B6 Inheritance, variation and evolution

Reproduction can be sexual or asexual. Sexual reproduction involves two parents and the fusion of male and female gametes during fertilisation. This mixes genetic information from both parents, so the offspring show variation.

Asexual reproduction involves only one parent. There are no gametes and no fusion of cells, so there is no mixing of genetic information. The offspring are genetically identical to the parent and to each other. These identical offspring are called clones.

Gametes are produced by a type of cell division called meiosis, which makes genetically different cells. In animals, the gametes are sperm and egg cells. In flowering plants, the gametes are pollen and egg cells.

Asexual reproduction relies on a different type of cell division called mitosis, which produces genetically identical cells. Examples of asexual reproduction include daffodil bulbs, potato tubers, spider-plant plantlets, and a single-celled organism dividing into two.

A gardener takes a cutting from a prize-winning plant and grows a new plant from it. Will the new plant have the same flower colour as the parent?

Yes. Growing a plant from a cutting is a form of asexual reproduction.

Because only one parent is involved and the cells divide by mitosis, there is no mixing of genetic information. The new plant is a clone, so it will have the exact same alleles for flower colour as the parent plant.

A common mistake is thinking pollen is a plant's seed. Pollen is the male gamete, while the seed develops later from the fertilised egg.

Remember that plants can reproduce both sexually (using flowers to make seeds) and asexually (using structures like bulbs or tubers).

Sexual reproduction gives variety in the offspring; asexual reproduction gives clones with no variety, so one change (such as a new disease) can affect them all.

Cells in the reproductive organs (such as the testes and ovaries in animals, or anthers and ovaries in plants) divide by meiosis to make gametes.

First, the genetic information is copied. The cell then divides twice, producing four gametes. Each gamete has a single set of chromosomes. This is half the number of chromosomes found in a normal body cell. All four gametes are genetically different from each other.

During fertilisation, two gametes join together. This restores the full paired number of chromosomes. The fertilised egg then divides by mitosis to increase the number of cells. As the embryo develops, these cells differentiate into specialised types.

Halving the chromosome number during meiosis is vital, because without it the chromosome number would double every generation.

Meiosis halves the chromosome number to make gametes. Fertilisation restores the full number, and then mitosis makes the embryo grow.

A dog's body cells contain 78 chromosomes. How many chromosomes are in a dog's gamete, and how many gametes are produced from one cell dividing by meiosis?

Gametes contain half the body-cell number of chromosomes, so a dog's gamete has 78 ÷ 2 = 39 chromosomes.

Meiosis involves two divisions, so one cell produces four gametes.

Do not confuse meiosis with mitosis. Mitosis involves one division to make two identical cells with the full chromosome number (for growth and repair). Meiosis involves two divisions to make four different cells with half the chromosome number (for gametes).

Human gametes have 23 chromosomes, not 23 pairs.

The fertilised egg divides by mitosis, not meiosis.

The genetic material in the nucleus is a chemical called DNA. DNA is a polymer: two long strands twisted around each other to form a double helix.

DNA is packed into structures called chromosomes. Each chromosome is one very long DNA molecule.

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which are joined to make a specific protein.

The genome is the entire genetic material of an organism: all of its DNA.

Zooming in: the nucleus holds the chromosomes, each chromosome is a DNA double helix, and a gene is one short section of that DNA.

The whole human genome has now been studied. Understanding it matters in three ways:

1. Searching for genes linked to different diseases, so people at higher risk can be identified.

2. Understanding and treating inherited disorders, such as cystic fibrosis.

3. Tracing how humans migrated in the past, by comparing the DNA of people living in different regions.

A newspaper says: "Scientists found that people in two distant countries share many unusual sections of DNA." Which use of the human genome is this, and what might it suggest?

This is tracing human migration from the past.

Shared sections of DNA suggest the two populations have common ancestors, so people probably moved from one region to the other long ago.

Learn the size order: nucleus → chromosome → gene. A gene is not a whole chromosome.

DNA has two strands, not one, and it is not a protein: it carries the code for making proteins.

Genome means all the genetic material, not one chromosome or one gene.

Link any use of the genome you give to the context in the question.

Gamete: a sex cell with one copy of each chromosome. Allele: a different version of a gene.

Dominant allele (capital letter): shows even if only one copy is present. Recessive allele (lower-case letter): shows only when two copies are present.

Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb).

Genotype: the alleles present. Phenotype: the characteristic that shows.

A few characteristics are controlled by a single gene (e.g. fur colour in mice), but most come from several genes interacting.

In a Punnett square, put one parent's gametes across the top and the other's down the side, then fill in each box.

Each box is an equally likely offspring, so a result can be given as a fraction, a percentage or a ratio such as 3 : 1.

A Punnett square for two heterozygous parents (Bb × Bb). There is a 3 in 4 chance of black fur, and a 1 in 4 chance of brown fur.

