• Overview
• Basic Genetic Concepts
• Patterns of Inheritance
• Variation and Mutation
• Selection and Breeding
• Genetic Modification
• Topic Questions

Overview

Basic Genetic Concepts

1. Genetic terminology and concepts

Genetics and heredity

• Genetics is the study of heredity in living organisms. Heredity is the way in which characteristics are inherited. Inherited characteristics are passed down from parents to offspring.

Genes

• Genes are short stretches of DNA which code for the production of a specific protein or enzyme. The genes of animals and plants are arranged into groups on structures called chromosomes.

Chromosomes

• A chromosome is a threadlike structure or strand of DNA in the nucleus of most living cells. Made up of nucleic acids and protein, chromosomes carry genetic information in the form of genes and are responsible for the transmission of hereditary information. Each species has a particular number of chromosomes (e.g. cattle have 60, humans have 46, peas have 14 and crayfish have 200).

Alleles

• The homologous chromosome pair has genes for a particular characteristic. But they are not necessarily identical because each member of the chromosome pair is derived from a different parent. The two equivalent genes derived from each parent are called alleles.
• Homozygous alleles have the same type of characteristic, for example both genes for height code for tallness.
• Heterozygous alleles that code for height will have an allele coding for tallness and one for shortness.

Genotype and phenotype

• We use different terms to describe the genes for a genetic characteristic and the appearance of the genetically determined characteristic.

The distinction between genotype and phenotype
Genotype

• The two alleles of the chromosome pair are referred to as the genotype for a characteristic. The alleles are represented by letters of the alphabet. Therefore the genotype is expressed by two letters to represent the characteristic of each member of the pair, for example TT, Tt or tt. The capital letter denotes a dominant gene and the small letter denotes the recessive or dominated gene.

Phenotype

• The phenotype is the physical expression of the genotype of an organism, in other words the appearance of the characteristic. So the phenotypes of TT, Tt and tt are tall, tall and short, respectively.

2. Monohybrid and dihybrid inheritance
In 1856 and 1868 Gregor Mendel, an Austrian monk, was the first person to use experiments to show how the inheritance of characteristics takes place. He did this by cross breeding pea plants with different characteristics. The laws that he discovered, which are named after him, revealed the basic mechanisms of inheritance.

Monohybrid inheritance – Mendel’s First Law
Mendel’s First Law, the Law of Segregation, was discovered when Mendel performed a monohybrid cross experiment. He selected one characteristic (or phenotype) and studied the result when two variations of this characteristic were crossed.

Details of monohybrid cross experiment
Mendel carried out monohybrid experiments with pea plants.

• In one of these experiments he crossed tall and short pea plants.
• In another experiment, he investigated the characteristic flower colour by studying the result of crossing pea plants with red flowers and white flowers.
  
  • In this experiment, Mendel cross-pollinated red flowered and white flowered pea plant parents (P).
  • He collected the seeds, planted them and then looked at the colour of the flowers of the progeny (F1).
  • He found that all the F1 progeny had red flowers, which indicated to him that red was a dominant characteristic.
  • But when he crossed the F1 peas with each other, 75% of the F2 generation had red flowers while 25% had white flowers.

Explanation of monohybrid cross experiment
Mendel wondered why the white flowers suddenly reappeared when the red ones had seemed to dominate. To explain the findings, we can represent the genotype of the F1 and F2 generations. We can assign a capital R to indicate the dominant red allele and a small r to indicate the recessive (dominated) allele.
Different combinations of these arise during meiosis when the gametes are formed, namely RR, Rr, rR and rr. So Mendel discovered that some genes have a dominance-recessive relationship.

Statement of Mendel’s First Law
Mendel First Law (Law of Segregation) states that the alleles for a specific characteristic (such as flower colour) separate during the process of meiosis.
They alleles recombine during fertilisation and result in various genotype combinations.

3. Three methods to illustrate genetic crosses
Genetic diagram of genetic crosses
Figure 1 represents the outcome of the genetic cross of two individuals. P stands for parent and the F represents the offspring from filial (daughter). Each successive generation is given a number: first offspring = F1; next offspring = F2 and so on.

Dihybrid inheritance – Mendel’s Second Law
Mendel’s Second Law (Law of Independent Assortment) was discovered when he decided to try a more complicated experiment and used pea plants with two unrelated characteristics. This is called a dihybrid cross experiment.
Details of dihybrid cross experiment
He chose plants with different seed appearances. They were yellow and smooth, or green and wrinkled.

