Patterns of inheritance

Mutations have contributed to the process of evolution. A mutation is a change to the genetic material. This may involve changes to the structure of DNA, or changes to the number or gross structure of chromosomes. Sexual reproduction may also lead to genetic variation. 

Types of mutagenic agent 

Mutagen 

Physical agents 

• X-rays
• Gamma rays
• UV light

Chemical agents 

• Benzopyrene (found in tobacco smoke)
• Mustard gas
• Nitrous acid 
• Aromatic amines - in some synthetic dyes
• Reactive oxygen species - free radicals 
• Colchicine 

Biological agents 

• Some viruses
• Transposons - jumping genes, remnants of viral nucleic acid that have become incorporated into our genes. 
• Food contaminants such as mycotoxins from fungi, e.g. aflatoxins in contaminated nuts, chemicals in charred meat, and alcohol. 

• deletion - part of a chromosome, containing genes include regulatory sequences, is lost 
• inversion - a section of a chromosome may break off, turn through through 180 degrees and then join again; although all the genes are still present, some may now be too far away from their regulatory nucleotide sequences to be properly expressed 
• translocation - a piece of one chromosome breaks off and then becomes attached to another chromosome. This may also interfere with the regulation of the genes on the translocated chromosome 
• duplication - a piece of a chromosome may be duplicated. Overexpression of genes can be harmful, because too many of certain proteins or gene-regulating nucleic acids may disrupt metabolism 
• non-disjunction - one pair of chromosomes or chromatids fails to separate, leaving one gamete with an extra chromosome. When fertilised by a normal haploid gamete, the resulting zygote has one extra chromosome. Down syndrome, or trisomy 21, is caused by non-disjunction.

• aneuploidy - the chromosome number is not an exact multiple of the haploid number for that organism. Sometimes chromosomes or chromatids fail to separate during meiosis (e.g. trisomy.
• polyploidy - if a diploid gamete is fertilised by a haploid gamete, the resulting zygote will be triploid (it has three sets of chromosomes). The fusion of two diploid gametes can make a tetraploid zygote. Many cultivated plants are polyploid (they have more than two sets of chromosomes). 

Genetic variation resulting from sexual reproduction has contributed to evolution. 

• Meiosis produces genetically different gametes. During meiosis, genetic variation may result from: allele shuffling (swapping of alleles between non-sister chromatids) during crossing over in prophase 1.
• independent assortment of chromosomes during metaphase/anaphase 1 
• independent assortment of chromatids during metaphase/anaphase 2 

Gametes produced by meiosis are individual and genetically dissimilar. They are also haploid, containing only one of each pair of homologous chromosomes and one allele for every gene. 

The random fusion of gametes creates more genetic diversity. Any male gamete can potentially combine with any female gamete from an organism of the same species. The random fertilisation of gametes, that are already genetically unique, produces extensive genetic diversity among the resulting offspring.

Variation caused solely by the environment 

Some phenotypic variation is caused by the environment and not passed on through genes. E.g. speaking with a particular regional dialect. A person's offspring would not inherit the dialect through their genes, although they might learn to speak in this way by listening to other people. 

Variation caused by the environmental interacting with genes 

If plants are kept in dim light after germination, or if the soil in which they are grown contains insufficient magnesium, then the leaves do not develop enough chlorophyll and are yellow or yellow-white. The plant is described as chlorotic, or suffering from chlorosis. The plant cannot photosynthesise. Chlorotic plants have the genotype for making chlorophyll, but environmental factors are preventing the expression of these genes.

A monohybrid cross involves the crossing of individuals and the examination of one (mono) character (flower colour, pod shape...) and different (hybrid) traits (red colour, white colour) in their offspring.

The Punnett square is a useful tool for predicting the genotypes and phenotypes of offspring in a genetic cross involving Mendelian traits. 

