Inheritance, Variation and Evolution

• Human cells contain 23 pairs of chromosomes in the nucleus. 23 from the father, and 23 from the mother.
• There are types of cells where the chromosomes are not paired and these are called gametes. An example of these are sperm and egg cells.
• Sexual Reproduction- Involves the fusion of male and female gametes, resulting in the formation of a zygote. Because there are two parents, the offspring are genetically different. Therefore, this produces variation.
• Asexual Reproduction- There is only one parent. Due to this, there is no mixing of genetic information and the offspring are genetically identical. Known as clones, as there is no variation.
• Continuous Variation- controlled by more than one gene or influenced by both genetic and environmental factors. No distinct categories and shows a normal distribution. Graph is usually a bell shaped curve. Examples include height, mass, number of leaves.
• Discontinuous Variation- usually controlled by one gene and not influenced by the environment. When graphed it shows distinct groups. Examples include blood groups, as there are only 4.
• DNA consists of two strands which wrap around eachother to form a double helix. It has a sugar-phosphate backbone, each base links to a base on the opposite strand in the helix. A always pairs with T, C always pairs with G. This is called complimentary base pairing.
• Gene- A small section of DNA found on a chromosome
• Alleles- Alternative versions of a gene that can be inherited from either parent. They determine what characteristics an organism will have.
• Phenotype- The physical expression of an individuals genotype.
• Genotype- The combination of alleles present in an individual's cells. Can be homozygous (both copies of the same) or heterozygous (different).
• Recessive Allele- An allele whose effect is masked by dominant alleles. Only expressed if an individual does not inherit any copies of the corresponding dominant allele.
• Dominant Allele- An allele that masks another when both are present in heterozygous condition. Example would be brown eyes (B) over blue eyes (b).
• Heterozygous Dominant- When one copy of a gene is dominant and one is recessive. For example Bb.
• The order of bases in a gene decides the order of amino acids in a protein. This determines the function and shape of the protein. Each amino acid is coded by a sequence of three bases in the gene called a triplet.
• A mutation is a change to DNA base sequences which can result in changes to proteins produced from genes. Mutations may occur spontaneously during cell division but they can also be caused by external factors such as radiation and chemicals.
• Cystic Fibrosis is a genetic disorder of the cell membrane. It is caused by the mutation of a single gene and results in the body producing lots of sticky mucus in the bronchioles and pancreas.
• Gene Therapy- involves altering a person's genes in an attempt to cure genetic disorders. As a treatment for cystic fibrosis, it involves inserting a normal copy of the non-mutated allele into the patient's lung tissue. An inhaler is used for this. It is not a cure as the cells are replaced over time.
• The allele which causes cystic fibrosis is recessive, so the person must inherit two copies of the allele to have the disorder. A person who is heterozygous is a carrier.
• Genome is the entire set of genetic material in an organism. In humans, there are about 20,000 different genes that code for proteins. The human genome project has mapped all these genes onto chromosomes.
• By understanding the human genome we can predict and prevent disease by knowing what genes are linked to it. We can understand and treat inherited disorders (cystic fibrosis) and better medicines.
• Drawbacks to the human genome- discrimination by life insurance companies and employers if you are more likely to get a disease, pressure not to have children, increased stress.
• Beneficial mutations- mutations which lead to a new phenotype, this can lead to evolution. Antibiotic resistance.
• GM crops- herbicide-resistant so farmers can spray their fields to kill weeds without affecting the crop. Bigger crop yield, resistant to disease and insect attack, and bigger and better fruit.
• Risks of GM crops- transplanted genes may get out into the environment- herbicide resistance could be picked up by weeds- reduced biodiversity, could affect human health and food chains.
• Ethical issues of GM crops- wrong to modify organisms purely for human benefit, irresponsible as we dont know long term effects, people believe GM might extend to humans.
• Genetic Engineering/ modification- to artificially transfer a gene responsible for a desired characteristic from one organism to another organism.
• Chromosomes are coiled up lengths of DNA molecules. DNA stands for deoxyribonucleic acid.
• Uses of genetic profiles- paternity testing, species classification, forensic science, medical diagnosis.
• Forensic scientists use DNA profiling to match blood or tissue samples found at crime scenes with suspects' DNA profiles.
• In medical diagnostics, doctors compare patients’ DNA profiles with those of known diseases to diagnose illnesses.
• Ethical issues of genetic profiling- invasion of privacy, data could be stolen, genetic profiling could be used to discriminate against people (insurance companies)
• Natural Selection- a mutation in a gene has led to a variation in the population of bacteria. Bacteria with the mutation are more likely to survive in the presence of antibiotics, whereas the non-resistant are more likely to die. Survival of the fittest. The bacteria with the more suitable characteristics reproduce and pass on their advantageous alleles to the next generation. This is repeated over many generations until the beneficial characteristics become the most common. The species then evolves.
• We can use a model to demonstrate how camouflaged organisms have an advantage against predation. First, arrange small pieces of plain and patterned card on a plain background. Then, use forceps to model a predator beak/jaw and record how many of each type of card you pick up in 15 seconds with the forceps. This model shows that you are more likely to select prey that is more easily seen (less camouflaged). This leaves the camouflaged organisms to breed and pass on their genes.
• Limitations of the natural selection model- The model prey does not move. The model background is flat not textured
• Genetic factors - These are characteristics that are caused by genes, inherited from parental DNA. For example, eye colour, blood type, being able to roll tongue, flower colour.
• Environmental factors - These are characteristics that change due to the environment. For example, language development, tattoos and piercings, competition in plants reducing access to light or water restricting growth.
• Variation - Small differences within a species make some individuals better adapted to the environment.
• If environmental change is too quick for species to adapt by natural selection extinction may occur.
• Variations in species that can be inherited (are heritable) are the basis of evolution. Charles Darwin and Alfred Russel Wallace worked on the theory of evolution by natural selection.