• He cross bred the two varieties and grew pea plants from the progeny.
• All the F1 progeny had yellow, smooth seeds which indicated that these two characteristics were dominant.
• Then he crossed plants from these seeds with each other (F1 × F1).
• They produced four different seeds: 9 round yellow seeds, 3 round green seeds, 3 wrinkled yellow seeds and 1 wrinkled green seed.

Explanation of dihybrid cross experiment
This dihybrid cross result is very different from the results obtained with the monohybrid cross because of the larger number of genotypic combinations which are possible. The various allele combinations are shown in the Punnett square diagram on the next page. The experiment showed the appearance of two new phenotypes, namely the round green and wrinkled yellow seeds.

Statement of Mendel’s Second Law
Mendel's Second Law ( Law of Independent Assortment) states that there is independent reassortment and recombination of characteristics. This means simply that two unrelated characteristics operate separately from each other.

Schematic representation of genetic crosses
The cross between tall and short pea plants can also be shown by a schematic representation. This method shows the genotypes of the parents. This allows us to predict the genotypes of the offspring by representing the possible combination of alleles which result from the gametes. There are four possible outcomes of this cross.

Punnett Square
The Punnett Square is a grid or box which is used to predict the outcome of a particular genetic cross. It shows all the possible allelic combinations in a cross of parents with known genotypes. This allows the probability of offspring possessing certain sets of alleles to be predicted. The Punnett square is useful for crosses which involve more than one characteristic.

4. Independent recombination of characteristics
Mendel’s Law of Segregation states that:

• Every individual possesses a pair of alleles (assuming diploidy) for any particular trait.
• Each parent passes a randomly selected copy (allele) of only one of these to its offspring.
• The offspring then receives its own pair of alleles for that trait.
• Whichever of the two alleles in the offspring is dominant determines how the offspring expresses that trait (e.g. the colour and height of a plant, or the colour of an animal’s fur).

However, due to the fact that each parent passes a randomly selected copy (allele) to its offspring instead of a pre-defined copy, there is an independent recombination of characteristics (traits) arising from the passing on the alleles from parents to offspring.

• Therefore, the offspring can end up with any recombination of paternal or maternal chromosomes.

5. Quantitative and qualitative characteristics
Hereditary genetic characteristics, passed down from parents to their offspring, can be placed in two main categories.

• Qualitative characteristics
  
  • Characteristics such as eye colour or gender (i.e. whether the organism is male or female). In other words, qualitative characteristics involve a particular quality of the organism.
• Quantitative characteristics
  
  • Involve a quantity such as height, weight or milk production. The number genes an organism has for a particular trait determine this type of characteristic. This is called polygenic inheritance.
  • In this case, the genes have an additive effect. For example, if a person has a few genes for shortness and a few genes for tallness, then that person will be of medium height.

Patterns of inheritance

1. Patterns of inheritance that lead to different phenotypes
Geneticists later discovered that variations exist on the two patterns of inheritance discovered by Mendel. These lead to different phenotypes that cannot be explained by the simple dominant/recessive relationship shown by Mendel’s pea plant experiments.

Types of dominance
Mendel demonstrated simple dominant/recessive inheritance, but there are other types.

• Incomplete dominance
• Occurs when both alleles of a heterozygous pair influence the phenotype. This means that the phenotype is halfway between the two homozygous phenotypes.
• Co-dominance
• Is a pattern of inheritance in which both alleles are expressed equally in heterozygous individuals.

Multiple alleles
The example of Mendel’s tall and short pea plants involves two types of alleles, namely T and t. Some genetic characteristics involve more than two types of alleles. An example of a characteristic determined by multiple alleles is that of coat colour in rabbits. There are four different alleles that each code for a different amount of pigmentation: C+ codes for brown/wild type colouration, c for an albino/no pigmentation, Cch for grey/ chinchilla, and Ch for Himalayan/white with dark hair on ears, feet and noses. These various alleles have a dominance hierarchy. For example, C+ will dominate over all the others. Although there are four possible alleles for coat colour, a rabbit can only have two of these.

Polygenic inheritance
Polygenic inheritance occurs when a characteristic is determined by the additive effect of a number of genes. For example, milk production in dairy cattle is determined by the presence of a number of genes that influence the quantity of milk produced. Cows with a large number of genes for low production will be poor producers, while those with a large number of genes for high production will produce more milk.