Mendel crossed true-breeding plants that differed for a given character. Pollen from true-breeding (homozygous) pea plants with purple flowers (one trait) was placed on stigmas of true-breeding plants with white flowers (another trait).

The F1 seeds were all purple; the white flower trait failed to appear at all.

Because the purple flower trait completely masks the white flower trait when true-breeding plants are crossed, the purple flower trait is called dominant, and the white flower trait is called recessive. 

The F1 plants were allowed to self-pollinate. This step was the monohybrid cross (or the F1 cross). The progeny, called F2, were examined: roughly 1/4 were white, and 3/4 were purple.

Dihybrid crosses look at the pattern of inheritance when two genes (not linked on the same chromosome) are considered at the same time. In one dihybrid cross, Mendel examined the inheritance of seed shape and seed colour in pea plants. He crossed true-breeding pea plants with yellow and round seeds (YYRR) with true-breeding pea plants that had green and wrinkled seeds (yyrr). All the F1 generation were hybrids, having the phenotype of yellow and round seeds (YyRr). Each plant in the F1 generation is heterozygous for both genes (seed colour and shape). Therefore, yellow and round are both dominant traits.

When allowing the F1 generation to self pollinate, the F2 generation have a ratio of 9:3:3:1.

When three or more alleles at a specific gene locus are known, then the gene is said to have multiple alleles. However, any individual can possess two alleles, one on each gene locus, in a pair of homologous chromosomes. 

The inheritance of human ABO blood groups it's a good example of multiple alleles. It also demonstrates both dominance and co-dominance of the alleles involved. The four blood groups (phenotypes) - A, B, AB and O - are determined by three alleles of a single gene on chromosome 9. The gene encodes an isoagglutination, I, on the surface of erythrocytes. The alleles present in the human gene pool are IA, IB and IO. IA and IB are both dominant to IO which is recessive. IA and IB are codominant. If both IA and IB are present in the genotype, they will both contributed to the phenotype. Any individual will have only two of the three alleles within their genotype. 

In humans, sex is determined by one of the 23 pairs of chromosomes, called the sex chromosomes. The other 22 pairs are called autosomes. Each of the autosomal pairs is fully homologous - they match for length and contain the same genes at the same loci. The sex chromosomes are XY in males and ** in females. The X and Y chromosomes are not fully homologous. A small part of one matches a small part of the other, so that these chromosomes can pair up before meiosis.

The human X chromosome contains over 1000 genes that are involved in determining many characteristics, or metabolic functions, not concerned with sex determination, and most of these have no partner alleles on the Y chromosome. If a female has one abnormal allele on one of her X chromosomes, she will probably have a functioning allele of the same gene on her other X chromosome. If a male inherits, from his mother, an X chromosome with an abnormal allele for a particular gene, he will suffer from a genetic disease, as he will not have a functioning allele for that gene. Males are functionally haploid, or hemizygous, for X-linked genes. They cannot be heterozygous or homozygous for X-linked genes. Sex linked characteristics in humans include haemophilia A and colour blindness. 

A person with haemophilia A is unable to clot blood fast enough. Injuries may cause bleeding or an internal haemorrhage. 

One of the genes on the non-homologous region of the X chromosome codes for a blood-clotting protein called factor 8. A mutated form of the allele codes for non-functioning factor 8.

A female with one abnormal allele and one functioning allele produces enough factor 8 to enable her blood to clot normally when required. However, this female is a carrier for the disease. If such a female passes the X chromosome containing the faulty allele to her son, he will have no functioning allele for factor 8 on his Y chromosome. As a result, he will suffer from haemophilia A. 

One of the genes involved in coding for a protein involved in colour vision is on the X chromosome, but not on the Y chromosome. A mutated allele of this gene may result in colour blindness - an inability to distinguish between red and green. A female with one abnormal allele and one functioning allele will not suffer from colour blindness, but a male with an abnormal allele on his X chromosome will not have a functioning allele on his Y chromosome and will therefore suffer from red-green colour blindness. The inheritance pattern is the same as for haemophilia A - that of a recessive sex-linked disorder.

