Scope, Sequence, and Coordination
A Framework for High School Science Education
Based on the National Science Education Standards
The Human Genotype
Heredity, Traits, Genes, Chromosomes, and DNA
In all organisms, the instructions for specifying the characteristics of the organism are carried in DNA, a large polymer formed from subunits of four kinds (A, G, C, and T). The chemical and structural properties of DNA explain how the genetic information that underlies heredity is both encoded in genes (as a string of molecular Aletters@) and replicated (by a templating mechanism). Each DNA molecule in a cell forms a single chromosome.
Changes in DNA (mutations) occur spontaneously at low rates. Some of these changes make no difference to the organism, whereas others can change cells and organisms. Only mutations in germ cells can create the variation that changes an organismís offspring.
SS&C Inferred Generalization (p. 185)
Recombinations and crossing over are also factors affecting mutation rates.
Before DNA and chromosomes can be described, an understanding of variations, how these variations are induced, and the discreteness of these variations must be developed. This leads to correlating Mendelís observations with certain cellular events identified by Sutton.
Observing Traits. Examination of characteristics, the identification of a trait and how a trait varies, is of prime importance. Measuring the heights of humans provides a common experience for most individuals and lays the groundwork for distinguishing between continuous and discontinuous variations. These experiences lead to the concept of phenotype as an observable characteristic. No mention of DNA, genes, or chromosomes is necessary at this time. This type of study can be done in the middle level although it can be repeated in grade nine.
A question that evolves from this study of traits is one of whether traits are caused by heredity or caused by some environmental factor such as nutrition, exercise, or temperature. In addition, can there exist environmentally induced traits that can be passed on to subsequent generations? Studies of certain plant growth episodes and modern examples of this problem can be examined to provide evidence to reject these hypotheses. Lamarckism can be used as a hypothetical explanation for inheritance of traits. For example, the development of the giraffe=s long neck as explained by Lamarck would involve each generation stretching and developing its neck and passing the development onto its offspring. This example should include the line of evidence that leads to its rejection as a viable explanation.
By examining traits, it is easy to observe some that are discrete and others that seem to demonstrate a blending. Analysis of human pedigrees will often reveal that some characteristics skip generations and do not produce individuals with an intermediate version of the trait. Other characteristics seem to exhibit a blending, somewhat as the blending of colored light or pigments. Certain plants offer easy systems to observe this blending and discreteness. It is also important to realize that traits are not always present in a single generation and may or may not be associated with gender. These observations provide evidence for sex-linked traits to be discussed later.
Mendelian Laws and Chromosomal Behavior. Students need to derive an understanding of chromosomal behavior from direct experiences. They can begin with F2 generations of green albino corn seedlings, soybean mutant strains, Tenebrio variants, fast plants, and, of course, the time-honored fruit flies. These studies would focus on Mendel=s laws of segregation and independent assortment. Observing these Achance@ events will provide students with a direct method of deriving these laws.
After deriving these basic rules, one must deal with the exceptions, that is, ratios that depart from 3:1 and 9:3:3:1, and demonstrate that these exceptions are just variations of the basic rules. Epistatic ratios and linkage are the two simple but interesting exceptions. It is not the deviation from the rules of independent assortment that is important. Rather, it is the interaction of gene pairs to produce epistatic ratios and the spatial relationship of genes to the linear configuration known as the chromosome.
By grade 10, evidence providing a correspondence between chromosomes and these Mendelian laws needs to be developed. This involves some model building. Meiosis and the behavior of chromosomes during anaphase I and anaphase II provide an explanation for independent assortment. Models using pipe cleaners and other materials could be used to represent the different chromosomes and traits. Examining the haploid systems of Neurospora or Sordaris increases the depth of understanding of these Mendelian laws.
Differences in chromosome number in humans and the resulting syndromes are interesting phenomena to students. However, unless students understand how chromosomes are gained or excluded in gamete production these syndromes lose much of their significance. Nondisjunction in fruit flies can be connected to sex-linked traits as the ultimate correlation of genes with chromosomes.
All organisms including humans have a specific number of chromosomes that helps define the species. Humans have 22 pairs of autosomes and two sex chromosomes. Chromosomes carry genes that will determine the characteristics of organisms. In humans the two sex chromosomes have an additional functionCdetermining the sex of the individual. A female carries chromosomes designated as XX while a male carries chromosomes designated as XY. In the production of sex cells a female will produce an egg with 23 chromosomes. The 23rd chromosome will always be an X chromosome. Male sex cells or sperm will carry 23 chromosomes including either an X or a Y chromosome. If the X sperm cell fertilizes the egg the individual produced is a female (XX). If the Y sperm cell fertilizes the egg the individual produced is a male (XY). Therefore, in humans the male sex cells determine the sex of offspring.
