bio topic 4

Dominant allele Recessive allele Codominant alleles:

Dominant allele an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state Recessive allele: an allele that only has an effect on the phenotype when present in the homozygous state Codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote

4.3.2: Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.

Make sure to define letters used to represent alleles and offspring phenotype.

4.3.3: State that some genes have than two alleles (multiple alleles).

More than two alleles exist in a population. Eg → hair color (brown, red, blonde)

4.2.2: Define Homologous chromosomes:

chromosomes of the same size and shape which carry genes for the same characteristic at the same loci Any Y-chromosome = man Nondisjunction → 45 or 47 chromosomes (3 or 1 in a pair) Chromosome 13, 21 and sex

Heterozygous

having two different alleles of a gene

Homozygous

having two identical alleles of a gene

4.3.6: State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.

4.3.7: Define Sex linkage:

when a gene controlling a characteristic is located on the X or Y chromosome and so we associate the characteristic with gender

Carrier

an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele

4.3.11: Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.

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4.3.9: State that a human female can be homozygous or heterozygous with respect to sex-linked genes.

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Genome

the whole genetic information of an organism

4.4.4 Describe the application of DNA profiling to determine paternity and also in forensic investigation

A DNA sample is collected (blood, saliva, semen, etc.) and amplified using PCR Satellite DNA (non-coding) is cut with specific restriction enzymes to generate fragments Individuals will have unique fragment lengths due to the variable length of their short tandem repeats (STR) The fragments are separated with gel electrophoresis (smaller fragments move quicker through the gel) The DNA profile can then be analysed according to need Two applications of DNA profiling are: Paternity testing (comparing DNA of offspring against potential fathers) Forensic investigations (identifying suspects or victims based on crime-scene DNA)

4.3.10: Explain that female carriers are heterozygous for X-linked recessive genes.

A carrier is an individual which has both dominant and recessive alleles in their genotype. However, only the dominant allele is expressed in the phenotype. With X-linked recessive genes, only females can be carriers as the genes in question are located on the X-chromosome and not the Y-chromosome. Females have two X chromosomes, which means they can have both a dominant and a recessive allele of the gene. Males have only one X chromosome and can therefore only have one allele. This means that if the male has one recessive allele, it is expressed in the phenotype. Therefore, only women can be carriers of an X-linked recessive gene.

4.4.11 Define clone

A clone is a group of genetically identical organisms or a group of cells derived from a single parent cell

4.4.13 Discuss the ethical issues of therapeutic cloning in humans

Arguments for Therapeutic Cloning May be used to cure serious diseases or disabilities with cell therapy (replacing bad cells with good ones) Stem cell research may pave the way for future discoveries and beneficial technologies that would not have occurred if their use had been banned Stem cells can be taken from embryos that have stopped developing and would have died anyway (e.g. abortions) Cells are taken at a stage when the embryo has no nervous system and can arguably feel no pain Arguments Against Therapeutic Cloning Involves the creation and destruction of human embryos (at what point do we afford the right to life?) Embryonic stem cells are capable of continued division and may develop into cancerous cells and cause tumors More embryos are generally produced than are needed, so excess embryos are killed With additional cost and effort, alternative technologies may fulfil similar roles (e.g. nuclear reprogramming of differentiated cell lines)

4.3.8: Describe the inheritance of color blindness and hemophilia as examples of sex linkage.

Color blindness and hemophilia are both examples of sex linked recessive conditions. This means that the gene loci for these conditions are found on the X-chromosome, but not the Y chromosome. This means females may have two alleles for these characteristics, but males only have one (hemizygous). Because of this, males have a higher probability of expressing the trait. For example, a male can have hemophilia if the X-chromosome he receives from his mother contains that allele (Xh), regardless of whether she is homozygous recessive or heterozygous. A female, however, must have received the allele from both her parents. The same is true for color blindness. Hemophilia: XH = unaffected; Xh = affected Color blindness: XB = unaffected; Xb = affected

4.4.9 State two examples of current uses of genetically modified crops or animals

Crops 1. Engineering crops to extend shelf life of fresh produce Tomatoes (Flavr Savr) have been engineered to have an extended keeping quality by switching off the gene for ripening and thus delaying the natural process of softening of fruit 2. Engineering of crops to provide protection from insects Maize crops (Bt corn) have been engineered to be toxic to the corn borer by introducing a toxin gene from a bacterium (Bacillus thuringiensis) Animals 1. Engineering animals to enhance production Sheep produce more wool when engineered with the gene for the enzyme responsible for the production of cysteine - the main amino acid in the keratin protein of wool 2. Engineering animals to produce desired products Sheep engineered to produce human alpha-1-antitrypsin in their milk can be used to help treat individuals suffering from hereditary emphysema

