Biology 2404 A&P Basics Lab Exercise 25 Genetics Dr. Weis
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Students should be able to
* Work basic genetics problems
* Define related genetics terms
* Understand and describe the structure and location of DNA
* Discuss the heredity of genetic disorders
* Explain the similarities and the difference between mitosis and meiosis
Many biologists throughout the years have contributed important concepts and theories that form the foundation of classical genetics, modern genetics, and molecular biology. From the early studies of the cell and mitosis done by Flemming, Hertwig, and Weismann to the 1900's work on hereditary principles done by an Australian monk Gregor Mendel, the principles of classical genetics began.
Genetics is the study of heredity, the transmission of traits from parents to offspring.
The traits that are passed along are called genes and the mechanisms involve several types of inheritance patterns. Understanding genes and inheritance patterns can help explain many traits and conditions that are seen in health and disease. The expression of the genes can be regulated by the environment. The study of this control is called epigenetics and the current research shown has demonstrated the importance of chemical changes by methylation in turning various gene segments on and off.
Genes are regions of DNA that code for cellular proteins. Recall that DNA is a double stranded sequence of nucleotides consisting of a sugar-phosphate backbone with nitrogen bases that form base pairs between the two strands. These nucleotide strands are wound around regulatory proteins called histones and this combined structure is called a nucleosome. Nucleosomes are then coiled several times to make up helical fibers that eventually form the strands of chromatin that form chromosomes.
In most human cells, the nucleus contains 46 chromosomes, the 2n or diploid number. Chromosomes are paired and the process of identifying the 23 paired chromosomes is called karyotyping.
Drawing of DNA->Nucleosome->chromosome
Picture of Karyotyping and Karyotyping Activity
Recall that meiosis results in half the chromosome number for each of the gametes, the ova and the sperm. During fertilization, the haploid (half or 1n) number of chromosomes of the ova and the sperm combine to form the diploid (normal) chromosome number and establish the genetic makeup and sex of the individual. The fertilized egg called the zygote divides by mitosis which enables the continuation of the diploid (2n) chromosome number. The daughter cells of the zygote called blastomeres will eventually form the embryo and supporting tissues. Blastomeres are currently being used in stem cell research to grow various tissue types to help in treatments such as bone marrow replacement. Future research is looking at the possibility of growing complete organs from these stem cells.
The 46 chromosomes form 23 pairs of chromosomes called homologous pairs. One member of the pair is maternal because it comes from the egg (ovum) and the other member is paternal because it comes from the sperm (spermatozoa). Of the 23 pairs of homologous chromosomes, 22 pairs are autosomes and the last pair are sex chromosomes. In women these sex chromosomes are shown as XX and in men are shown as XY. Every diploid cell has two copies of a gene that is located at a specific location (loci) on a particular chromosome (1-23). The genetic copies may have a different molecular form and the various or different forms of a gene is called an allele. Therefore, you receive a pair of alleles for each trait, one from your mother and one from your father.
As stated previously, the alleles for each protein or trait may be the same or different in molecular structure. If the two alleles are the same, that is, if they code for the same information or same characteristic they are said to be homozygous for the trait. If the two alleles are different, they are said to be heterozygous for the trait. The alleles form the genetic make up of an individual and can have various expressions depending on the inheritance pattern and environmental factors.
The genotype of an individual is the actual genetic makeup and consists of the actual alleles that are present. The phenotype of an individual is the expression of the alleles to create the outward appearance of that trait. Genotypical and phenotypical ratios for the probability of a trait and its expression can be determined by using parent crosses shown in the Punnet square. For example, if the genetic trait being studied is height, the trait can be represented by “T” for tall. T is dominant over the recessive, dwarf, represented by “t”. If the genotype of a tall plant is homozygous dominant, the allelic representation would be TT. If the genotype of the plant is homozygous recessive, the allelic representation would be tt. If the genotype of the plant is heterozygous, its allelic representation would be Tt. If the inheritance pattern is complete dominant-recessive, then the dominant trait will mask or hide the recessive trait phenotypically.
TT phenotype is tall, tt phenotype is dwarf, and Tt phenotype is tall. If the inheritance pattern was incomplete dominance, then the heterozygous Tt would have a height in between the dwarf and the tall individual. The homozygous TT would still be tall and tt would still be short.