A heterozygous black mouse (Bb) is crossed with a brown mouse (bb). Black (B) is dominant. Out of 40 offspring, how many are expected to be brown?

Gametes: B or b from the black parent; only b from the brown parent.

Punnett square: Bb, Bb, bb, bb.

2 out of 4 are bb, so the chance of brown fur is 1/2 (50%), a 1 : 1 ratio.

1/2 of 40 = 20 brown mice expected.

Genotype is the letters; phenotype is the feature you can see.

A 3 : 1 ratio is a probability, not a promise: a litter of 4 may not contain exactly 3 black mice.

Each fertilisation is independent, so earlier offspring do not change the chance for the next one.

Some disorders are caused by inheriting particular alleles.

Polydactyly (having extra fingers or toes) is caused by a dominant allele. One copy is enough to cause the disorder, so an affected person can be heterozygous (e.g. Dd) or homozygous (DD). An unaffected person must be homozygous recessive (dd).

Cystic fibrosis (a cell membrane disorder causing thick mucus) is caused by a recessive allele. Only a homozygous recessive person (ff) has the disorder. A heterozygous person (Ff) is a carrier: they have no symptoms but can pass the allele on.

Use a Punnett square to find the probability of inheriting a disorder.

Embryo screening detects alleles before birth, reducing suffering. It raises economic (cost of screening vs lifelong care), social (discrimination) and ethical (destroying embryos) issues.

Parents 1 and 2 are carriers (Ff) with no symptoms. Their daughter 4 has cystic fibrosis (ff), son 5 is a carrier (Ff) and son 3 is unaffected (FF).

A man is a carrier of cystic fibrosis (Ff). His partner has two normal alleles (FF). What are the chances that their child is a carrier, and that their child has cystic fibrosis?

Gametes: the man makes F or f; the woman makes only F.

Punnett square outcomes: FF, FF, Ff, Ff.

Carrier (Ff): 2 out of 4 = 1 in 2 (50%).

Cystic fibrosis (ff): 0 out of 4, so 0%. No child can inherit two f alleles.

Carriers have no symptoms at all. Cystic fibrosis is genetic, not caused by a virus.

A dominant disorder is not necessarily common. Polydactyly is rare despite being dominant.

Probabilities apply independently to each pregnancy. If the chance of an affected child is 1 in 4, having an affected child does not change the risk for the next.

Normal human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only, but one pair carries the genes that determine sex. These are called the sex chromosomes.

Females have two identical sex chromosomes (XX). Males have two different sex chromosomes (XY).

Two of the four boxes are XX (girl) and two are XY (boy), so each child has a 1 in 2 chance of being a boy.

When cells divide by meiosis to form gametes, the chromosome number halves. Every egg cell contains an X chromosome. Half of the sperm cells contain an X chromosome, and the other half contain a Y chromosome.

The sex of the child is determined by the father's sperm. If an X sperm fertilises the egg, the child is XX (female). If a Y sperm fertilises the egg, the child is XY (male).

A maternity unit expects 360 births next year. Use the sex-inheritance cross to predict how many boys are expected.

Gametes: mother X and X; father X and Y.

Punnett square: XX, XX, XY, XY, so the chance of a boy is 2 in 4 = 1/2 (a 1 : 1 ratio).

Expected boys = 1/2 × 360 = 180. The real number will probably be close to 180, not exactly 180.

Remember that the Y chromosome is smaller and is only found in males. Females never produce eggs with a Y chromosome.

A common mistake is to think that after having several children of one sex, a child of the opposite sex is 'due'. The chance is always 50% for every pregnancy.

Sex chromosomes are found in every body cell with a nucleus, not just in gametes.

Variation is the differences in characteristics between individuals of a population.

It has three causes: genetic variation (the alleles an individual has inherited, such as blood group or natural eye colour), environmental variation (conditions during development, such as a scar or the language spoken), or a combination of both (such as body mass or height).

All genetic variation originally arises from mutations (changes in the DNA). Mutations happen continuously.

The phenotype of an organism develops from its genome interacting with its environment.

To understand the cause of variation, scientists look at cases where one factor is fixed. Identical twins share exactly the same alleles, so any differences between them must be environmental. Plant clones are genetically identical, so differences in their growth are caused by their conditions.

Blood group is genetic. Body mass is affected by genes and environment. A tattoo is environmental. Explain how you could tell which cause applies to the length of a person's hair.

Ask whether the feature can change with conditions and whether it is inherited.

Hair length is changed by cutting (environment), but how fast hair grows and its natural thickness are partly inherited (genes).

So hair length is best described as a combination of genetic and environmental causes.

Most mutations have no effect on the phenotype. Some influence it, and very few determine it on their own.