One of the genes, C, for coat colour in cats is sex-linked. It is on the non-homologous region of the X chromosome. 

• The allele Co produces orange (ginger) fur.
• The allele CB produces black fur. 

These alleles are codominant, as cats with the genotype Xco XCB are tortoiseshell and have patches of black fur and patches of orange fur. Both the orange and black alleles contribute to the phenotype, but the orange allele is only expressed in cells where the X chromosome bearing the black coat colour allele is inactivated, and vice versa. Male cats may be either black or ginger but not tortoiseshell, as they only have one X chromosome.

It may appear that females have twice the number of X-linked genes being expressed as do males. However, a mechanism prevents this disparity. In every female cell nucleus, one X chromosome is inactivated. Determination of which member of the pair of X chromosomes becomes inactivation is random and happens during early embryonic development. 

When both alleles of a gene in the genotype of a heterozygous individual contribute to that individual's phenotype, the alleles are described as codominant. The two alleles are responsible for two distinct and detectable gene products. 

The phenotype of heterozygotes is different from the phenotype of the homozygotes.

Coat colour in shorthorn cattle is an example of codominant inheritance. The one gene for coat colour has two alleles: CR (red) and CW (white).

Cattle that are homozygous for the red-coat allele, CR, have a red (chestnut) coat. 

Cattle homozygous for the white-coat allele, CW, have a white coat. 

Heterozygous cattle, genotype CRCW, have both red and white hairs - a roan coat.

If red and white shorthorn cattle are interbred, all the offspring are roan. If roan cattle are mated, then the offspring will show all three phenotypes in the ratio 1 white : 2 roan : 1 red.

MN blood groups 

The MN blood group system is controlled by a single gene with two alleles, G^M and G^N. The gene codes for a particular protein on the surface of erythrocytes. The G^M allele codes for one version of the protein and the G^N allele codes for a slightly different version of the protein. These alleles are codominant. The children of a couple, one being blood group M, and the other having blood group N, will all have blood group MN.

ABO blood groups 

The alleles I^A and I^B are codominant to each other. An individual of genotype I^AI^B expresses both and has both types of isoagglutinogen protein on their erythrocytes. The inheritance of these blood groups also shows dominance, as both I^A and I^B are dominant to the allele I^O. 

Sickle cell anaemia

Sickle cell anaemia is caused by a mutation in the gene that codes for the B-globin chain of haemoglobin. The mutant allele is given the symbol Hb^s and the normal allele is given the symbol Hb^N. In heterozygous people, at least half the haemoglobin in their red blood cells is normal and half is abnormal. However, heterozygous people do not suffer from sickle cell anaemia. If we consider the type of haemoglobin as the phenotype, then these alleles are considered as codominant. However, if we take sickle cell anaemia to be the phenotype, the Hb^s allele is considered to be recessive, as this disorder has a recessive inheritance pattern. 

Some types of camellia have red flowers, and some have white flowers. If these two types are crossed, the offspring will have red and white spotted flowers. Both alleles of the gene for petal pigment, P^R and P^W, are expressed in the phenotype of the heterozygotes. 

When Mendel investigated the simultaneous inheritance of two characteristics, he chose seven characteristics for which the genes were on different chromosomes. Hence the genes assorted independently. 

There are many more genes in a genome than there are chromosomes. When two or more gene loci are on the same chromosome, they are said to be linked. 

The chromosome, not the gene, is the unit of transmission during sexual reproduction, therefore linked genes are not free to undergo independent assortment; they are usually inherited together as a single unit.

So autosomal linkage is where genes are linked by being on the same autosome.

In some cases different genes, at different loci on different chromosomes, interact to affect one phenotypic characteristic. When one gene masks or suppresses the expression of another gene, this is termed epistasis.

