Classical transmission genetics

SIGNIFICANCE: In sexual reproduction, parents produce specialized cells (eggs and sperm) that fuse to produce a new individual. Each of these cells contains one copy of each of the required units of information, or genes, which provide the blueprint necessary for the offspring to develop into individual, functioning organisms. Transmission genetics refers to the passing of the information needed for the proper function of an organism from parents to their offspring as a result of reproduction.

Discovery of Transmission Genetics

The desire to improve plant and animal production is as old as agriculture. For centuries, humans have been using selective breeding programs that have resulted in the production of thousands of varieties of plants and breeds of animals. The Greek philosopher-scientist Hippocrates suggested that small bits of the body of the parent were passed to the offspring during reproduction. These small bits of arms, heads, stomachs, and livers were thought to develop into a new individual. Following the development of the microscope, it became possible to see the cells, the small building blocks of living organisms. Study of the cell during the 1800s showed that sexual reproduction was the result of the fusion of specialized cells from two parents (eggs and sperm). It was also observed that these cells contained chromosomes (“color bodies” visible when the cells reproduced) and that the number and kind of these chromosomes was the same in both the parents and the offspring. This suggested that the chromosomes carried the genetic information and that each parent transmitted the same number and kinds of chromosomes. For example, humans have twenty-three kinds of chromosomes. The offspring receives one of each kind from each parent and so has twenty-three chromosome pairs. Since the parents and the offspring have the same number and kinds of chromosomes, and since each parent transmits one complete set of the chromosomes, it was thought that there must be a process of cell division that reduces the parent number from two sets of chromosomes to one set in the production of the egg or sperm cells. The parents would each have twenty-three pairs (forty-six) chromosomes, but their reproductive cells would each contain only one of each chromosome (twenty-three).

In the 1860s, the Austrian botanist Gregor Mendel repeated studies of inheritance in the garden pea and, using the results, developed a model of genetic transmission. The significance of Mendel’s work was not recognized during his lifetime, but it was rediscovered in 1900. In that same year, the predicted reductional cell division during reproduction was fully described, and the science of genetics was born.

A Study of Variation

In many respects, genetics is the study of variation. It is recognized that a particular feature of an animal or plant is inherited because there is variation in the expression of that feature, and variation in expression follows a recognizable inheritance pattern. For example, it is known that blood types are inherited, both because there is variation (blood types A, B, and O) and because examination of family histories reveals patterns that show transmission of blood-type information from parents to children.

Variation in character expression may have one of two sources: environmental conditions or inherited factors. If a plant is grown on poor soil, it might be short. The same plant grown on good soil might be tall. A plant that is short because of an inherited factor cannot grow tall even if it is placed on richer soil. From this example, it can be seen that there may be two different ways to determine whether a specific character expression is environmentally or genetically determined: testing for environmental influences and testing for inherited factors. Many conditions are not so easily resolved as this example; there may be many complex environmental factors involved in producing a condition, and it would be impossible to test them all. Knowledge of inheritance patterns can, however, help in determining whether inherited factors play a role in a condition. Cancer-associated genes have been located using family studies that show patterns consistent with a genetic contribution to the disease. There are certainly environmental factors that influence cancer production, but those factors are not as easily recognized.

The patterns of transmission genetics were discovered because the experimenters focused their attention on single, easily recognized characteristics. Mendel carefully selected seven simple characteristics of the pea plant, such as height of the plant, color of the flower, and color of the seeds. The second reason for success was the use of carefully controlled crosses. The original parents were selected from varieties that did not show variations in the characteristic of interest. For example, plants from a pure tall variety were crossed with plants from a pure short variety. Control of the information passed by the parents allowed the experimenter to follow the variation of expression from parents to offspring through a number of generations.

Transmission Patterns

The classic genetic transmission pattern is the passing of information for each characteristic from each parent to each offspring. The offspring receives two copies of each gene. (The term “gene” is used to refer to a character-determining factor; Mendel’s original terminology was “factor.”) Each parent also had two copies of each gene, so in the production of the specialized reproductive cells, the number must be reduced. Consider the following example. A tall pea plant has two copies of the information for height, and both copies are for tall height (tall/tall). This plant is crossed with a plant with two genes for short height (short/short). The information content of each plant is reduced to one copy: The tall plant transmits one tall gene, and the short plant transmits one short gene. The offspring receive both genes and have the information content tall/short.

The situation becomes more complex and more interesting when one or both of the parents in a cross have two different versions of the gene for the same characteristic. If, for example, one parent has the height genes tall/short and the other has the genes short/short, the cells produced by the tall/short parent will be of two kinds: ½ carry the tall gene and ½ carry the short gene. The other parent has only one kind of gene for height (short), so all of its reproductive cells will contain that gene. The offspring will be of two kinds: ½ will have both genes (tall/short), and ½ will have only one kind of gene (short/short). Had it been known that the one parent had one copy for each version of the gene, it could have been predicted that the offspring would have been of two kinds and that each would have an equal chance of appearing. Had it not been known that one of the parents had the two versions of the gene, the appearance of two kinds of offspring would have revealed the presence of both genes. The patterns are repeatable and are therefore useful in predicting what might happen or revealing what did happen in a particular cross. For example, blood-type patterns or DNA variation patterns can be used to identify the children that belong to parents in kidnapping cases or in cases in which children are mixed up in a hospital.

