What you'll learn
Inheritance, variation and evolution is the unit of AQA GCSE Combined Science: Trilogy that explains how characteristics are passed from one generation to the next and how species change over time. It begins with the molecule responsible — DNA, packaged into chromosomes inside the nucleus — and moves through sexual and asexual reproduction, genetic crosses and inherited disorders, to variation, natural selection, selective breeding, genetic engineering and the evidence for evolution. By the end you should be able to compare meiosis with mitosis, use a Punnett square to predict the outcome of a monohybrid cross, use the terms dominant, recessive, genotype, phenotype, homozygous and heterozygous correctly, explain Darwin's theory of natural selection and why it was slow to be accepted, and evaluate selective breeding and genetic engineering. This unit is assessed on Biology Paper 2, and the genetic cross questions are among the most reliably scoring marks on the paper if the notation is learned properly.
Key terms and definitions
Gene — a small section of DNA on a chromosome that codes for a particular sequence of amino acids, and so for a specific protein
Genome — the entire genetic material of an organism
Allele — one of the different forms a gene can take
Dominant allele — an allele that is expressed in the phenotype even if only one copy is present, written as a capital letter
Recessive allele — an allele expressed only when two copies are present, written as a lower case letter
Homozygous — having two identical alleles for a gene
Heterozygous — having two different alleles for a gene
Genotype — the combination of alleles an organism has
Phenotype — the physical characteristics that result from the genotype
Meiosis — the type of cell division that produces four genetically different gametes, each with half the number of chromosomes
Mutation — a random change in the DNA that may alter the protein produced
Natural selection — the process by which organisms better adapted to their environment survive, reproduce and pass on their alleles
Selective breeding — choosing organisms with a desired characteristic and breeding them together over many generations
Genetic engineering — modifying the genome of an organism by transferring a gene from another organism
Core concepts
DNA, genes and the genome
DNA is a polymer made of two strands forming a double helix, and it is contained in structures called chromosomes inside the nucleus. A gene is a small section of DNA that codes for a sequence of amino acids, which fold to form a specific protein.
The genome is all the genetic material of an organism. Understanding the whole human genome has three important applications you should be able to state: it helps scientists search for genes linked to particular diseases, it helps in understanding and treating inherited disorders, and it allows human migration patterns from the past to be traced.
Sexual and asexual reproduction
Sexual reproduction involves the fusion of male and female gametes — sperm and egg in animals, pollen and egg cells in flowering plants. Because there are two parents and the gametes are genetically different, the offspring show variation.
Asexual reproduction involves only one parent and no gametes, with all the cells produced by mitosis. The offspring are genetically identical clones, so there is no variation.
Each has advantages. Sexual reproduction produces variation, which gives a survival advantage if the environment changes and allows natural selection to act; it is also the basis of selective breeding. Asexual reproduction needs only one parent, is faster and uses less energy since no mate is required, and many identical offspring can be produced when conditions are favourable.
Some organisms use both. The malarial parasite reproduces asexually in the human host and sexually in the mosquito. Many fungi reproduce asexually by spores and sexually to give variation. Many plants produce seeds sexually and also reproduce asexually, as strawberry plants do by runners and daffodils by bulb division.
Meiosis
Gametes are produced by meiosis in the reproductive organs. The process begins with a cell copying its genetic information, then dividing twice. The result is four gametes, each with a single set of chromosomes — half the number of the parent cell — and each genetically different from the others.
At fertilisation the gametes fuse, restoring the normal number of chromosomes, and the new cell then divides by mitosis to form an embryo. Keeping meiosis and mitosis distinct is essential: mitosis gives two identical cells with the full chromosome number for growth and repair; meiosis gives four different cells with half the number, for reproduction.
Genetic crosses
Most characteristics are the result of many genes interacting, but a few are controlled by a single gene, and these are the ones used in crosses. Each person has two alleles for such a gene, one inherited from each parent.
A Punnett square sets one parent's gametes along the top and the other's down the side, and filling in the grid gives the four possible combinations in the offspring. Two heterozygous parents, each Bb, produce offspring in the ratio one BB, two Bb, one bb — a three to one ratio of the dominant phenotype to the recessive one.
Sex is determined by one pair of chromosomes. Females are XX and males are XY. Because the mother can only pass on an X, and the father passes on either an X or a Y, the probability of a male or female child is one half each time.
Inherited disorders
Polydactyly, in which a baby is born with extra fingers or toes, is caused by a dominant allele, so it can be inherited from just one parent who has the condition.
Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so a child must inherit it from both parents. A person with one copy is a carrier: they do not have the disorder but can pass the allele on.
Embryo screening allows embryos produced by in vitro fertilisation to be tested for the alleles of such disorders before implantation. The arguments are worth rehearsing. In favour: it reduces suffering, treating disorders costs the health service money, and there are laws to stop the process going too far. Against: it implies that people with genetic disorders are undesirable, the screened embryos that are not implanted may be destroyed, it is expensive, and there is concern that in future parents might select other characteristics.
Variation
Differences between individuals of the same species arise from differences in their genes, from the conditions in which they have developed, or most commonly from a combination of the two.
Mutations occur continuously. Most have no effect on the phenotype. Some influence it slightly. Very rarely, a mutation determines the phenotype so strongly that it gives a survival advantage, and this is where new variation useful to natural selection comes from.
Evolution by natural selection
The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago. Evolution occurs through natural selection, and the reasoning has four steps worth learning in order. There is variation within a population, caused by mutation. Individuals with characteristics best suited to the environment are more likely to survive. Those survivors breed successfully. The alleles for the advantageous characteristic are passed to the offspring, so over many generations they become more common in the population.
If two populations become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have become two separate species.
