Inheritance, Variation and Evolution

Sexual reproduction involves the fusion of male and female gametes. A gamete is a reproductive cell containing one set of chromosomes: sperm and egg cells are animal gametes, while pollen and egg cells are plant gametes. Sexual reproduction mixes genetic information from two parents, producing genetically different offspring. Asexual reproduction involves one parent, no fusion of gametes and only mitosis, producing genetically identical offspring called clones.

Meiosis is the type of cell division that produces gametes in reproductive organs. Before meiosis the genetic material is copied, then the cell divides twice to produce four genetically different gametes, each containing one set of chromosomes. Fertilisation joins two gametes and restores the normal chromosome number. Human body cells normally contain 46 chromosomes arranged in 23 pairs, whereas human gametes contain 23 chromosomes.

Sexual reproduction produces genetic variation, which may help populations survive changing environmental conditions and provides variation for natural selection and selective breeding. Asexual reproduction is faster, requires only one parent, uses less energy and can rapidly produce many offspring when conditions are favourable, but the lack of genetic variation may make a population more vulnerable to environmental change or disease.

DNA stands for deoxyribonucleic acid and is the genetic material found mainly in chromosomes in the nucleus of eukaryotic cells. DNA is a polymer made from two strands forming a double helix. Each strand contains repeating nucleotide units. A nucleotide contains a sugar, phosphate group and one of four bases: adenine (A), thymine (T), cytosine (C) or guanine (G). In complementary DNA strands A pairs with T and C pairs with G.

A gene is a small section of DNA on a chromosome that contains the information needed to produce a particular protein or influence a characteristic. The genome is the complete genetic material of an organism. Studying the human genome helps scientists identify genes associated with disease, understand inherited disorders and investigate patterns of human migration and ancestry.

Proteins are synthesised on ribosomes. The sequence of bases in a gene determines the sequence of amino acids used to build a polypeptide. A polypeptide is a chain of amino acids that folds into a particular three-dimensional shape to become a functional protein. The shape of a protein is important because it determines its function, for example the specific active site of an enzyme.

Protein synthesis involves using genetic information in DNA to determine the order of amino acids in a protein. At GCSE Higher Tier, students should understand that genetic information is transferred from DNA and used at ribosomes to assemble amino acids in the correct sequence. A change in the DNA base sequence can therefore alter the amino acid sequence, protein shape and ultimately the phenotype of an organism.

A mutation is a change in DNA. Mutations occur continuously and most have little or no effect on phenotype, but some alter the structure or amount of a protein produced. A mutation may therefore be harmful, neutral or occasionally beneficial. Mutations create new genetic variants and are an important source of variation on which natural selection can act.

An allele is an alternative form of a gene. The genotype is the combination of alleles an organism possesses, while the phenotype is the observable characteristic produced by the genotype together with environmental influences. A dominant allele is expressed when one or two copies are present, whereas a recessive allele is normally expressed only when two recessive copies are present. Homozygous means having two identical alleles and heterozygous means having two different alleles.

Genetic crosses can be represented using Punnett squares to predict possible genotypes and phenotypes of offspring. These predictions are probabilities, not guarantees. In humans, some disorders are caused by inherited alleles; polydactyly is caused by a dominant allele whereas cystic fibrosis is caused by a recessive allele. Genetic screening may identify inherited disorders, but its use involves medical, social, economic and ethical considerations.

Human sex is determined by the sex chromosomes. Females normally have XX chromosomes and males normally have XY chromosomes. The egg always contributes an X chromosome, whereas sperm may contribute either X or Y, so the sperm determines the genetic sex of the offspring. A Punnett square can be used to show that the probability of an XX or XY combination is approximately equal.

Variation means differences between individuals of the same species. Variation may be caused by inherited genes, environmental factors, or a combination of both. Genetic variation arises through mutation and sexual reproduction. Evolution is the gradual change in the inherited characteristics of a population over many generations. Natural selection favours individuals whose inherited characteristics provide a survival or reproductive advantage.

Natural selection occurs because individuals within a population show variation, more offspring are produced than can survive, and individuals with advantageous phenotypes are more likely to survive and reproduce. Their advantageous alleles are therefore more likely to be passed to the next generation. Over many generations these alleles become more common. If populations become sufficiently different that they can no longer interbreed successfully, a new species may form through speciation.

Selective breeding, sometimes searched as genetic breeding, is the human selection of organisms with desirable characteristics to reproduce. It can be used to produce disease-resistant crops, animals with higher meat or milk yields, animals with desirable temperament, or plants with particular flowers. Repeated breeding of closely related organisms can reduce genetic variation and increase the risk of inherited defects through inbreeding.

Genetic engineering modifies an organism's genome by transferring a useful gene from another organism. The required gene is isolated, inserted into a vector such as a bacterial plasmid, and transferred into target cells. Genetically modified crops may be made resistant to diseases, insects or herbicides, while genetically engineered bacteria can produce substances such as human insulin. Cloning produces genetically identical organisms through methods including plant tissue culture, cuttings, embryo transplants and adult cell cloning. Evolution is supported by fossils and antibiotic resistance; extinction occurs when no individuals of a species remain. Modern classification uses the Linnaean hierarchy, the three-domain system and evolutionary relationships to show how living organisms are related.