In a second example, the pattern is more complex, because both parents carry both versions of the gene: a tall/short to tall/short cross. Each parent will produce ½ tall-gene-carrying cells and ½ short-gene-carrying cells. Any cell from one parent may randomly join with any cell from the other parent, which leads to the following patterns: ½ tall × ½ tall = ¼ tall/tall; ½ tall × ½ short = ¼ tall/short; ½ short × ½ tall = ¼ short/tall; ½ short × ½ short = ¼ short/short. Tall/short and short/tall are the same, yielding totals of ¼ tall/tall; ½ tall/short; and ¼ short/short, or a 1:2:1 ratio.

This was the ratio that Mendel recognized and used to develop his model of transmission genetics. Mendel used pure parents (selected to breed true for the one characteristic), so he knew when he had a generation in which all of the individuals had one of each gene.

As in the previous example, if it had been known that each of the parents had one of each gene, the ratio could have been predicted; conversely, by using the observed ratio, the information content of the parents could be deduced. Using a blood-type example, if one parent has blood-type genes AO and the other parent has the genes BO, the possible combinations observed in their offspring would be AB, AO, BO, and OO, each with the same probability of occurrence (½ A gene-bearing and ½ O gene-bearing cells in one parent × ½ B gene-bearing and ½ O gene-bearing cells in the other parent).

Reductional Division

Transmission genetics allows researchers to make predictions about specific crosses and explains the occurrence of characteristic expressions in the offspring. In genetic counseling, probabilities of the appearance of a genetic disease can be made when there is an affected child in the family or a family history of the condition. This is possible because, for most inherited characteristics, the pattern is established by the reduction of chromosome numbers that occurs when the reproductive cells are produced and by the random union of reproductive cells from the two parents. The recognition that the genes are located on the chromosomes and the description of the reductional division in which the like chromosomes separate, carrying the two copies of each gene into different cells during the reductional division of meiosis, provide the basis of the regularity of the transmission pattern. It is this regularity that allows the application of mathematical treatments to genetics. Two genes are present for each character in each individual, but only one is passed to each offspring by each parent; therefore, the 50 percent (or ½) probability becomes the basis for making predictions about the outcome of a cross for any single character.

The classical pattern of transmission genetics occurs because specialized reproductive cells, eggs and sperm, are produced by a special cell reproduction process (meiosis) in which the chromosome number is reduced from two complete sets to one set in each of the cells that result from the process. This reduction results because each member of a pair of chromosomes recognizes its partner, and the chromosomes come together. This joining (pairing) appears to specify that each chromosome in the pair will become attached to a “motor” unit from an opposite side of the cell that will move the chromosomes to opposite sides of the cell during cell division. The result is two new cells, each with only one of the chromosomes of the original pair. This process is repeated for each pair of chromosomes in the set.

Independent Genes

Humans have practiced selective breeding of plants and animals for centuries, but it was only during the nineteenth and twentieth centuries that the patterns of transmission of inherited characters were understood. This change occurred because the experimenters focused on a single characteristic and could understand the pattern for that characteristic. Previous attempts had been unsuccessful because the observers attempted to explain a large number of character patterns at the same time. Mendel expanded his model of transmission to show how observations become more complex as the number of characteristics examined is expanded. Consider a plant with three chromosomes and one simple character gene located on each chromosome. In the first parent, chromosome 1 contains the gene for tall expression, chromosome 2 contains the gene for expression of yellow seed color, and chromosome 3 contains the gene for purple flower color. In the other parent, chromosome 1 contains a gene for short height, chromosome 2 contains a gene for green seed color, and chromosome 3 contains a gene for white flower color. Each parent will transmit these genes to their offspring, who will have the genes tall/short, yellow/green, and purple/white. In the production of reproductive cells, the reductional division of meiosis will pass on one of the character expression genes for each of the three characters. (It is important to remember that the products of the reductional cell division have one of each chromosome. If this did not occur, information would be lost, and the offspring would not develop normally.) The characteristics are located on different chromosomes, and during the division process, these chromosome pairs act independently. This means that the genes that came from any one parent (for example, the tall height, yellow seed, and purple flower expression genes from the one parent) do not have to go together during the division process. Since chromosome pairs act independently, different segregation patterns occur in different cells. The results from one meiosis may be a cell with the tall, green, and purple genes and one with the short, yellow, and white genes. In the same plant, another meiosis might produce a cell with the short, yellow, and purple genes, and the second cell would have the tall, green, and white genes.