The genes in question may work together antagonistically (against each other) or in a complementary fashion. Because the gene loci are not linked, they assort independently during gamete formation. Epistasis reduces the number phenotypes produced in the F2 generation of dihybrid crosses and therefore it reduces genetic variation.

The homozygous presence of a recessive allele at the first locus prevents the expression of another allele at a second locus. The alleles at the first locus are epistatic to those at second locus, which are hypostatic to those at the first locus. An example of recessive epistasis is the inheritance of flower colour in Salvia. Two gene loci, A/a and B/b, on two different chromosomes, are involved.

If a pure-breeding pink-flowered variety of Salvia, genotype AAbb, is crossed with a pure-breeding white-flowered variety, genotype aaBB, all the offspring of the F1 generation have purple flowers. Their genotype is AaBb. 

Interbreeding members of the F1 generation results in plants that bear purple, pink and white flowers, in the ratio of 9:3:4. This is a modified version of the dihybrid 9:3 (3:1) ratio. 

The homozygous aa is epistatic to both alleles of the B/b gene. Neither the allele B for purple nor the allele b for pink, when in the homozygous state, can be expressed if no dominant A allele is present.

The inheritance of feather colour in chickens is an example of dominant epistasis. There is an interaction between two gene loci, I/i and C/c. The hypostatic gene, C/c, codes for coloured feathers. The I allele of the epistatic gene, I/i, prevents the formation of colour, even if one C allele is present.

Individuals carrying at least one dominant allele, I, have white feathers, even if they also have one dominant allele for coloured feathers.

Birds that are homozygous for the recessive allele, c, are also white, as this mutated allele does not cause pigment to be made.

Pure-breeding White Leghorn chickens have the genotype IICC. Pure-breeding White Wyandotte chickens have the genotype iicc. 

If White Leghorn chickens, genotype IICC, are crossed with White Wyandotte chickens, genotype iicc, the offspring are all white. They are heterozygous at both gene loci, having the genotype liCc. If the progeny interbreed, they produce, in the F2 generation, white-feathered chickens and coloured-feathered chickens in the ratio of 13:3, respectively. 

In mice, the gene locus C/c determines that the coat will have colour. The genotype CC or Cc produces coloured fur. However, in the recessive homozygous state, genotype cc, no pigment develops and the mice are albino. 

A/a determines what that colour is by determining the distribution of pigment. The dominant allele, A, produces agouti colour - each hair has black pigment and a yellow band producing an overall greyish colour. The recessive allele, a, when homozygous produces black fur as the yellow band from each hair is lacking. However, if there are two c alleles, no colour will develop, as there is no pigment to be distributed. 

This can be regarded as an example of recessive epistasis. However, can be explained if we consider what each gene governs. 

• In the presence of a C allele, the black pigment can be made from a colourless substance. 
• In the presence of an A allele, this black pigment is deposited during the development of hair in a pattern, combined with a yellow band on each hair that produces the agouti coloration.

Two geneticists, William Bateson and Reginald Punnet, crossed two strains of true-breeding white-flowered sweat peas. 

They were surprised when all the F1 progeny produced purple flowers. When they allowed these F1 plants to interbreed by self fertilisation, the F2 phenotypes contained white-flowered plants and purple-flowered plants in the ratio 9:7. 

Two gene loci, A/a and B/b, may yield such results if one gene locus codes for an enzyme that catalyses the production of a colourless intermediate product from a colourless precursor substance, and the second gene locus codes for an enzyme that catalyses the production of a purple pigment from the intermediate product. 

At least one dominant allele for both gene loci has to be present for the flowers to be purple. 

Domestic chickens have 4 types of cone: single, rose, walnut and pea. 

Two gene loci, P/p and R/r, interact to affect comb shape. The effect of the P/p alleles depends upon which R/r alleles are present in the bird's genotype. When true-breeding pea-combed chickens, genotype PPrr, are bred with true-breeding rose-combed chickens, genotype ppRR the progeny all PpRr, have walnut combs. When the walnut-combed progeny are interbred, their progeny show four phenotypes: walnut comb, rose comb, pea comb and single comb, in the classic Mendelian dihybrid ratio of 9:3:3:1.

