AQA

Life Processes and Living Things

Variation, Inheritance and Evolution

10.16 Variation	FT and HT	Young plants and animals resemble their parents (have similar characteristics) because of information passed on to them in the sex cells (gametes) from which they developed. This information is carried by genes. Different genes control the development of different characteristics.
		Candidates should be able, when provided with appropriate information, to explain: why Mendel proposed the idea of separately inherited factors that we now call genes; why the importance of Mendel's discovery was not recognised until after his death. Differences in the characteristics of different individuals of the same kind may be due to differences in: the genes they have inherited (genetic causes); the conditions in which diey have developed (environmental causes); or a combination of both. New forms of genes result from changes (mutations) in existing genes. Mutations occur naturally. The chance of mutations occurring is increased by: exposure to ionising radiations, including ultra-violet light, X-rays and radiation from radioactive substances; the greater die does of radiation, the greater the chance of mutation; certain chemicals. There are two forms of reproduction: sexual reproduction - which involves the joining (fusion) of male and female gametes; asexual reproduction - where there is no fusion of cells and only one individual is needed as the single parent. Asexual reproduction gives rise to individuals whose genetic information is identical with that of the parent. These genetically identical individuals are known as a clone. Sexual reproduction results in individuals that have a mixture of the genetic information from two parents. These individuals show more variation than offspring from asexual reproduction.
	HT	The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same genes as the parents. Sexual reproduction gives rise to variation because: the gametes are produced from the parental cells by meiosis; when gametes fuse, one of each pair of alleles comes from each parent; the alleles in a pair may vary and therefore produce different characteristics. Most mutations are harmful: if mutations occur in reproductive cells, young may develop abnormally or die at an early stage of their development; if mutations occur in body cells, these may start to multiply in an uncontrolled way and invade other parts of the body; this is cancer. Some mutations are neutral in their effects and, in rare cases, a mutation may increase the chances of survival of an organism and any offspring which inherit the mutant gene.
10.17 Genetics and DNA	FT and HT	In human body cells, one of the 23 pairs of chromosomes carries the genes which determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY). Chromosomes have long molecules of a substance called DNA. A gene is a section of a DNA molecule. For certain characteristics the characteristic is controlled by one gene. Some genes have two different forms called alleles. An allele which controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele. An allele which controls the development of characteristics only if the dominant allele is not present is a recessive allele. Some disorders are inherited: [Attention is drawn to the potential sensitivity needed in teaching about inherited disorders.] Huntington's disease — a disorder of the nervous system — is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder. cystic fibrosis — a disorder of cell membranes — must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder. sickle-cell anaemia is a disorder of the red blood cells which reduces the oxygen-carrying capacity of the blood. Being a carrier of the allele can be advantageous in countries where malaria is prevalent.
	HT	If both chromosomes in a pair contain the same allele of a gene, the individual is homozygous for that gene. If the chromosomes in a pair contain different alleles of a gene, the individual is heterozygous for that gene. Candidates should be able, when provided with appropriate information: to predict and/or explain the outcomes of crosses between individuals for each possible combination of dominant and recessive alleles of the same gene; to construct and/or interpret genetic diagrams. DNA contains coded information that determines inherited characteristics. DNA is made of very long strands which have four different compounds called bases. A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein. [Candidates are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place].
10.18 Controlling Inheritance	KS3	Selective breeding in agriculture has resulted in varieties of plants and breeds of animals that have increased yields.
	FT and HT	New plants can be produced quickly and cheaply by taking cuttings from older plants. These new plants are genetically identical to the parent plant. Cuttings are most likely to grow successfully if they are grown in a damp atmosphere until roots develop. We can use artificial selection to produce new varieties of organisms. We do this by choosing individuals which have characteristics useful to us and breeding from them. Selective breeding greatly reduces die number of alleles in a population. Widespread use of clones in agriculture also reduces the number of alleles available for further selective breeding. Selective breeding to produce new varieties for changed conditions may not then be possible. Modern cloning techniques include: tissue culture - using small groups of cells from part of a plant; embryo transplants - splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers. Genes from the chromosomes of humans and other organisms can be 'cut out' using enzymes and transferred to bacterial cells. The transferred gene continues to make the same protein in a bacterial cell. By culturing the genetically engineered bacteria on a large scale, commercial quantities of the protein can be produced. This process is used in the manufacture of drugs and hormones, including human insulin. Genes can also be transferred to the cells of animals or plants at an early stage in their development so that they develop with desired characteristics.
		Candidates should be able to make informed judgements about the economic, social and ethical issues concerning cloning and genetic engineering that they have studied or from information that is presented to them.
10.19 Evolution	FT and HT	Fossils are the 'remains' of plants or animals from many years ago which are found in rocks. Fossils may be formed in various ways including: from the hard parts of animals which do not decay easily; from parts of animals or plants which have not decayed because one or more of the conditions needed for decay are absent; when parts of the plant or animal are replaced by other materials as they decay; as preserved traces of animals or plants, e.g. footprints, burrows or rootlet traces. We can learn from fossils how much (or how little) different organisms have changed since life developed on Earth. The theory of evolution states that all species of living things which exist today — and many more which are now extinct — have evolved from simple life-forms which first developed more than three billion years ago. Evolution occurs via natural selection: individual organisms within a particular species may show a wide range of variation because of differences in their genes; predation, disease and competition cause large numbers of individuals to die; individuals with characteristics most suited to the environment are more likely to survive and breed successfully; the genes which have enabled these individuals to survive are then passed on to the next generation. Candidates should be able to: explain how fossils provide evidence for the theory of evolution; explain how over-use of antibiotics can lead to evolution of resistant bacteria. Candidates should be able, when provided with additional information, to interpret evidence relating to evolutionary theory. The environment that species need to be able to survive may change or successful new predators, new diseases or new competitors may arise. Unless evolution occurs, and species become better adapted to survive these changes, they may become extinct.
		Candidates should be able, when provided with appropriate information, to: suggest reasons why Darwin's theory of natural selection was only gradually accepted;
	HT	identify the differences between Darwin's theory of evolution and conflicting theories, e.g. Lamarck's; suggest reasons for the different theories.