Since these genes are independent, height does not influence seed color or flower color, nor does flower color influence seed color or height. The determining gene for each characteristic is located on a different chromosome, so the basic transmission model can be applied to each gene independently, and then the independent patterns can be combined. The tall/short height genes will segregate so that ½ of the cells will contain the tall gene and ½ will contain the short gene. Likewise, the yellow/green seed color genes will separate so that ½ of the cells will contain the yellow gene and ½ will contain a green gene. Finally, ½ of the cells will contain a purple flower gene and ½ will contain a white gene. These independent probabilities can be combined because the probability of any combination is the product of the independent probabilities. For example, the combination tall, purple, white will occur with a probability of ½ × ½ × ½ = . This means that one should expect eight different combinations of these characters. The possible number of combinations for n chromosome pairs is 2ⁿ. For humans, this means that any individual may produce 2²³ different chromosome combinations. This is the same idea as tossing three coins simultaneously. Each coin may land with a head or a tail up, but how each coin lands is independent of how the other coins land. Knowledge of transmission patterns based on chromosome separation during meiosis allows researchers to explain the basic pattern for a single genetic character, but it also allows researchers to explain the great variation that is observed among individuals within a population in which genes for thousands of different characters are being transmitted.

Continuous Variation

The principles of transmission genetics were established by studying characters with discrete expressions—plants were tall or dwarf, seeds were yellow or green. In 1903, Danish geneticist Wilhelm Johannsen observed that characteristics that showed continuous variation, such as weight of plant seeds, fell into recognizable groups that formed a normal distribution. These patterns could also be explained by applying the principles of transmission genetics.

Assume a plant has two genes that influence its height and that these genes are on two different chromosomes (for example, 1 and 3). Each gene has two versions. A tall gene stimulates growth (increases the height), but a short gene makes no contribution to growth. A plant with the composition tall-1/tall-1, tall-3/tall-3 would have a maximum height because four genes would be adding to the plant’s height. A short-1/short-1, short-3/short-3 plant would have minimum height because there would be no contribution to its height by these genes. Plants could have two contributing genes (tall-1/short-1, tall-3/short-3) or three contributing genes (tall-1/short-1, tall-3/tall-3). The number of offspring with each pattern would be determined by the composition of the parents and would be the result of gene segregation and transmission patterns. Many genes contributing to a single character expression apply to many interesting human characteristics, such as height, intelligence, amount of skin pigmentation, hair color, and eye color.

Linkage Groups

Mendel’s model of the transmission of genes was supported by the observations of chromosome pair separation during the reductional division, but early in the twentieth century, it was recognized that some genes did not separate independently. Work in American geneticist Thomas Hunt Morgan’s laboratory, especially by an undergraduate student, Alfred Sturtevant, showed that each chromosome contained determining genes for more than one characteristic and established that genes located close together on the same chromosome stayed together during the separation of the paired chromosomes during meiosis. If a pea plant had a chromosome with the tall height gene and, immediately adjacent to it, a gene for high sugar production, and if the other version of this chromosome had a gene for short height and a gene that limited the sugar production, the most likely products from meiosis would be two kinds of cells: one with the genes for tall height and high sugar production and one with the genes for short height and limited sugar production. These genes are said to be “linked,” or closely associated on the same chromosome, because they go together as the chromosomes in the pair separate. It is generally accepted that humans contain approximately 21,000 genes, but there are only twenty-three kinds of chromosomes. This means that each chromosome contains many different genes. Each chromosome is considered a linkage group, and one of the goals of genetic study is to locate the gene responsible for each known characteristic to its proper chromosome.

A common problem in medical genetics is locating the gene for a specific genetic disease. Family studies may show that the disease is transmitted in a pattern consistent with the gene being on one of the chromosomes, but there is no way of knowing its location. Variations in DNA structure are also inherited in the classic pattern, and these DNA pattern modifications can be determined using current molecular procedures. DNA variation patterns are analyzed for linkage to the disease condition. If a specific DNA pattern always occurs in individuals with the disease condition, it indicates that the DNA variation is on the same chromosome and close to the gene of interest because it is transmitted along with the disease-producing gene. This information locates the chromosome position of the gene, allowing further work to be done to study its structure. With the completion of the Human Genome Project, it is predicted that tracking down the genes responsible for genetic defects will be a much faster process than before. Many more genetic markers have now been identified, which, in theory, should greatly enhance the techniques used to locate a faulty gene.

Key terms

• chromosomesstructures in haploid cells (eggs and sperm) that carry genetic information from each parent
• crossthe mating of parents to produce offspring during sexual reproduction
• genea sequence of base pairs that specifies a product (either RNA or protein); the average gene in bacteria is one thousand base pairs long
• linkagea relation of gene loci on the same chromosome; the more closely linked two loci are, the more often the specific traits controlled by these loci are expressed together
• meiosisthe process of nuclear division during sexual reproduction that produces cells that contain half the number of chromosomes as the original cell
• sexual reproductionreproduction that requires fusion of haploid gametes, each of which contains one copy of the respective parent’s genes, as a first step

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