Topic 3 Genetics
3.1. Chromosomes, Genes, Alleles and Mutations
3.1.1. State that eukaryote chromosomes are made of DNA and protein.
3.1.2. Define gene, allele, and genome.
- Gene: A heritable factor that controls a specific characteristic. It is a portion of the DNA.
- Allele: one specific form of a gene, differing from other alleles by one of few bases only and occupying the same gene locus as other alleles of the gene.
- Genome: the whole genetic information of an organism.
3.1.3. Define gene mutation
- A change in the base sequence of a gene. Point mutations affect only individual bases. Bases can be deleted, inserted or substituted.
3.1.4. Explain the consequences of a base substitution in relation to the process of transcription and translation , using the example of sickle cell anemia.
- Base substitution (gene mutation) from A to T in the triplet coding of a sixth amino acid. The mutation changes the allele HbA into a new allele, HbS
- One codon in the mRNA is different and therefore one amino acid in the polypeptide is altered. Therefore, the hemoglobin of people with sickle cell anemia contain valine instead of the normal glutamate.
- These valine may change the phenotype from normal. Sometimes the cells become sickle shaped (hence the name). The problem with this is that it might cause clotting since the cells hook on and clot areas. Also, the person might suffer a severe lack of oxygen in their body tissues.
- The allele HbS is quite common in southern Africa, because it actually helps resist malaria.
State that meiosis is a reduction division in terms of diploid and haploid chromosomes
Define homologous chromosomes
- Chromosomes which have the same arrangement of genes as each other. The actual base sequence of these genes may differ, however, resulting in different alleles of those genes. For example, homologous chromosomes carry a gene coding for eye-color on the same location, the actual information carried on these genes (the base sequence) may be different.
Outline the process of meiosis including pairing of chromosomes followed by two divisions, which results in four haploid cells.
- Homologous chromosomes pair up. Nuclear membrane will break down. Spindle fibres extended from each pole of the equator.
- The pairs of chromosomes line up on equator. Spindle fibres attach to different chromosomes in each pair ensuring that each is pulled to opposite poles.
- The spindle fibres pull the chromosomes to opposite poles halving the chromosome number. Each chromosome still consists of two chromatids.
- The cell membrane around the equator is pulled inwards to divide the cell to form two haploid cells. New spindle fibres grow from the poles to the equator.
- Steps 2 and 3 repeats (separating the chromatids).
- Four new haploid cells are formed. Nuclear membranes reform. Each nucleus now has half of many chromosomes as the nucleus of the parent cell.
Explain how the movement of chromosomes during meiosis can give rise to genetic variety in the resulting haploid cells
- The position of each pair of chromosomes in the nucleus when the spindle fibres become attached is random (random orientation). They attach to whichever chromosome of the pair is closer.
- Each pole could receive either chromosome of a pair. In total, each pole could receive four possible combinations of two chromosomes.
- The number of possible combinations of chromosomes that can be formed during random orientation is expressed through 2n (n being the number of pairs of chromosomes).
- Each combination is genetically different, so the movements of chromosomes in meiosis generate a lot of genetic variety.
Explain how the non-disjunction can lead to changes in chromosome number, illustrated by reference to Down's Syndrome (trisomy 21)
- Sometimes chromosomes that separate and move to opposite poles move to one pole instead. When chromosomes do not separate it is called non-disjunction.
- When gametes that contain an extra chromosome fertilize, the zygote produces three chromosomes of one type instead of the normal two. This is called trisomy of chromosome 21. This is when there are 3 chromosome 21's instead of just 2.
State Mendel's Law of Segregation
- Two alleles of each gene separate into different gametes.
Explain the relationship between Mendel's Law of Segregation and Meiosis
- This segregation occurs during meiosis.
3.3. Theoretical Genetics
3.3.1 Define: genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier, and test cross
Genotype- the alleles of an organism
Phenotype- the characteristics of an organism
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 it is present in the homozygous state
Codominant allele- pairs of alleles that both affect the phenotype when present in a heterozygote
Locus- the particular position of a gene on homologous chromosomes
Homozygous- having two identical alleles of a gene
Heterozygous- having two different alleles of a gene
Carrier- an individal that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele
Test Cross- testing a suspected heterozygote by crossing it with a known homozygous recessive
Construct a Punnett grid
Construct a Pedigree chart
State that some genes have more than two alleles (multiple alleles)
3.3.4 Describe ABO blood groups as an example of codominance and multiple alleles
Phenotype Genotype O ii A I^A I^A or I^A i B I^B I^B or I^B i AB I^A I^B
Outline how the sex chromosomes determine gender by referring to the inheritance of X and Y chromosomes in humans
State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans
Sex linkage is the association of a characteristic with gender, because the gene controlling the characteristic is located on a sex chromosome.
State two examples of sex-linkage
Color blindness and hemophilia- both these conditions are produced by a recessive sex-linked allele on the X chromosome.
State that a human female can be homozygous or heterozygous with respect to to sex-linkage genes.