Epistasis
Epistasis is the phenomenon which occurs when genes interact and hide the action of another. This can be seen in the example of coat colour in horses. As with rabbit coat colour, there are a number of alleles that can influence coat colour in horses. Horses have genes that block the pigmentation coded for by other genes. This results in the appearance of albino (no pigment) or the various other coat colours, namely red, black or brown.

2. Prepotency and atavism
These refer to the frequency at which genes appear in a population of plants or animals.

Prepotency

• Is a greater than normal ability of an organism to carry over their genes to their offspring. These can either be external characteristics or production characteristics.
• Prepotency is seen sometimes in pure bred animals because they have a large number of dominant genes.

Atavism

• Occurs when a homozygous recessive gene suddenly appears in a population.
• This can be shown by the sudden appearance of a red calf in herd of Black Angus cattle or the sudden appearance of a different coloured flower after several generations.

3. Sex chromosomes and sex-linked characteristics
Sex (gender) can be classified as a qualitative genetic characteristic.

• Geneticists have found that female humans and mammals have two X chromosomes while the males have one X chromosome and a small Y chromosome.
• Maleness or femaleness is therefore a genetic phenotype. Extra or missing X and Y chromosomes can cause various problems.
• Humans and animals have a 50:50 chance of producing male or female offspring, which is shown by the genetic diagram below.
• Sex-linked genes are located on the sex chromosomes.

Variation and mutation

1. Variation, mutation and selection

• Variation in populations causes offspring to be slightly different from their parents.
• Mutation is a change which takes place in the original DNA sequence of an organism during the process of replication.
• Selection is the process that breeds organisms for certain genetic characteristics.
• This is done by nature or by humans.

2. The importance of variation and selection
Changes in the DNA of an organism occur as a result of spontaneous mutations,
chromosome defects and sexual reproduction.

• These changes cause variation in individual organisms.
• Through a process of selection, variations can result in new breeds and varieties of plants and animals.

Internal and external causes of variation
Internal causes of variation

• These affect the composition of the DNA of an organism and are genetic
• Can also be caused by chromosomal abnormalities such as translocation, replication and deletion.
• All these internal causes of variation result in offspring having DNA that differs from that of their parents.

External causes of variation

• The environment causes non-genetic or non-hereditary variation of the individual.
• Genetic potential will only be reached in a suitable environment.
• External caused of variation are therefore an important consideration that agriculturalists must take into account.

3. Mutagenic agents, types of mutations and their effects
Approximately one DNA replication fault occurs in about every million divisions.

• DNA Mutations that take place during replication in the somatic cells will only change the individual.
• DNA mutations that occur in the gametes during meiosis will be inherited.

Mutations can be induced by certain substances called mutagens/mutagenic agents.

Types of mutagenic agents
A mutagen/mutagenic agent is a substance or process which causes a mutation or change in the structure of DNA. Examples of mutagens include chemicals and various types of radiation, such as x-rays and UV radiation.

• These mutagens increase the frequency of changes in genetic material because they cause mutations in DNA.
• Mutagens are also usually carcinogens because they alter the DNA structure of somatic cells.
• Mutagens have been used to alter organisms for the study of genetics.
• They are also used to produce plant mutants such as colour variations in ornamental flowers or new cereal varieties.
  
  • Chemicals
  • There are various mutagenic chemicals. They can be classified into three groups.
  • Base analogues are similar in structure to the purine and pyrimidine bases in DNA. This means that they can become incorporated into DNA during replication. The base analogue, 5-bromouracil (5BU), resembles thymine so it substitutes for this base and then pairs with adenine. At the next replication the 5BU is mistaken for cytosine and gets paired with guanine. An AT pair therefore becomes a CG pair.
  • Alkylating agents also induce base substitution. Alkylating agents do this by adding chemical groups to the molecules of the bases. Mustard gas is an example of an alkylating agent. It is sometimes used as a chemical weapon.
  • Intercalating agents are chemical agents that have a flat ring structure which becomes wedged between the bases. This is called intercalation and it tends to distort the helix. The distortion causes insertions or deletions to occur during DNA replication.
  • Radiation
  • Radiation such as X-rays and UV rays can break down DNA strands. A gene or chromosome can be lost during the repair process if both strands are broken. Radiation also produces dimers or unwanted bonds between bases which results in the insertion of two complementary bases instead of one.