At least one dominant allele for both gene loci has to be present in the bird's genotype for it to exhibit the phenotype walnut comb. 

At least one dominant R allele in the presence of two recessive p alleles produces a rose comb. 

At least one dominant P allele in the presence of two recessive r alleles produces a pea comb 

Being recessive homozygous at both gene loci produces a single comb. 

The null hypothesis states: 'There is not statistically significant difference between the observed and expected data. Any difference is due to chance.' 

Where phenotype classes are distinct and discrete, each clearly discernible from the others in a qualitative way this is discontinuous variation. There are no or very few intermediates between the different phenotypes. For example, you are either male or female and have only one of the four possible ABO blood groups. Ear lobes are another example: they may be attached or free-hanging.

Characteristics that exhibit discontinuous variation are usually determined by the alleles of a single gene locus. They are monogenic. Sometimes the alleles of two genes interact to govern a single characteristic. In either case: 

• different alleles at a single gene locus have large effects on the phenotype 
• different gene loci have quite different effects on the characteristic. 

In tomato plants, many genes determine features of the plant's leaves. One gene locus codes for leaf shape and another determines whether the leaves have hairs. A third gene locus determines the presence or absence of chlorophyll Genes at different loci may interact to influence one characteristic and produce discontinuous variation, as in epistasis. 

Where the genetic variation between individuals, even if they are related, within a population shows a range with a smooth gradation between the many intermediates, it is described as continuous variation. Examples include, birth mass, foot size, finger length, height, skin colour, hair colour, eye colour, mass and heart rate in humans, cob length in maize plants, leaf length in many plants and tail length in mice.

Many genes are involved in determining such characteristics. Therefore such characteristics are described as polygenic. The alleles of each gene may contribute a small amount to the phenotype, therefore the alleles have an additive effect on the phenotype. As a result, the phenotypic categories vary in a quantitative way. The greater the number of gene contributing to the determination of the characteristic, the more continuous the variation (the greater the range). The study of the genetics of such inherited characteristics is called quantitative genetics. Many characteristic of crop plants are polygenic, so plant breeders need to apply knowledge of quantitative genetics.

Genetic analysis of the inheritance of such traits become more complicated as the number of gene loci increases above two. 

The environment has a greater effect on the expression of polygenes/polygenic characteristics than it does on monogenic characteristics. For example, each person has a genetic potential for height and intelligence, but without proper nutrition and also, for intelligence, mental stimulation, these potentials will not be reached.

Mutations and migration introduce new alleles into populations. Some individuals within a population will be better adapted than others to the environment, due to differences in their genotypes and phenotypes. These individuals are more likely to survive and reproduce, passing on the advantageous alleles. Over time, allele within the will change. This is natural selection. Natural selection may also maintain constancy of a species, as well as leading to new species. There are three main types of selection: stabilising, directional and disruptive selection.

Stabilising selection normally occurs when the organisms' environment remains unchanged. It favours intermediate phenotypes. 

In humans, babies if birth mass close to 3.5kg are more likely to survive. Their offspring inherit alleles from them, also leading to this mean birth mass.

If the environment changes, for example by becoming colder, there may now be an selection pressure so a new larger mass becomes the ideal and will be selected for. If more larger individuals survive and reproduce, they will be more likely to pass genes and alleles for larger size to their offspring. Over several generations, there is a gradual shift in the optimum value for the a trait. In nature, within a population, periods of directional selection may alternate with periods of directional selection may alternate with periods of stabilising selection. 

If a population descends from a small number of parents, the gene pool will lack genetic variation. Some alleles resulting from confer advantage nor advantage on the individual, so there will be no selection pressure upon them. However, chance events may drastically alter the allele frequency. 

