Inheritance, variation & evolution

• Reproduction can be asexual, where there is only one parent and no gamete fusion, resulting in genetically identical offspring, or sexual, where there are two parents and the fusion of male and female gametes, resulting in a mix of genetic information and variation in offspring.
• Asexual reproduction is more time and energy efficient as it doesn't need to find a mate, only requires one parent, is faster, and makes best use of good conditions with identical offspring.
• Sexual reproduction produces variation, is more likely for some of the species to survive if the environment changes, and allows for selective breeding, which can increase food production.
• Meiosis is used in sexual reproduction and involves a diploid cell with two pairs of chromosomes, each chromosome replicates itself, separates and moves to opposite poles, divides, and copies.
• Four haploid cells (gametes) are left with half the number of chromosomes after meiosis.
• DNA, or deoxyribonucleic acid, is the chemical that makes up genetic material, found in the nucleus, contained in chromosomes which are made up of genes, each gene codes for a particular sequence of amino acids to make a certain protein.
• The Genome is the entire genetic material of an organism, the whole human genome has been studied and can be used to search for genes linked to certain disorders, help understand the cause of inherited disorders and how to treat them, investigate changes over time and how migration may have occurred.
• The theory of evolution is controversial as it challenges the religious idea that God made all organisms.
• Evolution: gradual change in inherited characteristics of a population over time, may lead to the formation of new species.
• Natural selection: most commonly supported theory, Charles Darwin, states all species have evolved from simple life forms that first developed more than 3 billion years ago.
• Selective breeding, also known as artificial selection, is a process by which humans breed plants and animals with desirable genetic characteristics, used by humans for thousands of years to produce food crops from wild plants and domesticated animals.
• Darwin observed that often organisms produce large numbers of offspring, populations stay the same size, and variation can be inherited.
• Evidence used to prove Darwin’s theory includes fossils, which are the remains of organisms from hundreds of thousands of years ago found in rocks, and resistant bacteria, which have high rates of reproduction and can undergo rapid evolution.
• Genetic engineering is a method of changing the characteristics of an organism by introducing a gene from another, more recently used.
• Darwin concluded that there’s a struggle for existence, with more born than survive, and those that survive and breed are best suited to the environment.
• Lamarck’s theory, an alternative to Darwin’s theory, is based on the idea that changes in an organism’s lifetime are inherited.
• Most mutations don't affect the phenotype, some do, and rarely a mutation can create a survival advantage.
• During the mid-20th century, scientists worked out the structure and mechanisms of genetics, but their work was not recognised in Mendel's lifetime as he was a monk in a monastery not a university scientist, and he didn't publish his work in a well-known book.
• Most characteristics are controlled by several genes, but some involve one (monohybrid inheritance), as predicted by genetic diagrams/Punnett squares.
• Late in the 19th century, chromosome behaviour in cell division was observed, leading to the realisation that Mendel's 'units' behaved similar to chromosomes and were genes.
• Embryo screening involves searching the DNA of an embryo to check if it carries the gene, but it raises ethical issues and can be expensive to screen.
• Sex determination involves one chromosome carrying the genes that determine sex, in females they're identical (XX), males they're different (XY), everyone inherits an X from their mother and an X or Y from their father, Y is shorter.
• Variation refers to the difference in characteristics of individuals within a population, due to genetics, environment or both, and sexual reproduction produces different combinations of alleles but new alleles only from mutations.
• Gregor Mendel investigated the belief that sexual reproduction blends characteristics in breeding experiments on pea plants, finding that characteristics are determined by inherited 'units' and not blended.
• Some disorders are caused by inheriting certain alleles, such as Polydactyly (extra fingers/toes), which is dominant, and Cystic fibrosis (cell membrane disorder), which is recessive.
• DNA structure consists of a polymer of repeating units (nucleotides), consists of a sugar, phosphate, a base (A, C, G or T), joined together to form long strands, molecules have 2 alternating sugar and phosphate strands twisted in a double helix, base attached to each sugar, attraction between bases holds the strands together, set pairs (C&G & A&T always together).
• Tissue culture involves growing a small group of cells from a plant to create a new one, which is used in nurseries commercially and to preserve rare species.
• Wallace proposed a theory of speciation, stating that populations are physically isolated, have large amounts of genetic variation between them, and natural selection operates differently between them, making successful interbreeding impossible.
• Embryo transplants involve splitting cells off embryos before differentiation and transplanting the embryos into host mothers.
• Evolutionary trees are a method to show how scientists think organisms are related, using current classification data for living organisms and fossil data for extinct organisms.
• Speciation is the development of new species, where two populations of one species that can’t interbreed to produce fertile offspring have formed two new species.
• Extinction occurs when no remaining individuals of a species are alive, caused by environmental change over geological periods, new predators, new diseases, new & more successful competitors, or a single catastrophic event.
• Adult cell cloning involves removing the nucleus from an unfertilised egg cell and inserting the nucleus from an adult body cell, which is then stimulated to divide by an electric shock, resulting in an embryo.
• Carl Linnaeus’ system is mainly used for classifying living things based on kingdom, phylum, class, order, family, genus, and species, with a Latin name based on the binomial system.
• Carl Woese proposed the 3 domain system due to new evidence, dividing organisms into archaea (primitive bacteria, usually in extreme environments), bacteria (true bacteria), and eukaryote (protists, fungi, plants & animals).
• Cloning is the production of genetically identical individuals, which can happen naturally in asexual reproduction or artificial methods can be used.
• DNA structure also consists of making proteins, where the order of bases on DNA controls the order amino acids are joined to make a protein, sequence of 3 bases codes for each amino acid, protein synthesis in ribosomes use template taken from the DNA, carrier molecules bring specific amino acids to add to the growing protein chain in the correct order, folds up to form a unique shape when the protein chain’s finished, allows proteins to do their job as enzymes, hormones or structural proteins (collagen).
• Mutations are changes in DNA structure, if any bases are changed it may change the order of amino acids coded for by the gene, mutations occur all the time, most don't make much difference, some may change shape- if its an enzyme the substrate might not fit, if it’s a structural protein it may lose strength.
• Genetic inheritance involves different forms of a gene, known as alleles, everyone has 2 alleles (1 from each parent), these can be dominant (always shows up) or recessive (only shows up if there’s no dominant), genotype is the combination of alleles present (bb), phenotype is how the alleles are expressed (characteristic that appears) (blue eyes), homozygous refers to the 2 alleles being the same (BB or bb), heterozygous refers to alleles being different (Bb).