Types of mutations
Single base loss/substitution

• A mutation that occurs when one base is replaced with another purine or pyrimidine.
• It is a spontaneous chemical change which occurs during replication.
  Apurination refers to the loss of a purine base by DNA. A base can also be replaced by the process of deamination which involves the loss of an amino group (NH2).
• The loss or substitution of a base causes a point mutation.

Strand slip-up

• Occurs when the DNA helix unzips.
• Both DNA chains are exposed and therefore replicated at the same time
• Sometimes one of the strands forms a loop which causes the insertion of bases on the one side and the deletion of bases on the other side.

Effects of mutagenic agents
Effect on DNA structure: Mutagens can affect the structure of DNA in two ways.

• Point mutation This occurs when one base pair in the DNA sequence is substituted for another. A point mutation usually has little effect on the individual gene where it occurs because it has a minor effect on the protein for which the gene codes.
• Sequence mutation: This occurs when a whole gene sequence changes. This causes a change in the protein for which the gene encodes. The effect of this mutation depends on the importance of the gene to the survival of the organism.

Effect on function: A mutagenic agent can result in a loss or gain of function.

• Loss of function: A mutation can severely affect gene function or even cause a gene to stop functioning. Indels often cause a loss of function.
• These mutations are usually recessive because the other allele of the pair can produce the protein. But there will not be any function at all when the individual is homozygous for the mutation.
• Gain of function: A new trait or phenotype is created when a change in gene function produces a new protein. This gain in function can be harmless. But the new protein can be harmful if it interferes with some other function.

Effect on the organism: Mutagens can affect the organism in three different ways:

• Neutral effects
• Neutral mutations change the amino acid produced by the mutated gene but they do not affect the protein function.
• Harmful effects
• This type of change in the gene compromises its function which has a harmful effect on the organism and it can be lethal in homozygous form.
• Beneficial effects
• Sometimes mutations produce a new allele which makes the organism more successful in its environment, as shown by the example of the English peppered moth (Biston betularis).

4. Changes in chromosome structures
Chromosomes can be deleted, duplicated, inverted, translocated or crossed over during meiosis. Translocation defects of chromosomes can cause fertility problems in cattle. Chromosomes often duplicate in plants which results in a condition called polyploidy. This means that they have extra chromosomes which can give the plant new characteristics.

Selection and breeding

1. Natural and artificial selection
Natural selection
Occurs in nature. The combination of climate and the environment selects individuals that are best adapted to the current conditions in a specific habitat. This process occurs because the best adapted organisms survive and are able to breed and produce offspring.

Natural selection in plants

• The plants that survive in desert areas have developed special ways to find and store water. For example, desert plants have fleshy leaves which store water, waxy leaves that prevent evaporation and very long roots that can access underground water. Many of these plants have spines on their leaves so that thirsty animals cannot eat the leaves.

Natural selection in animals

• Desert animals also have special adaptations to survive extreme heat and dry conditions. For example, desert rats do not sweat and their kidneys can reabsorb water from the urine. Camels store water in the humps on their backs which allows them to go without drinking for long periods. The gemsbok is a desert antelope which has a special blood vessel system in its sinuses. This protects the brain from harm caused by very high temperatures. Natural selection over very long periods of time leads to the evolution of new varieties and even new species.

Artificial selection
This is the selection of plants and animals by humans. It occurs when plants and animals are bred because of certain desirable characteristics. Artificial selection can cause visible changes within a relatively short period of time. In agriculture we select characteristics which will increase disease resistance and yields/production (such as milk production), and also provide a good feed conversion in animals.

General principles of artificial selection

• Measurability (biometrics): You must be able to measure the selected trait, for example milk production, weight at weaning age or weight at slaughter age
• These measurements must be made accurately so that they can be used for the selection process.
• The science that measures and studies traits used for artificial selection is called biometrics.
• Heritability: The traits selected by the breeder must be heritable.
• This means that it must be possible to transfer them to the next generation
• Some traits are highly heritable while others have poor heritability.

• Economic importance: The selected traits must be of economic importance.
• This means that they must increase productivity such as milk production, weight gain, egg production, wool yield or disease resistance.