Mutations happen at random all the time; they are not caused by an organism needing a new feature.

Identical twins and clones have the same alleles, so any difference between them is environmental.

Evolution is a change in the inherited characteristics of a population over time through a process called natural selection.

The theory of evolution states that all living species evolved from simple life forms that first appeared more than three billion years ago.

Natural selection happens in steps:

1. Variation already exists in a population because of random mutations.

2. Individuals with phenotypes best suited to the environment (like better camouflage against predators) are more likely to survive.

3. The survivors breed and pass on their advantageous alleles.

4. Over many generations, the advantageous allele becomes more common in the population.

Pale moths are eaten because they stand out on the soot-darkened tree. The better-camouflaged dark moths survive, breed, and become more common.

Arctic foxes have white winter fur. Explain how this feature could have become common in the population.

Foxes varied in fur colour because of random mutations.

Foxes with whiter fur were better camouflaged against snow, so they caught more prey and were less likely to starve.

They were more likely to survive, breed and pass on the alleles for white fur.

Over many generations, the allele for white fur became more common in the population.

Individuals do not evolve or adapt within their lifetime. A rabbit does not grow thicker fur because it is cold; the population changes over generations.

Mutations happen randomly, not because an organism 'needs' them to survive.

Survival of the 'fittest' means the best suited to the environment, not necessarily the physically strongest.

Speciation happens when two populations become so different they can no longer interbreed to produce fertile offspring.

Selective breeding, or artificial selection, is the process where humans choose plants or animals with a desired genetic characteristic and breed them. This has been done for thousands of years, since crops were first bred from wild plants and animals were domesticated.

Organisms are chosen for usefulness or appearance, such as disease resistance in food crops, animals that produce more meat or milk, domestic dogs with a gentle nature, and large or unusual flowers.

1. Choose parents from a mixed population that have the desired characteristic.

2. Breed them together.

3. From the offspring, choose those that best show the desired characteristic and breed them.

4. Repeat this process over many generations until all offspring show the characteristic.

A grower has a mixed population of pepper plants. A few plants are less affected by a leaf disease. Describe how she could produce a variety that resists the disease.

1. Choose the plants that are least affected by the disease and cross them.

2. Grow the seeds, expose the young plants to the disease, and choose the most resistant offspring.

3. Breed these together and repeat for many generations until all the offspring resist the disease.

Selective breeding does not add new genes to a species. It only selects for alleles that are already present. This is different from genetic engineering.

The process is slow and takes many generations; it is never finished in one generation.

Breeding close relatives is called inbreeding. This reduces genetic variation, making the breed more prone to disease and inherited defects. A whole population could be wiped out by a change in conditions or a new disease because there is little variation.

Genetic engineering is the process of modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.

Genes can be cut out of the chromosomes of humans or other organisms and transferred to the cells of different organisms.

Bacteria have been genetically engineered to produce human insulin, providing large amounts of cheap, safe insulin to treat diabetes. Medical researchers are also exploring gene therapy to treat inherited disorders.

In agriculture, genetically modified (GM) crops can be engineered to be resistant to diseases, insect attack or herbicides (weedkillers). This generally gives higher yields.

There are concerns about GM crops. These include the effect on populations of wild flowers and insects (like resistance genes spreading to wild relatives), ethical objections, and that the long-term effects of eating GM crops on human health are not fully explored.

A company makes a GM tomato plant that carries a gene from another species, so it produces bigger fruit. Give one benefit and one concern.

Benefit: more food from the same area of land, so a higher yield for the farmer.

Concern: the gene might spread to wild plants, or some people feel the long-term health effects of eating GM food have not been fully explored.

Do not confuse genetic engineering with selective breeding. Genetic engineering is much faster and can add a gene from a completely different species.

In evaluation questions, give a benefit AND a risk, and only claim what the data actually show.

GM crops: resistant to insects, herbicides or diseases, or with bigger and better fruit. GM bacteria: make human insulin.

Evolution by natural selection is widely accepted. It is supported by genetics, the fossil record and the evolution of antibiotic resistance in bacteria.

Fossils are the remains or traces of organisms from millions of years ago, found in rocks. They show how much or how little organisms have changed.

Mineral replacement: the remains are buried in sediment, minerals slowly replace the bone, and later the rock wears away to expose the fossil.

Fossils form in three ways:

1. No decay: Parts have not decayed because a condition for decay is absent (e.g. insects in amber, mammoths in ice).

2. Mineral replacement: Hard parts like bones are replaced by minerals as they decay.

3. Traces: Preserved traces are left behind, such as footprints or burrows.

The fossil record is incomplete. Early life forms were mostly soft-bodied, so they left few traces. Also, many traces were destroyed by geological activity (rocks folding or melting). Therefore, scientists cannot be sure how life began.

