Inheritance, variation and evolution

1. Two ways to make offspring

Reproduction makes new organisms. In sexual reproduction, two sex cells called gametes join. This joining is fertilisation. In animals, sperm joins an egg. In flowering plants, the male gamete comes from pollen and joins an egg cell.

Each gamete carries genetic information. Joining gametes mixes information from two parents, so the offspring differ from one another. The gametes are made by a type of cell division called meiosis.

In asexual reproduction, there is one parent and no joining of gametes. Cells divide by mitosis. The offspring have the same genetic information as the parent: they are clones, apart from any new mutations.

2. Halving the chromosomes, then restoring them

A chromosome is a structure made of DNA. Human body cells normally have 46 chromosomes arranged in 23 pairs. Gametes have one chromosome from each pair: 23 chromosomes.

1. A cell in a reproductive organ copies its genetic information.
2. The cell divides twice during meiosis.
3. Four gametes form. Each has a single set of chromosomes, and the gametes are genetically different.

At fertilisation, a sperm and egg join: 23 + 23 = 46. The first cell of the new organism therefore has the normal number. It divides by mitosis to increase the number of cells. Cells then differentiate — become specialised for different jobs — as the embryo develops.

3. DNA, genes and the genome

DNA is the chemical that carries genetic information. It is a polymer: a large molecule made from many smaller units joined together. Two strands twist around each other to form a double helix, like a twisted ladder.

In the nucleus, DNA is organised into chromosomes. 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. Proteins help build the body and carry out jobs such as acting as enzymes.

The genome is all the genetic material of an organism. Think of a gene as one set of instructions and the genome as the complete collection.

Studying the human genome helps scientists:

• find genes linked to diseases;
• understand inherited disorders and develop possible treatments;
• compare DNA to investigate human migration in the past.

A genetic result can show increased risk, but it does not always predict whether someone will become ill. Other genes and the environment can also matter.

4. Making sense of inheritance words

An allele is a version of a gene. For most genes, you inherit one allele from each parent. Your pair of alleles is your genotype. The characteristic that develops is your phenotype.

For a simple example, suppose a mouse’s fur colour is controlled by one gene. Let B be a dominant allele for black fur and b a recessive allele for brown fur.

A dominant allele affects the phenotype when just one copy is present. A recessive allele affects the phenotype in this two-allele example only when both copies are recessive.

Homozygous means the two alleles are the same. Heterozygous means they are different.

5. Read a genetic cross one step at a time

A Punnett square shows possible allele combinations when gametes join. Each gamete carries just one allele of the gene.

Here, two heterozygous black mice are crossed: Bb × Bb. Each parent can produce B gametes or b gametes. Combine the row and column letters in each box.

The four equally likely combinations are BB, Bb, Bb and bb. The genotype ratio is 1 BB : 2 Bb : 1 bb. The phenotype ratio is 3 black : 1 brown.

Probability of brown fur = 1 ÷ 4 = 0.25 = 25%. Out of 80 offspring, the expected number with brown fur is 0.25 × 80 = 20.

This is a prediction, not a guarantee. Each fertilisation is a new event. Four offspring do not have to include exactly three black mice and one brown mouse.

6. When an allele causes an inherited disorder

Some disorders are caused by inherited alleles. Two examples show why dominant and recessive inheritance matter.

A carrier has one copy of a recessive disorder allele. They do not have that disorder but can pass the allele to a child. Two carriers of cystic fibrosis can have an affected child if each passes on f.

A family tree shows how a characteristic appears across generations. Read its key before drawing conclusions. In this example, shaded symbols show cystic fibrosis.

Embryo screening checks embryos for particular genetic conditions before an embryo is selected for transfer to a uterus during IVF. It can reduce the chance of a child inheriting a tested disorder. It does not guarantee a child will have no health problems.