Related Items

Difficulties with artificial selection

• Too many selected characteristics
• The aim of the selective process will be achieved slowly if too many characteristics are chosen by the breeder.
• So it is better to select a few important traits.
• Negative characteristics
• Negative traits can be selected when you choose production characteristics for artificial selection.

2. Selection methods and breeding values
Breeders can use various selection methods for breeding to obtain the plants and animals with the desired characteristics. Modern selection methods use breeding values (BV).

Mass selection

• Also called individual selection. The individuals are selected for breeding based on their performance.

Pedigree selection

• Breeding stock is selected based on the performance of their forebears.

Family selection

• Similar to pedigree selection but involves comparing siblings. They can either be full siblings that have the same mother and father or half siblings that share only one parent.

Progeny selection

• Based on the performance of the offspring of the individual. The individual is selected for breeding if the progeny show the desired characteristics during progeny testing. BVs are used to determine the suitability of the parents.

Estimated breeding value
EBV is a value assigned to the production trait of an individual. The EBV can be calculated in two ways. In the first method, individuals are compared with their contemporaries in the same environment to exclude external effects on variations. In the second method, information from all relatives of the individual is used. The selection must be based on proper records of the performance of individuals, groups or families.

3. Breeding systems
Inbreeding

• The mating of plants/animals that are more closely related to each other than the rest of the population. This is usually done by breeding an individual with a close relative, for example a female animal with either a brother or father.
• The aim is to produce a population of plants or animals with the homogenous traits which characterise the breed or variety. This means that they are genetically similar or pure-bred.
• Inbreeding is an essential method to maintain pure-bred lines in plants in order to produce hybrid seeds.
• An example of inbreeding in livestock is the selective breeding which done in cows for larger udders and therefore the ability to produce more milk.

Line-breeding

• This is a less intense form of inbreeding. It makes use of individuals that are less closely related but still of the same breed or variety for mating.
• In animals this means mating a related male ancestor with a pure-bred female.
• An example of line breeding is when a bull is selected to mate with a distant relative (granddaughters) rather than with a close relative (his daughters).

Out crossing

• This method introduces new characteristics to inbred animals. A distantly related relative of the same breed with desirable characteristics is brought in to cross breed with inbred animals.
• An example of outcrossing is when one improves the vigour of an inbred herd of cattle by using a bull distantly related to the cows in the herd.

Cross breeding

• Involves the crossing of two or more different animal breeds or plant varieties.
  The result is a hybrid animal/plant with enhanced qualities, which is called hybrid vigour or heterosis.
• Cross breeding in cattle
  → The development of the Bonsmara breed is an example of effective cross breeding in cattle. This involved the crossing of British cattle like the Shorthorn and Hereford breeds, which are good beef producers, with the indigenous Afrikaner breed, which is better adapted to local conditions. This cross breeding led to good carcass quality, disease-resistance and tick- resistance, increased fertility and docility, and early sexual maturation.
• Cross breeding in crops
  → Used extensively to improve the quality of crops. Hybrids of various cultivars produce plants that are more suited to commercial requirements (e.g. beet plants with higher sugar levels and plants that are more disease- resistant or have a higher oil content in their seeds). F1 hybrids perform best and successive generations do not always have the same uniform characteristics. This means that the farmer needs to buy new seeds each year.

Upgrading

• Used to improve a poor quality herd. This is done by mating a good sire with females of inferior quality or using semen of good quality animals to inseminate these females.
• This has a positive effect for the first few generations but the benefits decrease in later generations.
• It is therefore an excellent way to produce animals for slaughter rather than for breeding.

Species-crossing

• Refers to the crossing of different animal species to produce offspring.
• Species-crossing in animals
  → The crossing of two different animal species usually produces non-fertile offspring because each species has a unique chromosome number. This means that non-viable gametes form after meiosis. However, agriculture has benefited by the cross of a horse with a donkey to produce a mule.
• Species-crossing in plants
  → Used to produce unusual ornamental plants and crops with a new characteristic. The cross between a Brassica or cabbage species with a Raphanus or radish species has produced a cold-resistant plant called radpole which is grown for fodder in some parts of the world. Species crosses in plants can result in new and fertile species. This is because they have a tendency to polyploidy or the production of additional chromosomes. If chromosomes replicate and form a compatible number, then this will result in a viable species cross.