Shark teeth 50 million years old are common fossils, but fossils of jellyfish-like animals 550 million years old are very rare. Suggest two reasons.

Teeth are hard and decay slowly, so they are often preserved or replaced by minerals; jellyfish-like animals were soft-bodied and decayed before they could fossilise.

Older rocks have had longer to be folded, worn away or melted by geological activity, destroying more of the fossils they held.

The three types of evidence for evolution: genes (how characteristics are inherited), the fossil record, and antibiotic resistance in bacteria (evolution seen in our lifetime).

Know the three ways fossils form and an example of each: no decay (amber, ice, peat bog), mineral replacement (bones, shells), traces (footprints, burrows, root traces).

Deeper rock layers are usually older, so their fossils are usually older too.

A species is extinct when no individuals of it are left alive anywhere.

Factors that can cause extinction: new predators, new diseases, more competition (for example from an introduced species), habitat destruction, climate change, or a single catastrophic event such as a huge volcanic eruption or an asteroid impact.

How antibiotic resistance evolves:

1. Bacteria reproduce very fast, so they can evolve quickly.

2. A random mutation produces a new strain that is resistant to an antibiotic.

3. The antibiotic kills the non-resistant bacteria, but the resistant ones survive and reproduce.

4. The resistant strain spreads, because people are not immune to it and there is no effective treatment. MRSA is one such strain.

The antibiotic does not cause the mutation: it removes the non-resistant bacteria, so the resistant ones take over.

Why should antibiotic use on farms be restricted?

Giving antibiotics to large numbers of animals exposes huge numbers of bacteria to them.

Any resistant bacteria survive and reproduce, so resistant strains become more common and can spread to people.

To slow resistance: doctors should not prescribe antibiotics for viral or non-serious infections; patients should finish the whole course so no bacteria survive to form resistant strains; farm use should be restricted.

New antibiotics are slow and costly to develop, so they are unlikely to keep up with new resistant strains.

Never write that bacteria 'learn' or 'get used to' an antibiotic.

Carl Linnaeus classified living things into groups based on their structure and characteristics. The groups from largest to smallest are: kingdom, phylum, class, order, family, genus and species.

Organisms are named by the binomial system. This gives each organism a two-part name: its genus followed by its species. The genus starts with a capital letter and the whole name is written in italics, for example Panthera leo.

New models of classification were proposed as science developed. Improvements in microscopes allowed scientists to see internal structures, and their understanding of biochemical processes improved.

Using chemical analysis, Carl Woese proposed the three-domain system:

Archaea: primitive bacteria that often live in extreme environments like hot springs or very salty water.

Bacteria: true bacteria.

Eukaryota: protists, fungi, plants and animals.

Each branch point is a common ancestor. A and B share the most recent common ancestor (8 million years ago), so they are the most closely related; D split off earliest, 30 million years ago.

The brown bear is Ursus arctos and the polar bear is Ursus maritimus. What can you say about how they are classified?

Both names begin with Ursus, so they are in the same genus but are different species.

Because they share a genus, they must also share every larger group: kingdom, phylum, class, order and family. So they are closely related.

A species is the smallest classification group, not the largest.

Evolutionary trees use current classification data for living organisms and fossil data for extinct ones.

Organisms on neighbouring branch tips do not evolve from each other. For example, humans did not evolve from chimpanzees; instead, they both evolved from a shared common ancestor.

Higher tier: construct a Punnett square from the information given, then use it to predict probabilities as fractions, percentages or ratios.

Genetic engineering adds a gene from another organism to an organism's genome to give a desired characteristic.

The main steps of genetic engineering are:

1. Enzymes are used to cut out (isolate) the required gene.

2. The gene is inserted into a vector, usually a bacterial plasmid or a virus.

3. The vector is used to insert the gene into the required cells.

4. The gene is transferred at an early stage of development (such as an embryo) so the organism develops with the desired characteristic in all of its cells.

A plasmid is a small circle of DNA found in bacteria. Once the new gene is inserted, the bacteria multiply and all the new cells carry the gene.

A guinea pig breeder wants to know whether a rough-coated guinea pig (rough R is dominant) is RR or Rr. She crosses it with a smooth-coated (rr) guinea pig. What results would show each genotype?

If it is RR: every gamete carries R, so all offspring are Rr and rough.

If it is Rr: half its gametes carry r, so the Punnett square gives Rr, Rr, rr, rr: a 1 : 1 ratio of rough to smooth.

Even one smooth offspring shows the parent is Rr. A large number of rough-only offspring suggests (but cannot prove) RR.

Each fertilisation is independent: earlier offspring do not change the chance for the next one.

If you already know an offspring shows the dominant phenotype, only three of the four boxes are possible (for Rr × Rr, the chance of RR is then 1/3).

Enzymes cut the gene out; the vector (plasmid or virus) carries it into the cells.