Decisions involve evidence and values. Families may consider the severity of a disorder, emotional effects, cost and access to treatment. Some people object to selecting embryos or to embryos not being used. A fair evaluation explains both potential benefits and concerns without assuming everyone will make the same choice.

7. Sex chromosomes: a probability example

Human body cells normally have 23 pairs of chromosomes. One pair is the sex chromosomes. In the usual pattern, females have XX and males have XY.

An egg carries X. A sperm carries X or Y. The cross below shows the usual inheritance pattern.

Half the boxes are XX and half are XY: a 1 : 1 ratio and approximately 50% probability of each for each fertilisation. This does not mean every family has equal numbers of boys and girls, or that earlier births change the probability of the next.

8. Why individuals are different

Variation means differences between individuals in a population. There is usually a lot of genetic variation within a species.

The genome interacts with the environment to influence the phenotype. For example, plants with the same genes may grow to different heights if one receives less light.

Mutations are changes in DNA. They happen continually and are the original source of new genetic variants. Most variants have no effect on the phenotype, some influence it, and very few determine it.

A mutation does not happen because an organism needs it. Very rarely, a new phenotype is helpful when the environment changes. The individuals with that phenotype may then leave more offspring.

9. Natural selection: which variants leave offspring?

Evolution is a change in the inherited characteristics of a population over generations. Natural selection can cause that change.

1. Individuals already vary. Some differences are inherited.
2. Conditions create a selection pressure, such as predators, disease or a shortage of food.
3. Individuals with a helpful inherited characteristic are more likely to survive and reproduce.
4. They pass the helpful allele to offspring.
5. Over many generations, the allele and characteristic become more common.

Imagine light and dark moths. On dark bark, dark moths may be harder for birds to see. More may survive to breed, so the dark-colour allele becomes more common.

The theory of evolution by natural selection explains how living species evolved from simple life forms more than three billion years ago. If populations become so different that they can no longer interbreed to produce fertile offspring, they are different species.

10. Selective breeding: people choose the parents

In selective breeding, humans choose which organisms reproduce because they want a particular inherited characteristic. It is also called artificial selection.

1. Choose parents with the desired characteristic from a varied population.
2. Breed them together.
3. Choose offspring that show the characteristic most strongly.
4. Breed those offspring and repeat over many generations.

Examples include disease-resistant crops, animals that produce more milk or meat, dogs with a gentle nature, and plants with large or unusual flowers. People have used selective breeding for thousands of years.

Benefits depend on the aim: more food, fewer losses to disease or a suitable temperament. But repeatedly using closely related parents causes inbreeding. Harmful recessive alleles can meet more often, making some breeds more likely to have inherited disorders. Reduced genetic variety can also make a population vulnerable to a new disease.

11. Genetic engineering: changing the instructions

Genetic engineering changes an organism’s genome by introducing a gene from another organism to give a wanted characteristic.

A useful example is putting the human insulin gene into bacteria. The bacteria can then make human insulin, which is collected and purified for use as a medicine. The bacteria do not become human: they have received a particular gene.

Genetically modified (GM) crops may resist insect attack, disease or a herbicide. Some are developed to give larger or better-quality fruit. Yields can increase because fewer plants are damaged or because weeds can be controlled.

Possible concerns depend on the crop and how it is grown. For example, reducing wild flowers may reduce food for insects. A useful evaluation weighs evidence about yield, cost, wildlife and health rather than assuming every GM crop has the same effects. Some people feel that possible long-term effects of eating GM food need further investigation.

Researchers are also investigating genetic modification to treat some inherited disorders. A possible benefit is tackling the genetic cause; reliable delivery and safety must be checked.

12. How do we know evolution happens?

Scientific explanations are tested against evidence. Several lines of evidence support evolution by natural selection:

• Genes: we understand how inherited characteristics pass from parents to offspring.
• Fossils: organisms in rocks of different ages show changes over time.
• Antibiotic resistance: bacterial populations can change over relatively short times.