1. Advantages and disadvantages of breeding systems

Genetic Modification

1. Genetic modification and its aims
The DNA of organisms can be changed or manipulated in the laboratory. We call this process genetic modification/engineering and the organisms involved are known as genetically modified (GM) organisms or GMOs. They are developed to produce an organism with new characteristics which will benefit agriculture:

• improved adaptation to the environment
• increased resistance to disease or predators
• increased and more consistent yields
• the development of new products which are useful as foods or medicines.

2. Advantages of genetic modification

• You can select characteristics that are not available by natural variation.
• You can use DNA from difference species to confer or give a desired characteristic.
• There are more ways to improve organisms that are used in agriculture.

3. Applications of GMOs

Pest-resistant plants

• A gene from a bacterium called Bacillus thuringiensis (Bt) has been introduced into crops like maize plants and potatoes to produce pest-resistant plants.
• The Bt gene codes for the production of a crystal that is toxic to insects.
• The plant then produces the Bt crystal which is toxic to the insects that attack it.
• Bt resistant cotton, maize and potato cultivars are examples of pest-resistant plants.

Herbicide-resistant plants

• The application of herbicides to fields of crops can be dangerous since the weed killer itself can kill the crops.
• Herbicide-resistant crops can be treated with herbicides that effectively eradicate the weeds but do not kill the crops.
• Canola and cotton are examples of herbicide-resistant plants.

Disease-resistant plants

• Various vegetables and crops have been engineered to resist diseases caused by viruses, bacteria and fungi. These diseases can affect their productivity and they are difficult to treat.
• Some examples are virus-resistant tomatoes, bananas, cucumbers and rice, and fungus-resistant peppers and cucumbers.

Vegetables with longer shelf lives

• GM varieties of tomatoes have been developed which last longer after they have been picked. This is an advantage because it prevents spoiling before the product gets to the market. It therefore reduces the losses due to softening and rotting tomatoes.

Healthier tobacco

• Genetic modification has been used to develop tobacco plants that produce less nicotine or none at all. Cigarettes made with the tobacco still contain harmful carcinogenic chemicals but they are less addictive so people smoke less.

Bacterial protein production

• Genetic manipulation is very easy to achieve in bacteria. The E. coli species, which has been widely researched, grow fast and can be grown in large tanks on an industrial scale. When genes for the production of various proteins are inserted into E. coli, the bacteria produce the proteins in large amounts.
• GM bacteria are currently being used to produce hormones, for example insulin and bovine somatotropin (BST).
• BST is responsible for milk production in cattle and it can be produced when the BST gene is inserted into E. coli bacteria. The bacteria produce BST which can be harvested and used in cattle to increase their milk production.

Transgenic salmon

• A gene for a growth hormone has been inserted into Atlantic salmon.
• These transgenic salmon are sold under the trade name AquAdvantage® and they grow six times faster than non-transgenic fish.
• Therefore they are the right size for market in 16–18 months instead of three years.

Fluorescent fish

• Scientists transferred a bioluminescence gene from jellyfish into a fish species called the zebra fish.
• This gene allows the zebra fish to fluoresce or glow when they are in distress, for example when toxic substances are added to water.
• These GloFish® can be used by scientists to detect water pollution.

Recombinant vaccines

• Various vaccines for livestock have been developed using GMOs. These are referred to as recombinant vaccines.
• The advantage of these vaccines is that they contain very little of the infectious agent. This eliminates the potential to cause the disease which is being vaccinated against.
• Various recombinant vaccines are currently used in the poultry industry and many are being developed for mammalian livestock.
• A recombinant rabies vaccine has been used effectively to vaccinate wild animals and dogs and reduce the incidence of the disease.

4. Techniques used in genetic modification

Identifying the gene

• The gene with the desirable characteristic must be selected.
• These characteristics include protein production and herbicide resistance.

Modifying the gene

• The selected gene is usually from a different species, for example a bacterium
• Therefore the required gene must be modified in order to be accepted by the plant.
• Scientists use promoter and terminator gene sequences to ensure that the foreign proteins can be correctly translated.
• The gene sequence is usually inserted into a vector.
• The vector can be a plasmid or a bacterium called Agrobacterium which tends to insert itself into plant DNA.

Inserting the gene

• The DNA can be inserted into plants with a gene gun or Agrobacterium.
• The gene gun shoots out microscopic particles of gold or other metals propelled by compressed air. The micro-particles are coated in a solution which contains the transgenes and these are shot into the plant tissue. Some of these genes are carried into the nuclei of the plant cells which allow them to replicate.
• Agrobacterium can carry an implanted gene into the DNA of the plant which allows the desired protein or substance to be produced.