With small population descended from one set of heterozygous parents. There are only two alleles, A and a, in the population. If a catastrophic event occurs, such as an earthquake, flood, outbreak of a disease or a severe shortage of food, which leads to the death of many of the already small population, one of the alleles may disappear from this population. When the population recovers and increases in size, it will have less genetic diversity than before and may lack particular alleles. The allele(s) in question did not disappear due to selection pressures, but due to genetic drift. 

Genetic drift can arise after a genetic bottleneck or as the result of the founder effect. 

When a population size shrinks and then increases again, it has said to have gone through a genetic bottleneck. 

After this event, the genetic diversity within that population will be reduced. There may be loss of some advantageous alleles or a disproportionate frequency of deleterious alleles, putting that population's chances of long-term survival at risk. Sometimes, after a genetic bottleneck, a population shrinks to such a small size that its fertility is affected, leading to the species becoming endangered and then extinct. 

However, if the ones that survive are those that have a particular advantage, for example resistance to a particular pathogen, then a bottleneck could improve the gene pool whilst also shrinking genetic diversity. Species that have been selectively bred for certain traits have also been through a genetic bottleneck.

If a new population is established by a very small number of individuals who originate from a larger, parent population, the new population is likely to exhibit loss of genetic variation. 

Some groups of migrating humans, not fully genetically representative of the parent population, have set up populations in new areas. If they have remained isolated from other human populations, for example because of religious and cultural differences or due to geographic isolation, then the new population will have a small gene pool. This has happened in Iceland, the Faroe Islands, Pitcairn Island, Easter Island and among the Amish people of North America. Founder effect is a special case of genetic drift.

Population genetics attempts to study the changes in allele frequencies within a population, over time. 

If a species is to succeed and not become extinct, it needs genetic variation between the individuals in its populations. Individuals inherit their genomes from their parents and pass on some of their genetic material to their offspring. 

Population genetics studies the variation in the alleles and genotypes within the gene pool and how their frequencies vary over time. 

It is a fundamental concept in population genetics. It describes and predicts a balanced equilibrium in the frequencies of alleles and genotypes within a breeding population. It can also be used to determine the frequencies of those carrying a recessive allele (heterozygotes) for a genetic disorder with a recessive inheritance pattern, if we know the incidence of affected babies born each year in that population.

Over time, one species may evolve into another, or it may evolve two new species. For a species to evolve into two species, it must be split into two isolated populations. If this happens, then any mutations that occur in one population are not transmitted by interbreeding to the other population. In each location, there will be different selection pressures and each population will accumulate different allele frequencies. Hence each population can evolve along its own lines. 

At times during the evolutionary process, the two populations will be different, but still able to interbreed. They are then called sub-species. When there have been sufficient genetic, behavioural and physiological changes in the two populations so that they can no longer interbreed, they are then separate species. The process by which new species are formed is called speciation.

If populations are separated and isolated from each other by geographical features such as lakes, rivers, oceans and mountains, these also act as barriers to gene flow between the populations.  The isolated populations, being subject to different selection pressures in the two different environments, then undergo independent changes to the allele frequencies and/or chromosome arrangements within their gene pools. These genetic changes may be the result of mutation, selection and genetic drift. 

As a result of natural selection, each population becomes adapted to its environment. This type of speciation is called allopatric speciation (allopatric means "in different countries').

Biological and behavioural changes within a species may lead to reproductive isolation of one population from another. 

If a mutation leads to some organisms in a population changing their foraging behaviour and becoming active at dawn, dusk or at night rather than during the day, enabling them to exploit a new niche, the members of the diurnal population will be unlikely to mate with members of either the crepuscular or the nocturnal populations. 

Genetic changes can also lead to reproductive isolation. A change in chromosome number may, prevent gamete fusion, make the zygotes less viable, so that they fail to develop and lead to infertile hybrid offspring with an odd number of chromosomes, so that chromosome pairing during meiosis cannot occur. 