These independent sources of evidence support one another. Evidence for inherited characteristics and the fossil record helped Darwin’s theory become widely accepted. Scientists continue to refine explanations when new evidence is found.

13. Fossils: clues preserved from the past

A fossil is a preserved remain or trace of an organism from the past, often millions of years old. Fossils can form in several ways:

• Parts do not decay because conditions needed for decay are missing, for example a lack of oxygen.
• Minerals replace parts of an organism as it decays.
• Traces such as footprints, burrows or root traces are preserved.

The fossil record is incomplete. Many early organisms were soft-bodied and left few traces. Other traces were destroyed by geological activity such as heat, pressure or erosion. Scientists therefore cannot be certain exactly how life first began.

Fossils can show how much organisms have changed or stayed similar. When you compare fossil data, check both the features and the ages. A gap in the record does not show that no organisms lived during that time.

14. When a species disappears

A species is extinct when no individuals remain alive anywhere. A population disappearing from one place is not necessarily global extinction.

Possible causes include rapid environmental change, a new predator, a new disease, stronger competitors or a catastrophic event. Human actions such as hunting and habitat destruction can add pressure.

Link the cause to survival and reproduction. For example, if a wetland dries out, an animal may lose its food and breeding sites. If too few survive and reproduce, its population may fall to zero.

15. Antibiotic resistance is evolution in action

Bacteria reproduce quickly, so their populations can change over many generations in a short time. A random mutation may give a bacterium resistance to an antibiotic.

1. A population contains variation, including a few resistant bacteria.
2. An antibiotic kills susceptible bacteria. Resistant bacteria survive.
3. Survivors reproduce and pass on resistance.
4. The proportion of resistant bacteria rises, and the strain may spread.

MRSA is an example of antibiotic-resistant bacteria. Resistance makes infections harder to treat. New antibiotics take time and money to develop, so development may not keep pace with resistant strains.

Unnecessary antibiotic use creates more opportunities for resistant strains to be selected. Antibiotics should not be used for viral infections or other situations where they are not appropriate. Patients should follow the prescribed course and advice, rather than stopping or changing treatment themselves. Agricultural use should also be restricted.

16. Putting living things into groups

Classification groups organisms using similarities and differences. Carl Linnaeus developed a system based on their structures and characteristics.

The groups run from broad to specific: kingdom → phylum → class → order → family → genus → species.

The binomial system gives each species a two-part scientific name: its genus and species. In Homo sapiens, Homo is the genus. The genus starts with a capital letter; the second word is lower case.

Organisms in the same genus are in the same family, but organisms in the same family need not be in the same genus. Classification helps people communicate clearly even when common names differ between countries.

17. New evidence can change classification

Better microscopes revealed more internal detail. Studies of chemical processes and genetic material gave scientists new evidence about relationships. Classification systems changed as a result.

Carl Woese developed a three-domain system using chemical evidence, including comparisons of genetic material. A domain is a very broad group.

Similar appearance alone can be misleading. Chemical and genetic evidence can reveal relationships that were not obvious from the outside.

18. Read an evolutionary tree

An evolutionary tree shows a scientific explanation of how groups are related. A branching point represents a shared ancestor. Groups that share a more recent common ancestor are more closely related.

Trees are built using current classification information, genetic and chemical comparisons, and fossils of extinct organisms. New evidence may lead to a revised tree.

Do not decide relatedness from how close the names sit on the page. Follow the branches back to the most recent shared branching point. Living species at the tips are not shown as ancestors of the other living species.

Use evidence from a genetic cross

In this example, B for black fur is dominant to b for brown fur. Two Bb mice have 80 offspring: 58 black and 22 brown. A Punnett square predicts a 3:1 phenotype ratio.

Read a family tree

Use the key, identify affected individuals and work back to the alleles their parents must have supplied. An unaffected phenotype does not always reveal a unique genotype.

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One final thought

Use a scientific idea and a specific example in your reflection.