Topic B: Genetics and Evolution

This part explains how characteristics are inherited and how life has changed over millions of years. You move from the molecular level, DNA and protein synthesis, to whole-organism inheritance using genetic crosses, then to the sources of variation and finally to evolution by natural selection. DSE rewards clear genetic diagrams and accurate use of terms such as allele, genotype and phenotype, so precision matters as much as understanding.

DNA structure and replication

DNA is the molecule that carries genetic information. It is a double helix made of two strands, each a chain of nucleotides, with the strands held together by complementary base pairing: adenine pairs with thymine and guanine pairs with cytosine. The base sequence stores the instructions for making proteins. Before a cell divides, DNA is replicated so each daughter cell receives an identical copy. The two strands unwind and separate, and each acts as a template on which a new complementary strand is built, giving two identical double helices each containing one old and one new strand. This semi-conservative copying ensures genetic information is passed on accurately, while occasional errors are a source of mutation.

Genes, protein synthesis and the genetic code

A gene is a section of DNA that codes for one polypeptide or protein. The sequence of bases determines the sequence of amino acids in a protein, and because proteins include all enzymes, genes ultimately control the cell's chemistry and the organism's characteristics. In outline, the base sequence of a gene is first copied into messenger RNA in the nucleus (transcription), then the mRNA moves to a ribosome where the code is read in groups of three bases, each triplet specifying one amino acid (translation). A change in the base sequence can change the amino acid sequence and therefore the protein. You are expected to understand the gene-to-protein link at this level rather than memorise every molecular detail.

Monohybrid inheritance

Inheritance of a single characteristic controlled by one gene is studied using genetic diagrams. An allele is one version of a gene; an organism has two alleles for each gene, one from each parent. The genotype is the alleles present and the phenotype is the observable characteristic. A dominant allele is expressed even when only one copy is present, while a recessive allele is expressed only when both copies are present. In a monohybrid cross, you set out parental genotypes, the gametes, and a Punnett square or cross diagram to predict offspring ratios, for example a 3 to 1 ratio from two heterozygous parents. Always define your allele symbols clearly and state genotype and phenotype ratios separately.

Sex determination, codominance and sex-linkage

In humans sex is determined by a pair of sex chromosomes: females are XX and males are XY, so the father's gamete decides the sex of the child, giving a theoretical 1 to 1 ratio. Some characteristics show codominance, where both alleles are expressed in the heterozygote, as in the human ABO blood groups where alleles for A and B are codominant and both are dominant over O. Sex-linked characteristics are controlled by genes on the X chromosome, such as red-green colour blindness and haemophilia, so they appear more often in males who have only one X. You should be able to construct genetic diagrams for codominance and sex-linkage and explain why certain patterns appear.

Variation and mutation

Variation is the differences between individuals of the same species. Continuous variation, such as height or body mass, shows a range of values and is usually controlled by many genes and influenced by the environment, giving a smooth distribution. Discontinuous variation, such as blood group or the ability to roll the tongue, falls into distinct categories controlled mainly by genes. Genetic variation arises from mutation, the random change in the base sequence of DNA, and from the reshuffling of alleles during meiosis and fertilisation. Mutations are the ultimate source of new alleles; most are harmful or neutral but a few are beneficial. Mutation rate is increased by mutagens such as ionising radiation and certain chemicals.

Molecular genetics and biotechnology

Modern techniques allow scientists to study and manipulate DNA. Genetic engineering transfers a gene from one organism into another, for example inserting the human insulin gene into bacteria so they make human insulin, using enzymes to cut and join DNA and a plasmid as a vector. The polymerase chain reaction makes many copies of a DNA sample, and gel electrophoresis separates DNA fragments by size to produce patterns used in DNA profiling for identification and forensic work. You should understand the principles and applications, including the benefits and ethical concerns of genetically modified organisms, and be able to discuss issues in a balanced way rather than simply listing them.

Evolution and natural selection

Evolution is the gradual change in the inherited characteristics of a population over many generations. Darwin's theory of natural selection explains this: individuals vary, more offspring are produced than can survive, and there is a struggle for existence. Individuals with characteristics best suited to the environment are more likely to survive and reproduce, passing on the favourable alleles, so over time these alleles become more common in the population. Examples include antibiotic resistance in bacteria, pesticide resistance in insects, and the change in colour of the peppered moth during industrialisation. You should be able to construct a clear natural-selection argument and apply it to unfamiliar examples in the exam.

Evidence for evolution

Several independent lines of evidence support evolution. Fossils show that living things have changed over geological time and reveal intermediate forms linking groups. Comparative anatomy shows homologous structures, such as the pentadactyl limb, that share a basic plan despite different functions, suggesting common ancestry. The rapid evolution of antibiotic resistance in bacteria and pesticide resistance in insects provides directly observed evidence of natural selection acting today. More recently, comparing DNA and protein sequences shows that closely related species share more similar sequences, confirming relationships suggested by anatomy. When answering, link the evidence explicitly to common ancestry and selection rather than just describing the observations.