Mating between members of the reproductively isolated populations may also be prevented by mutations leading to changes in, courtship behaviour, e.g. time of year for mating or courtship rituals that precede mating, animal genitalia or plant flower structure. 

Speciation resulting from reproductive isolation is called sympatric (meaning "in the same country') speciation.

Humans have been practising animal and plant breeding via artificial selection for about 10,000 years, since the beginning of settled agriculture. 

Whereas when natural selection is operating the environment is the agent of selection, during artificial selection, humans are the agents of selection. Breeders select individuals with the desired characteristics and allowed them to breed, at the same time prevent those without the desired traits from breeding. 

Considering the number of species of organisms on the planet, humans have domesticated very few. Included are cereals, potatoes, vegetables and fruits, cattle, pigs, sheep and goats, horses, oxen, dogs cats, pigeons and poultry. 

Desirable characteristics in plants include increased yield, and pest and disease resistance. In livestock, desirable characteristics include docility, placidity and the ability to be trained. Animals that normally live in social groups (herds) with a dominance hierarchy may be able to be trained to accept human as the pack leader, and to tolerate being penned with other animals.

Artificial selection produces new breeds of organisms.

New breeds may be produced by selective breeding programmes. Breeders may grow many plants if a particular type under the conditions they wish these plants to withstand - such as low temperature or high salinity. They will then select those individuals that grow best under these conditions and cross pollinate them; collect and sow the seeds and repeat this process over many generations. A selective breeding programme takes 20 years. 

At each stage of selective breeding, the individuals with the desirable characteristics and no or few undesirable characteristics are selected. Inevitably, the genetic diversity in the gene selected breed is reduced. If related individuals are crossed, inbreeding depression can result. The chances of an individual inheriting two copies of a recessive harmful allele are increased.

Breeders sometimes outcross individuals belonging to two different varieties, to obtain individuals that are heterozygous at many gene loci. This property is termed hybrid vigour. 

Selective breeding, whilst developing bigger and better varieties of crop plants and animal breeds, has reduced the organisms' genetic diversity. The number of commercially grown varieties of crop has greatly reduced over the last 50-100 years. All commercial varieties are genetically similar; if a pathogen was introduced, most plants would succumb to the infection. Breeders may need to outcross the cultivated varieties with varieties more like their wild ancestors to increase hybrid vigour. Samples of such wild ancestral types need to be conserved, often in gene banks. 

Much of the wheat grown in the UK has a dwarfing allele introduced from a Japanese variety of wheat. If given extra fertiliser, the wheat does not grow taller and fall over in the wind, but uses the extra nutrients to increase seed size and yield. However, if the environmental temperature rises above 30oC, the effect of this allele is changed and yield is decreased. If climate change is likely to produce higher temperatures during the British summer, a new breed of wheat will have to be developed. Wheat breeders are looking, in a gene bank, for different dwarfing alleles. Gene banks store genomes, but in their organisms. Examples of gene banks include:

• Rare breed farms
• Wild populations of organisms 
• Crops in cultivation
• Botanic gardens and zoos
• Seed banks
• Sperm banks
• Cells in tissues culture 
• Frozen embryos 

• Domesticated animals retain many juvenile characteristics, making them friendly, docile and playful, but less able to defend themselves. The loss of their nervous disposition can also make them easy prey. 
• Livestock animals, such as pigs, selected to have more lean meat and less fat, might succumb to low environmental temperatures during winter if they were not housed. 

Dogs have been domesticated for many thousands of years and used by humans for hunting, companionship, protection, herding, transport, as guide dogs and to help deaf or disabled people, as well as for their aesthetic qualities. 

• The traits in dogs, considered desirable by humans, might put the dogs at a selective disadvantage if they had to survive in the wild. 
• Some breeds, through inbreeding from a limited number of pedigree dogs, have susceptibility to disease
• Some coat colours, selected because humans like the look of them, would also fail to camouflage the animals.