Verifying gene presence, monitoring and testing

• The GM plants are then checked by molecular analysis in the laboratory to confirm that the plant has taken up the gene (i.e. to verify gene presence).
• In monitoring, the performance of the modified plant is tested in isolation.
  This is to ensure that the plant has the required characteristics.
• The effectiveness of its new gene and the safety of the plant have to be tested under controlled conditions. This must take place before the GM plant can be released on a large scale.

5. Characteristics of GMOs

• They have genetic material from a donor organism inserted into their DNA. This gives them new or changed characteristics. These new or changed characteristics can be either quantitative or qualitative.
• GMOs with quantitative changes have increased abilities such as rate of growth or yield of food component.
• GMOs with qualitative changes have entirely new characteristics, for example being able to resist a herbicide or glow in the dark.
• GMOs – especially plants – often have a marker gene, such as those for antibiotic resistance, to distinguish them from the non-GMO plants.

6. Potential benefits of genetically modified crops

Potential productivity benefits Genetic manipulation in plants can increase their productivity. This can be due to an increase in yield, disease resistance, herbicide resistance or the production of more beneficial nutrients.

Potential environmental benefits GM crops are potentially more environmentally friendly because, for example, they are pest resistant and so no (or less) pesticides are used. This results in a decrease in contamination of air, soil and water. In addition, GM crop production can result in less deforestation needed to feed the world’s growing population. A decrease in deforestation decreases carbon dioxide in the atmosphere, which in turn slows global warming.

Potential economic benefits GM crops potentially have economic benefits, such as a decrease in production costs due to the reduced use of pesticides. This has the potential to increase the wealth of farmers while also decreasing food prices due to lower costs and higher yield. This can help to reduce poverty and starvation in developing countries.

Potential health benefits GM crops can be designed to be more nutritious than conventional crops. Potentially, this can lead to a reduction of illnesses, as GM crops provide necessary vitamins and minerals to vast populations who previously had limited access to these nutrients. GM crops may also be used to produce pharmaceuticals and vaccines in the future.

Potential to create ‘super food’ Scientific knowledge developed from producing GM crops can lead to the creation of super foods. These are types of food that are cheap to produce, grow fast in large quantities and are highly nutritious.

Potential of increased food security Scientific advances in GM crops can lead to the development of new kinds of crops that can be grown in extreme climates such as dry or freezing environments (like deserts). For example, scientists have already developed a type of tomato that grows in salty soil. Such a development would increase food security as people in extreme climates would be able to grow a wider range of cops.

Potential benefits to livestock Livestock are also beneficiaries of the higher nutritious value of GM crops. This has the potential to increase livestock’s resistance
to disease, their productivity and their hardiness.

7. Potential risks of GMOs
Scientific reasons why GMOs must be researched and produced with various controls.

Escape of transgenes

• There has been one incident in which a herbicide-resistant transgene escaped into a non-GM canola plant. Plants that are pollinated by wind often exchange genes and this is difficult to control. The consequences of the canola plant incident were not very serious. But there are concerns about the escape of transgenes if crops are to be used to produce pharmaceuticals which can be toxic to humans.

Resistance to transgene effects

• If weeds become resistant to herbicides or insects become resistant to the Bt toxin, then herbicide-resistance and insect-resistance will become useless GM characteristics. Various strategies have been suggested to prevent this outcome.

Food safety issues

• There have been fears about the safety of food produced by GMOs. However, no ill effects have been shown by GMOs despite the fact that millions of people have eaten the food produced from these crops.

Effect on non-target organisms

• There are concerns that insect-resistant GMOs like Bt maize plants will affect non-target organisms. Beneficial insect species like bees and butterflies are of particular concern.

Topic Questions

• Answer the questions below. Check your answers afterwards and do corrections.
• Give yourself one hour.
• Marks: 100

1. Study the diagram. Then choose the best answer for each question. Write down only the question number and the letter that represents the best answer.
   