It may now be obvious that in order to understand
how these traits will be expressed, one
must know the type of inheritance pattern affecting the trait. The work to
determine the inheritance patterns was first identified by Gregor
Mendel who used pea plants to study seven distinct characteristics with two
well defined, easily recognized traits.
Mendel designed his experiments based on the previous research and findings of other scientists. He needed a species that could provide a large number of offspring in a short period of time and a large population sample to prevent distortion of observations. The pea plants growing in the Abby garden provided the needed research tool. The pea plants are self pollinating and could be handled mechanically to aid in the pollination process. Pea plants could also be “made” into male or female plants by removing either the stamens or stigmas. During his research, Mendel used monohybrid crosses to follow one trait at a time.
The following chart shows a summary of what he studied using pea plants.
| Characteristic | Trait | Trait |
| height | tall | dwarf |
| seed shape | round | wrinkled |
| seed color | yellow | green |
| flower color | red | white |
| flower position | axial | terminal |
| pod color | green | yellow |
| pod shape | inflated | constricted |
From his work, Mendel determined that heriditary characteristics are determined by factors (genes) and these factors (genes) occur in pairs. He also described two forms (alleles) of the traits as either dominant or recessive. The dominant alllele will be the one expressed as the outward trait (phenotype), while the recessive allele may be hidden in the phenotype, but passed on to the offspring in their genetic makeup (genotype). To help follow these allelic traits, the dominant allele is represented by a capital letter, such as "D" and the corresponding recessive allele is represented by a lower case letter "d".
As Mendel continued his work, he also noted that dominance is not always absolute, as shown when each allele expressed itself to produce an intermediate effect. An example of this would be a flower color in which red and white are blended to create pink.
In summary, the inheritance patterns seen are:
Complete Dominance-Recessive Dominant allele masks recessive allele
Co-Dominance Both alleles are expressed
Incomplete Dominance Blending of the heterozygous alleles creating an
intermediate between the dominant and recessive
Multiple Allele More than one trait is followed
Sex-Linked Trait linked to X or Y chromosome
The basis of Mendel’s experiments and conclusions can help us identify various traits, determine their probability for occurring in offspring, and provide the basis of genetic counseling for potential health problems and disorders. Today, the human genome project’s activities center around finding the total number of genes and the genetic codes for corresponding alleles that are located on each of the chromosomes. This has and will continue to have impact on medicine and future treatments.
In the exercises that follow, you will work genetics problems involving the various types of inheritance patterns, find genotypical and phenotypical ratios, see how genetic traits can be passed from one generation to the next, and understand the effects of genotype and phenotype in health and disease.
-gen, -genic to produce heter/o- other
poly- many hom/o- same
dactyl/o- fingers, toes tox/i- poison
co, con- together a, an- without
II. Genetics Information [What did you learn from this site?]
III. Genetics research web site [What did you learn from this site?]
congenital anomaly, birth defect
genetic counseling
karyotyping
Klinefelter’s Syndrome
Down’s Syndrome
Turner syndrome
Dominant diseases: Huntington’s Chorea
Incomplete Dominant diseases : Sickle Cell
Recessive traits : PKU, Cystic Fibrosis, Tay-Sachs, Schizophrenia
Gene therapy, Genetic engineering
lethal allele
mutation: frameshift, point (nonsense, silent)
Human Genome Project
Cri-Du-Chat syndrome
X-linked diseases : Duchenne’s Muscular Dystrophy, Hemophilia
Geneticist
http://www.nlm.nih.gov/research/visible/visible_gallery.html visible human project
http://www.sciencedaily.com/news/health_medicine.htm
http://esg-www.mit.edu:8001/esgbio/chapters.html
1. Define the following terms:
a. chromosome
b. gene
c. heterozygous
d. homologous
e. homozygous
f. allele
2. Name three types of inheritance patterns and give an example of each.
3. Explain the differences between genotype and phenotype.
4. What human cells do NOT have 46 chromosomes ?
5. What mature human cells do NOT have DNA ?
6. Why does a human have two genes for each protein or trait ?
