin its tone. Albino individuals must be extremely careful about skin cancer and sunburn because of the absence of this protective pigment in their skin.
Phenylketonuria (PKU) is another disorder that occurs approximately 1 in 5-6,000 births. The condition is due to a missing enzyme that metabolizes an amino acid called phenylalinine which the body requires in order to function normally. Because individuals with PKU lack this enzyme, there is an accumulation of phenylalanine in blood and a related substance in urine; this is the condition of PKU. Forty states now require that infants be tested for this trait, as it is detected easily through a simple blood test measuring phenylalanine levels. The reason it is required is because after the first few weeks of birth, if it is not treated, the high phenylalanine levels can cause mental retardation. If found early enough, it can be treated through a special diet that monitors the intake of phenylalanine. There have been ethical questions presented about whether or not testing should be completely monitored because of the cost. The regulation of mothers with PKU who carry fetuses that might be damaged by the high phenylalanine in the mother's blood raises a difficult ethical question.
Cystic Fibrosis is another disease detectable in infants at birth. It is the most common genetic disorder in Caucasians in the U.S. where it afflicts approximately 1 in 2000 people. This is considered a very high frequency because most CF patients never reproduce and the disease occurs for unknown reasons. There is much research being conducted on this disease right now. In 1989, the gene for CF was isolated and sequenced. It has been found to be a large membrane protein that regulates water and salt balance in the cells. This leads to a thick dry mucous in the lungs and other secretory tissues which causes the lungs and intestines of these patients to become clogged. Bacteria can grow in these thick secretions and can cause fatal lung problems.
There is a test available to determine if a potential parent is heterozygous for the CF allele, but this test is only 75% accurate. Thus, it causes problems for the parents and poses questions such as "Should you take this test?" and "What will you do if the results are positive?" Researchers are continuing to research this disease to try to find a cure for CF patients.
Sickle-cell Trait: Sickle-cell anemia is another disease that afflicts mainly African-Americans. This occurs as a result of a mutation in a hemoglobin gene. Hemoglobin is a protein that has four subunits - two alpha and two beta. Hemoglobin carries oxygen throughout the body inside red blood cells (RBCs). The mutation causes the hemoglobin to crystallize inside the RBC, which leads to fragile sickle-shaped red blood cells that become entangled with one another and can lead to clotted blood vessels. This can lead to oxygen deficiency which may result in mental retardation or heart failure because the heart has to work harder to pump the thicker blood. Because of oxygen deficiency, there are often many other problems that can result - this is called a pleiotropic effect. Although malaria is a disease that has been eliminated in the U.S., it still causes many deaths in Africa. Malaria occurs as a result of a parasitic protozoan which lives in red blood cells. Malaria does not occur in individuals that are heterozygous for the sickle cell allele (or homozygous for the sickle cell allele) because the altered red blood cells do not provide a place where the malaria parasite flourishes. Thus, this deleterious trait has provided some survival advantage to those who carry it.
There are also traits that are a result of autosomal dominant inheritance, in which a single defective allele is dominant and can cause a heterozygote to be affected. Neurofibromatosis or NF is an autosomal dominant trait. This afflicts 1 in 3,500 people. The disease is characterized by hundreds of non-malignant tumors all over the body and by abnormal bone growth. This disease, unlike some others, cannot be diagnosed prenatally and cannot be cured. Patients can become blind and deaf if these tumors grow on the spinal cord. The gene was discovered in 1990, and they are working on a test and a cure.
The quality of earwax is also an autosomal dominant trait. It is an innocuous phenotype that has two forms. There is wet, sticky earwax and dry earwax. Wet earwax is a result of a single dominant gene (
Ww). Dry earwax is from the result of
wwcombination. Wet earwax is associated with armpit odor and is more common in Caucasians and Blacks (80-90% wet earwax). Dry earwax is more common in Asians (94% dry earwax). Mammals, including humans, tend to choose their mates based in part on scent, and it is postulated that Asians have chosen their mates (in part) based on body aroma. The gene for earwax influences secretions from apocrine glands, which include the mammary glands. Research shows that there is double the rate of occurrence for breast cancer in Japanese women with wet earwax as opposed to dry earwax, suggesting that the gene may somehow be involved in breast cancer. This hypothesis is supported elsewhere, because the occurrence of breast cancer in a population is directly related to the predominance of wet earwax.
Blood groups vary among individuals as well. Blood groups are a result of molecules on the surface of the blood cells. These surface molecules that can interact with antibodies are known as antigens. The ABO blood group is the most common, and it consists of three alleles. A and B are both codominant, and O is the absence of both A and B. Individuals do not have antibodies to their own RBC surface antigens. It is because of these blood groups that there emerged an initial understanding of why some blood transfusions worked and others were fatal. People have different blood types, and individuals can accept only certain types of blood - type OO being the universal donor, and type AB being the universal acceptor. Blood type is also used in criminal investigation.
Most people have very limited knowledge of genetics in general, and do not know about those diseases which are a result of genetic complications. Furthermore, they are not educated about the counseling available to them. There are new diagnostic and therapeutic treatments constantly being developed and made available. Genetic counselors help families understand the genetic disorder and provide counseling on how to deal with the knowledge. They also can advise on the risks to children or other relatives. The counselors are there solely to educate families, though, and not to make the decisions for them - that is still up to the parents. Genetic counselors try to help them make more educated decisions and understand all of their options.
Modern treatment of diseases and prevention of them are constantly evolving. The increased knowledge presents complex ethical issues and problems for the families. These dilemmas occur primarily in prenatal diagnosis of diseases. For example, this raises issues about abortion and whether or not a parent should have a right to terminate a pregnancy of a child who may be terminally ill.