4.4.3 State that gel electrophoresis of DNA is used in DNA profiling

DNA profiling is a technique by which individuals are identified on the basis of their respective DNA profiles Within the non-coding region of an individual's genome, there exists satellite DNA - long stretches of DNA made up of repeating elements called short tandem repeats (STRs) These repeating sequences can be excised to form fragments, by cutting with a variety of restriction endonucleases (which cut DNA at specific sites) As individuals all have a different number of repeats in a given sequence of satellite DNA, they will all generate unique fragment profiles These different profiles can be compared using gel electrophoresis

4.4.10 Discuss the potential benefits and potential harmful effects of one example of genetic modification

Example: Maize introduced with a bacterial gene encoding a toxin to the European Corn Borer (i.e. Bt Corn) Potential Benefits Allows for the introduction of a characteristic that wasn't present within the gene pool (selective breeding could not have produced desired phenotype) Results in increased productivity of food production (requires less land for comparable yield) Less use of chemical pesticides, reducing the economic cost of farming Can now grow in regions that, previously, may not have been viable (reduces need for deforestation) Potential Harmful Effects Could have currently unknown harmful effects (e.g. toxin may cause allergic reactions in a percentage of the population) Accidental release of transgenic organism into the environment may result in competition with native plant species Possibility of cross pollination (if gene crosses the species barrier and is introduced to weeds, may have a hard time controlling weed growth) Reduces genetic variation / biodiversity (corn borer may play a crucial role in local ecosystem)

4.4.2 State that, in gel electrophoresis, fragments of DNA can move in an electric field and are separated according to their size

Gel electrophoresis is a technique which is used to separate fragments of DNA according to size Samples of fragmented DNA are placed in the wells of an agarose gel The gel is placed in a buffering solution and an electrical current is passed across the gel DNA, being negatively charged (due to phosphate), moves to the positive terminus (anode) Smaller fragments are less impeded by the gel matrix and move faster through the gel The fragments are thus separated according to size Size can be calculated (in kilobases) by comparing against a known industry standard

4.1.4: Explain the consequences of a base substitution mutation in relation to the processes of transcription and translation, using the example of sickle-cell anemia.

Hemoglobin is a protein that carries oxygen and is found in erythrocytes (red blood cells). The hemoglobin protein has a quaternary structure made up of two alpha globin and two beta globin polypeptides. Sickle cell anemia is a genetic disease that is caused by a base substitution mutation in the beta globin gene. The beta globin polypeptide is made up of a linear sequence of 146 amino acids. The sickle cell mutation occurs in the part of the gene that codes for the sixth amino acid (glutamic acid) in beta globin. At this site in the sense strand of the beta globin gene, the nucleotides GAG are changed to GTG. This causes the antisense strand to change from CTC to CAC. This change is passed on to mRNA during transcription. The messenger RNA is supposed to have the codon GAG which codes for glutamic acid. In sickle cell anemia, the codon in the messenger RNA becomes GUG which codes for valine. During translation, this change causes valine to be placed into the linear sequence of amino acids in beta globin instead of glutamic acid. This change in the linear sequence of amino acids in beta globin causes it to change shape. The R group on glutamic acid has a negative charge and can form an ionic bond with a positively charged R group on a different amino acid which can help stabilize the tertiary structure of the beta globin polypeptide and therefore the structure of the hemoglobin protein. Valine has no charge. When glutamic acid is replaced with valine an ionic bond cannot be formed and the tertiary structure of the beta globin polypeptide, and therefore hemoglobin, can change. The hemoglobin cannot carry oxygen as efficiently, and the erythrocytes form a sickle shape and stick to other erythrocytes which can lead to blood clots.

4.4.1 Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA

PCR is a way of producing large quantites of a specific target sequence of DNA It is useful when only a small amount of DNA is avaliable for testing E.g. crime scene samples of blood, semen, tissue, hair, etc. PCR occurs in a thermal cycler and involves a repeat procedure of 3 steps: 1. Denaturation: DNA sample is heated to separate it into two strands 2. Annealing: DNA primers attach to opposite ends of the target sequence 3. Elongation: A heat-tolerant DNA polymerase (Taq) copies the strands One cycle of PCR yields two identical copies of the DNA sequence A standard reaction of 30 cycles would yield 1,073,741,826 copies of DNA (230)

4.4.5 Analyse DNA profiles to draw conclusions about paternity or forensic investigations

Paternity Testing: Children inherit half of their alleles from each parent and thus should possess a combination of their parents alleles Forensic Investigation: Suspect DNA should be a complete match with the sample taken from a crime scene if a conviction is to occur

4.1.1: State that eukaryote chromosomes are made of DNA and proteins.