   	Paternal
   Maternal		B	b
   	B	BB	Bb
   	b	Bb	bb

1.1 This diagram is called a _________.

1. Monohybrid cross
2. Mendelian inheritance
3. Punnett square
4. dihybrid cross (1)

1.2 The capital B in the diagram indicates a _________.

1. genotype
2. dominant allele
3. phenotype
4. gamete (1)

1.3 In the diagram, both the maternal and the paternal organism have:

1. the phenotype Bb
2. the phenotype BB
3. the genotype Bb
4. the allele Bb (1)

1.4 In the diagram, assuming the maternal organism is a dog with a black coat (B) and the paternal organism is a dog with a white coat (b), _________ of the offspring will be black while _________ will be white.

1. 25% ; 75%
2. 50% ; 50%
3. 75% ; 25%
4. None of the above. (2)

2. Fill in the missing words.
   2.1 Chromosomes divide during ________ into a pair of ________. (2)
   2.2 An homologous chromosome pair ________ during ________ resulting in a ________ cell. (3 × 1 = 3)
3. Cow parsley (Anthriscus sylvestris) produces a purple-leafed variant called
   Ravenswing. The purple colour is produced by the allele R which shows incomplete dominance over the allele for green leaves (r). Heterozygotes (Rr) produce green and purple spotted leaves.
   3.1 Show the genotypes that result from a cross between the Ravenswing variant and a green plant. (2)
   3.2 What are the resulting phenotypes? (2)
   3.3 Show what genotypes and phenotypes result from crossing the progeny of the cross mentioned in (3.1), namely F1 × F1. (3)
4. Tortoise shell cats (red and black colour combination) are always female because the alleles for the red and black colouring are linked to the X chromosome. This means that tom cats can only be red (O) or black (o).
   Use a Punnett square to determine how many tortoise shell cats will be produced by crossing red, black and tortoise shell females with red or black males. (Hint: You only need to show the female progeny.) (6)
5. Sex is a qualitative characteristic in humans and most animals.
   5.1 Which sex has homologous chromosomes?
   5.2 Name a sex-linked characteristic found in animals.
   5.3 What is the sex ratio of offspring after sexual reproduction? (3 × 1 = 3)
6. Some genetic characteristics are quantitative.
   6.1 Name an example of a quantitative genetic characteristic. (1)
   6.2 Quantitative genes have ________ effects. (1)
   6.3 What quantitative characteristics would be important in agricultural production? (2)
7. Explain briefly how external factors can cause variation in individuals. (2)
8. Polyploidy in plants is a chromosomal abnormality which results from ________ of chromosomes during replication. (1)
9. Spontaneous mutations arise in the DNA of all living organisms.
   9.1 Give four examples of spontaneous DNA mutations. (4)
   9.2 Explain how mutations can affect gene function. (5)
   9.3 What effects can mutations have on organisms? (3)
10. Explain the difference between inherited and non-inherited mutations. (2)
11. Explain why sexual reproduction is a cause of genetic variation. (4)
12. Name five mutagens that can be used to cause DNA mutations. (5)
13. Selection of the animals that have adapted the best to their environment is called ________. (2)
14. What adaptive characteristics could you expect to find in desert plants? (5)
15. Artificial selection is the process used by humans to improve plants and animals.
    15.1 What type of characteristics should be selected for breeding plants and animals? (3)
    15.2 What problems could be encountered when performing selective breeding? (2)
    15.3 Explain the principle of hybridization and its application in agronomy. (5)
    15.4 What are the two main disadvantages of pure bred species? (2)
    15.5 Why do most species crosses in animals result in infertility? (1)
    15.6 Why are species crosses possible in plants? (1)
16. What is the advantage of genetic engineering or transgenics over selective breeding? (2)
17. Name four aims of genetic engineering in crops. (4)
18. GMOs are used widely in agriculture.
    18.1 Identify the main environmental concerns about GMOs. (3)
    18.2 How can you identify an escaped GMO? (1)
    18.3 Name the bacterium used to transfer genes into GMOs. (1)
    18.4 Explain why the bacterium named in (18.3) is used. (4)
19. Various types of breeding systems are used in agriculture.
    19.1 Name one example of the result of a species crossing.
    19.2 Give an example of cross-bred cattle.
    19.3 Give an example of pure-bred cattle.
    19.4 What type of plant breeding system can be used to develop a new plant cultivar? (4 × 1 = 4)
20. Name the selection system used for sires in animal science. (1)
21. What value is used to rate sires with good production characteristics? (1)
22. Mention the role biometrics plays in performance testing. (1)