Darwin's theory was only gradually accepted, for three reasons you should be able to give. It challenged the idea that God made all the animals and plants. There was insufficient evidence at the time to convince many scientists. And the mechanism of inheritance and variation was not known until fifty years later, when genes were understood.
Evidence for evolution
Fossils are the remains of organisms from millions of years ago, found in rocks. They form when parts of the organism do not decay because one or more conditions for decay are absent, when parts are replaced by minerals as they decay, or as preserved traces such as footprints and burrows. The fossil record is incomplete because many early organisms were soft-bodied and left few traces, and because geological activity has destroyed much of what did form.
Antibiotic resistance provides evidence of natural selection happening now, and over a timescale short enough to observe. Bacteria mutate, resistant individuals survive treatment, and the resistant strain spreads.
Extinction occurs when there are no remaining individuals of a species still alive. New diseases, new predators, new competitors, catastrophic events and changes to the environment can all cause it.
Selective breeding and genetic engineering
Selective breeding is the process by which humans breed plants and animals for particular genetic characteristics. Parents with the desired characteristic are chosen from a mixed population and bred together, the best of the offspring are chosen, and the process is repeated over many generations. It is used for disease resistance in food crops, docility in animals, large or unusual flowers, and increased meat or milk yield. Its main drawback is inbreeding, which reduces the number of different alleles in the population and can make the group more vulnerable to a new disease and more prone to inherited defects.
Genetic engineering transfers a gene from one organism into another. Enzymes are used to cut out the required gene, which is inserted into a vector, usually a bacterial plasmid or a virus, and the vector introduces the gene into the required cells. In plants, genes are transferred at an early stage so that the whole organism develops the characteristic. Crops have been engineered to resist insect attack or herbicides, giving larger yields, and bacteria have been engineered to produce human insulin.
The concerns are also examinable: some people worry about the effect of engineered crops on populations of wild flowers and insects, and about long-term effects on human health that are not yet known.
Worked examples
Example 1: A monohybrid cross (4 marks)
In mice, black fur (B) is dominant to brown fur (b). Two heterozygous black mice are bred together. Determine the expected ratio of phenotypes in the offspring.
Both parents have the genotype Bb, so each produces gametes carrying B or b. Setting these in a Punnett square gives BB, Bb, Bb and bb. The genotype ratio is one BB to two Bb to one bb. Since B is dominant, the first three all have black fur and only bb has brown fur. The expected phenotype ratio is three black to one brown, or a 75 per cent probability of black.
Example 2: Working out a carrier (3 marks)
Two parents who do not have cystic fibrosis have a child who does. Explain how this is possible.
Cystic fibrosis is caused by a recessive allele, so the child must have inherited one copy from each parent and be homozygous recessive. Each parent must therefore carry one copy of the recessive allele alongside a dominant normal allele, making them heterozygous. Because the dominant allele is expressed, neither parent shows the disorder; they are carriers.
Example 3: Explaining natural selection (4 marks)
A species of moth living on pale tree bark is mostly pale, with a few dark individuals. Pollution darkens the bark. Explain what happens to the moth population over time.
There is variation in the population caused by mutation, with both pale and dark moths present. On darkened bark the dark moths are better camouflaged from predators, so they are more likely to survive. The surviving dark moths breed and pass on the alleles for dark colouring to their offspring. Over many generations the proportion of dark moths in the population increases.
Common mistakes and how to avoid them
The most common single error in this unit is careless case in genetic notation. B and b mean different things, and an answer written in ambiguous handwriting can lose every mark on a cross. Write capitals noticeably larger.
Students often describe an individual as having a disease when they mean carrying the allele. A carrier is heterozygous and healthy.
In natural selection answers, many students write that organisms adapt to their environment or that they evolve in order to survive. Individuals do not change; the population changes because the better-adapted individuals leave more offspring. Avoid any phrasing suggesting the organism chooses or tries.
Meiosis and mitosis are routinely swapped. Meiosis makes gametes and halves the chromosome number; mitosis makes body cells and keeps it the same.
Finally, in selective breeding questions, be careful not to describe it as genetic engineering. Selective breeding chooses parents; genetic engineering moves a gene between organisms.
Exam technique for "Biology: Inheritance, Variation and Evolution"
For any genetic cross, always draw the Punnett square even if you think you can do it in your head. Marks are available for the parental genotypes and for the gametes, so the working itself is worth more than the answer.
State probabilities in the form the question asks for. If it asks for a ratio, give a ratio; if it asks for a percentage or a probability, convert. Writing three to one when the question asked for a percentage can cost the mark.
Evaluation questions on embryo screening, genetic engineering and selective breeding follow the same pattern: give at least two points on each side and finish with a judgement referring to the specific context given.
When asked why Darwin's theory was only gradually accepted, give the historical reasons rather than criticising the science. Three reasons are expected and each is a separate mark.
Quick revision summary
DNA is a double helix in chromosomes; a gene codes for a protein and the genome is all of an organism's genetic material. Sexual reproduction uses gametes made by meiosis and produces variation; asexual reproduction uses mitosis and produces clones. Meiosis makes four genetically different gametes with half the chromosome number. Alleles may be dominant or recessive, and Punnett squares predict the outcome of crosses; two heterozygous parents give a three to one phenotype ratio. Sex is determined by XX in females and XY in males. Polydactyly is dominant and cystic fibrosis recessive. Variation comes from genes, environment or both, and from random mutation. Natural selection acts on that variation so advantageous alleles become more common, and enough change produces a new species. Evidence comes from fossils and from antibiotic resistance. Selective breeding chooses parents over many generations but risks inbreeding; genetic engineering transfers a gene using a vector such as a plasmid or virus.