Proteins → histones

4.3.5: Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

The gender of an individual is controlled by the inheritance of sex chromosomes. These are the 23rd pair of chromosomes in a human karyotype and can be either X or Y. Males have an X and a Y chromosome; females have two X chromosomes. During fertilization, the ovum will always contain an X chromosome, as it comes from the mother. The sperm which fuses with the ovum is what determines gender. If the sperm contains a Y chromosome, the zygote will be male; if the sperm contains an X chromosome, the zygote will be female. As shown in the Punnet square below, there is a 50% chance that the offspring will be male and a 50% chance that the offspring will be female.

4.4.8 Outline a basic technique used for gene transfer involving plasmids, a host cell (bacterium, yeast or other cell), restriction enzymes (endonucleases) and DNA ligase

Restriction endonucleases are used to isolate a gene of interest, in this case the human insulin gene, from the human genome. When restriction endonucleases cut DNA at specific recognition (target) sequences, they leave uneven cuts called sticky ends. The human insulin gene will have sticky ends. A plasmid is removed from a bacterial cell. Plasmids are small circular pieces of DNA that are found in bacteria that are not part of the main chromosome. The plasmid should have one recognition site for the same restriction endonuclease that was used to isolate the human insulin gene. The plasmid is cut with the same restriction endonuclease so it will have sticky ends that are complementary to the sticky ends of the human insulin gene. The cut plasmid and the human insulin gene are placed together in a test tube with DNA ligase. The sticky ends of the human insulin gene will form hydrogen bonds with the complementary sticky ends of the plasmid, and DNA ligase will help seal the insulin gene into the plasmid by forming covalent bonds. This new plasmid is an example of recombinant DNA because it contains DNA from two sources (humans and bacteria). The recombinant plasmid is then placed inside a host cell (usually E. coli) by the process of transformation. Transformation is the process where a cell takes up DNA from the external environment. As the host bacteria reproduces, the plasmid with the human gene is copied. Soon you have millions (or billions) of bacteria containing the human gene. The final step is to get the bacteria to express (transcription and translation) the human gene and produce the protein (insulin) of interest. The protein can then be extracted from the bacteria for use in humans.

4.4.12 Outline a technique for cloning using differentiated animal cells

Somatic Cell Nuclear Transfer (SCNT) is a method of reproductive cloning using differentiated animal cells A female animal (e.g. sheep) is treated with hormones (such as FSH) to stimulate the development of eggs The nucleus from an egg cell is removed (enucleated), thereby removing the genetic information from the cell The egg cell is fused with the nucleus from a somatic (body) cell of another sheep, making the egg cell diploid An electric shock is delivered to stimulate the egg to divide, and once this process has begun the egg is implanted into the uterus of a surrogate The developing embryo will have the same genetic material as the sheep that contributed the diploid nucleus, and thus be a clone

4.3.12: Deduce the genotypes and phenotypes of individuals in pedigree charts.

Squares = male Circles = female Shaded = affected Horizontal line = marriage Vertical line = offspring Offspring age in descending order from left (oldest is furthest to the left)

4.4.7 State that, when genes are transferred between species, the amino acid sequence of polypeptides translated from them is unchanged because the genetic code is universal

The genetic code is universal, meaning that for every living organism the same codons code for the same amino acids (there are a few rare exceptions) This means that the genetic information from one organism could be translated by another (i.e. it is theoretically transferable)

4.3.4: Describe ABO blood groups as an example of codominance and multiple alleles.

There are three alleles for blood type: IA, IB and i. Blood type is controlled by the presence of antigens on red blood cells, and each allele codes for a different antigen. i is recessive and does not produce any antigen, causing blood type O (ii). IA and IB are dominant; they code for antigen A (IAIA or IAi) and antigen B (IBIB or IBi) respectively. IA and IB are also codominant, that is they both affect the phenotype when present in a heterozygote. In a person who is heterozygous for these alleles, both A and B antigens will be present, giving them blood type AB (IAIB).

4.1.2: Define Gene:

a heritable factor that controls a specific characteristic

4.2.1: State the meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

meiosis is a reduction division of a diploid nucleus to form haploid nuclei.

Allele:

one specific form of a gene, differing form other alleles by one or a few bases only and occupying the same gene locus as other alleles of the gene

Test cross

testing a suspected heterozygote by crossing it with a known homozygous recessive

4.4.6 Outline three outcomes of the sequencing of the complete human genome

the Human Genome Project (HGP) was an international cooperative venture established to sequence the 3 billion base pair (~25,000 genes) in the human genome The outcomes of this project include: Mapping: We now know the number, location and basic sequence of human genes Screening: This has allowed for the production of specific gene probes to detect sufferers and carriers of genetic disease conditions Medicine: With the discovery of new proteins and their functions, we can develop improved treatments (pharmacogenetics and rational drug design) Ancestry: It will give us improved insight into the origins, evolution and historical migratory patterns of humans

Genotype: and Phenotype

the alleles of an organism the characteristics of an organism

locus

the particular position on homologous chromosomes of a gene