Pedigree analysis of human sex-linked (X-linked) traits such as color vision, hemophilia, and glucose-6-phosphate dehydrogenase deficiency would provide evidence for the cellular basis of heredity. White-eyed fruit flies would also provide an example of this phenomenon. The mechanism of sex determination is also of great interest. What determines malenessCthe absence of the Y chromosome or the presence of only one X chromosome? Do other animals have the same system?
Sex cell production in humans is a cellular process produced by specialized glands in the human body. In males the testes are responsible for producing sperm, and in females the ovaries are responsible for producing eggs. Sex cells are produced by meiosis, a process that reduces the number of chromosomes to exactly one-half the number of the original cell. Meiosis ensures that each sex cell will carry one but only one representative of each chromosome in the adult. Upon fertilization of egg and sperm, the offspring produced carries exactly half the traits of the male parent and half the traits of the female parent. During chromosomal movement, chromosomes segregate evenly into the sex cells through the process of meiosis.
Genes. At the lower level of abstraction, the gene has been described as simply a trait. This description is crude and inadequate when one has some understanding of the molecular basis of genetics. By grades 11 and 12, it is conceptually appropriate for the gene to be defined in terms of function. Evidence from nutritional mutant strains in yeast or molds and the study of biochemical pathways with accumulation products (PKU, alkaptonuria) provide evidence for this further definition. The empirical evidence of one gene equaling one enzyme (or better, protein) work of Beadle and Tatum can be done with appropriate microorganisms. The focus of these experiments is not to just repeat them but to build a model of how a gene functions.
Because chromosomes exist in pairs, genes will exist in pairs. Genes coding for the same trait can exist in different forms. Genes can be expressed in the outward appearance of an organism, or genes may be concealed. If a gene is expressed it is said to be dominant. If the gene is concealed it is said to be recessive. Many other interactions of genes may occur, for example, as incomplete dominance or blended characters and as codominance, where both characters are expressed. Many other types of gene expressions result in producing the tremendous variations found in one species.
DNA Structure. Building a model of the double helix does not provide much information for the model of the gene. From this discussion, genes have been correlated with chromosomes; the next question is one of structure and function. Of all the various chemical compounds within a cell, which is the genetic molecule and how can its structure support the requirements of genetic function? Experiments such as those of Avery; Meselson, Stahl, and Hershey; and Chargaff are beneficial even to just read about. Finally, after all of this evidence has been gathered, the structure of DNA can be used to demonstrate that (1) it provides information for the production of the gene product; (2) it has coding ability; and (3) it has a self-replication mechanism.
DNA (and in some cases RNA) codes for all genetic traits in all organisms because it carries specific code sequences that are translated into traits through protein synthesis. The structure of the molecule allows for countless rearrangements of its units or codes. These codes arrange themselves into specific sequences that then determine traits. The arrangement of the sequences is infinite, and therefore the possibilities of different kinds of traits are unimaginable. An organism is made up of thousands of characteristics specific to its species. The genes that produce these traits are attached to chromosomes. Chromosomes are the structures that will carry traits from cell to cell and from parent to offspring. In order for genes to be copied, the DNA molecule must replicate itself. DNA replication is a process by which DNA can produce exact replicas of itself. Because of replication, identical genetic information is replicated and can be passed on from one generation to the next.
Mutations and Recombinant DNA. Recombinant DNA studies at this point would be appropriate and would provide students with experiences in manipulating the gene. Biotechnology and the importance of gene splicing can be easily discussed.
A sudden change in the genetic composition of an organism is called a mutation. Mutations are the agents for change in characteristics of any species and therefore are important for the origins of new species. Mutations are the direct result of structural changes of chromosomes or changes in the gene itself. Chromosomal changes include deletion, where a segment of a chromosome is missing, translocations, where a portion of a chromosome is broken off and attached to another chromosome, and inversions, where part of a chromosome is inverted with respect to the rest of the chromosome.
Structural changes in the gene result from substitution of one or more nucleotides of the genetic code. Regardless of the type of mutation that is produced, mutations provide variations from which natural selection can work. When mutations occur in the production of male and female sex cells, these mutations may be inherited through recombination of sex cells in fertilization.
Discontinuous variation, continuous variation, genetically determined variation, trait, phenotype, probability
Chromosomes, autosomes, sex chromosomes, sex cells, heredity, diploid, haploid, genotype, dominant and recessive, sex determination, sex-linked traits, mean, standard deviation
One gene equals one protein, DNA structure, DNA replication, DNA transcription, RNA function, protein synthesis, nucleus
Replication, amino acid sequence, code sequence, mutations, detection, inversion, translocation, point mutation, nucleotide, DNA structure, germ cells, recombination
Principle of segregation, principle of independent assortment, meiosis, one geneBone enzyme, Chargaff rule, sexual reproduction, life cycle, polygenetic inheritance
Chromosome theory of inheritance, chromosome mapping, semiconservative hypothesis, base-pair complementarity, chi-square model, punnet squares