Explain that female carriers are heterozygous for X-linked recessive alleles
Calculate and predict the genotypic and phenotypic ratio of offspring of monohybrid crosses involving any of the above patterns of inheritance
Deduce the genotypes or phenotypes of individuals in pedigree charts
3.4 Genetic Engineering and Other Aspects of Biotechnology
State that PCR (Polymerase Chain Reaction) copies and amplifies minute quantities of nucleic acid.
State that gel electrophoresis involves the separation of fragmented pieces of DNA according to their charge and size.
- Based on idea that molecules move at different rates (and directions) when placed in an electric field due to different charges and sizes.
State that gel electrophoresis of DNA is used in DNA profiling.
Describe two applications of DNA profiling.
- Paternity suits (determining parent-child relationships).
- Criminal investigations (using blood or semen collected from the scene).
- Identification of people long dead (e.g. Egyptian mummies, Russian Tsars).
- Everyone's DNA is unique and so can be used in identification; however, samples can be contaminated.
Define genetic screening.
- Determining experimentally if a person has a specific gene or not.
Discuss three advantages and/or disadvantages of genetic screening.
- Advantages include: Checking unborn babies for genetic disorders, can gain knowledge about one's own genotype.
- Disadvantages include: Ethical issues (e.g. selection, abortion(?))
State that the Human Genome Project is an international cooperative venture established to sequence the complete human genome.
- The human genome has 30,000 to 40,000 genes. The HGP is attempting to find the location of all these genes and their base sequence.
Describe two advantageous outcomes of this project
- Easier to study how genes control human development.
- Allow identification of genetic diseases and allow for the production of a drug based on the normal DNA sequences of genes.
State that genetic material can be transferred between species because the genetic code is universal
- The genetic code is universal. This means that genes can be transfered from one organism to another, (and this has indeed been the case). Organisms that have had genes transferred to them are called genetically modified organisms.
Outline the basic technique for gene transfer involving plasmids, a host cell (bacterium, yeast or other cell) restriction enzymes (endonucleases) and DNA ligase.
- Messenger RNA coded for insulin is extracted from human pancreas cells.
- DNA copies of the messenger RNA coding for insulin are made using the enzyme reverse transcriptase.
- Plasmids: are small loops of DNA found in bacteria.
- Plasmids are cut open using restriction enzyme endonucleouse.
- The insulin gene and the plasmid are mixed.
- DNA ligase seals up the plasmid.
- The plasmid with the human insulin gene is inserted into a recombinant plasmid.
- The recombinant plasmid are mixed with a strain of E.Coli bacteria.
- The E.coli bacteria start to make insulin which is then extracted, purified and used by patients suffering from diabities.
State two examples of the current uses of genetically modified crops or animals
- Golden rice is a genetically modified rice crop that produces beta-carotene, which can be metabolized into Vitamin A within the body. Scientists hope that golden rice will eventually be a cheap source of beta-carotene in malnourished countries, reducing the number of children worldwide that go blind from Vitamin A deficiency.
- Bt maize is a genetically modified corn crop that produces a toxin that kills European corn borers feeding on the maize.
Discuss the potential benefits and potential harmful effects of one example of genetic modificiation
- Bt Maize: Contains a gene which releases toxin that kills insects feeding on the maize.
- Advantages: Less pest damage, and therefore better harvests.
- Less land needed for crop production.
- Less use of insecticides.
- Disadvantages: Humans or animals that eat BT Maize might be harmed by bacterial DNA in it.
- Insects that are not feeding on the maize could be killed. The maize pollen, which contains the toxin, could be blown onto nearby plants.
- Wild plants might also adapt the same gene.
- Overuse of Bt maize would lead to toxin-resistant corn borers.
Outline the process of gene therapy using a named example
- Gene Therapy: is the treatment of a genetic disease by altering the genotype. It is is still very experimental. However, if the allele of a gene is recessive, then perhaps the dominant gene could be inserted to prevent that disease. SCID (Severe combined immune deficiency) is caused by lack of the enzyme ADA. It produces lymphocytes, without it, the body cannot fight diseases. The gene causing SCID is recessive.
- Genetic screening shows a baby has SCID.
- The gene that codes for ADA is obtained. The gene is inserted into a retrovirus.
- The retrovirus is mixed with stem cells. They enter and insert the gene into the stem cell's chromosomes.
- Stem cells containing ADA are injected back into the baby's blood stream.
"SCID-ADA-l(ymphcytes)" / "skedaddle"
- A clone is a group of genetically identical organisms or a group of genetically identical cells derived from a single parent.
Outline the technique for cloning using differentiated cells
- Udder cells are taken from a donor sheep. The genes in the cell are made dormant.
- Unfertilised eggs are taken from another sheep.
- The nucleus is removed from the egg cell.
- The egg cell without a nucleus is fused with the udder cell using a pulse of electricity.
- The fused cells develop into zygotes.
- Inserted back into mother.
Discuss the ethical issues of cloning in humans
- Arguments against cloning humans
- Psychological problems from being a clone.
- Clones might suffer diseases, and there is a higher risk of diseases or abnormalities.
- Arguments for cloning in humans:
- Happens naturally, so nothing new.
- Might make screening for genetic diseases easier.
- Greater chance for IVF.