As mentioned in previous pages, the genetic information found in DNA is essential in creating all the characteristics of an organism. This remains the case when passing genetic information to offspring, that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell.
For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This can be done by passing on the information found in the chromosomes via the gametes that are created in meiosis.
Humans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another.
Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes.
Tetraploid organisms possess more than 3 sets of a particular chromosome.
Reproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringing together half the genetic information from the parents into one new cell, that is now genetically different from both its parents.
This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a unique genome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways, explained soon.
The process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again to form 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process
At this stage two haploid cells have been created from the original diploid cell of the parent.
Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species as a whole. Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased, all of which is elaborated upon on the next page.
The previous page investigates the process of meiosis, where 4 haploid gametes are created from the parent cell. Half the genetic information from a parent is present in these haploids, which fuse with gametes of the opposite sex to create a zygote, with a complete chromosome compliment that will create offspring after prolonged growth.
The process of meiosis increases genetic diversity in a species. The sex organs which produce the haploid gametes are the site of many occurrences where genetic information is exchanged or manipulated.
The top assortment to the left produces 2 blonde hair/blue eyes gametes while the below produces 2 brown hair/brown eyes gametes
The top assortment on the right produces 2 blonde hair/brown eyes gametes while the below produces 2 brown hair/blue eyes gametes
The above indicates that even though the two homologous chromosomes contain the same genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each of the 4 gametes produced. With 23 chromosomes in a human gamete, their are 223 combinations (8388608 combinations)
During meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the other pair. This point of contact is deemed the chiasmata, and can allow the exchange of genetic information between chromosomes. This further increases genetic variation.
There are also many other ways in which genetic variation is increased in a species gene pool, all of which are described in the following pages.
The next page investigates the work of Gregor Mendel, an Austrian monk famous for his work involving monohybrid and dihybrid crossing, alongside the continuation into looking at genetic diversity through meiosis and genetics in general.
Gregor Mendel, an Austrian monk, is most famous in this field for his study of the phenotype of pea plants, including the shape of the peas on the pea plants.
Mendel's goal was to have a firm scientific basis on the relationship of genetic information passed on from parents to offspring. In light of this he focused on how plant offspring acquired the phenotype of their seeds. In this example, there are two choices, round and wrinkled seeds.
The plants that were used in the experiment had to be true breeding, i.e. those plants with round seeds must have had parents with round seeds, who in turn had parents producing round seeds etc. This is done to increase the accuracy of results.
After successfully producing two generations from these true breeding plants, the following was evident
Mendel successfully hypothesised that the reason for this trend in phenotypes from generation to generation was down to the fact that genetic information was being passed on from their parents.
The fact that round seeds appeared more frequently than wrinkled seeds is due to round seeds being the dominant phenotype, which when present effectively 'masks' the phenotype of the recessive (wrinkled seed) gene.
The parents, one possessing wrinkled seeds the other possessing round were crossed together, for some reason in the first and second generation the presence of the round seed gene in offspring superceded the presence of the wrinkled seed. This is called dominance.
The next page investigates this dominance, and how it can successfully be predicted. The following page also has examples of a monohybrid and dihybrid cross.
The previous page investigated Gregor Mendel and how he found trends in the phenotypes of offspring produced by true breeding parents
Mendel paved the way to discovering that alleles that code for a particular characteristic, such as the shape of the seeds produced are expressed in dominant and recessive genes.
When dominant genes were present, they would supercede the presence of wrinkled and were deemed the dominant gene. For example;
If the genotype was rr (where both are recessive) there are no dominant genes therefore the recessive phenotype for wrinkled seed is expressed
The previous page mentioned that in the first generation all offspring produced were round seeds, and in the second generation for every three that were round seeded there would be one wrinkled seed. This can be expressed in a Punnett square as illustrated below.
All dominant genes are marked in red, and all recessive genes are marked in green. Whenever the dominant gene is present in an organism this will be expressed. We can summarise the above diagram in the following statements
This is an example of a monohybrid cross, where we are studying one respect of an animals genotype. The next page continues to look at dominance and examples of monohybrid and dihybrid crossing plus related info.
Previous pages have described how genetic information is passed along from parents to offspring. Mendel summarised this in his first law, the principle of segregation
When a particular gene possesses both dominant and recessive alleles, it is possible for incomplete dominance to occur, where the organism at hand expresses a phenotype morphed by the expression of both the dominant and recessive alleles.
In essence, heterozygous (possessing opposing alleles Rr) organisms derived from homozygous (possessing the same alleles RR or rr) are created, they possess a phenotype different to that of both their parents.
Some of the following examples of monohybrid and dihybrid crossing illustrate this incomplete dominance.
Diploid organisms naturally have a maximum of 2 alleles for each gene expressing a particular characteristic, one deriving from each parent. In some cases, however, more than two types of allele can code for a particular characteristic, as is the case of genetic coding for blood type in humans. Their are up to 6 possible genotypes that code for the four blood groups, A, B, AB and O.
The following dihybrid cross involves two true breeding pea plants, where two factors are looked at, the shape of the seed and the colour of the seed.
The past few pages have elaborated on the work of Gregor Mendel and how his work has paved the way to predicting the characteristics of offspring. However, a degree of randomness is involved, when involving factors such as independent assortment during meiosis and the possibility of genetic mutations (explained in further pages).
In light of this, Mendel's work allowed us to see that there is a degree of genetic inheritance from parents in offspring though modern biology indicates that more factors come into play to determine the final genotype and phenotype of an organism.
Sticking to the subject of genetics, the next page looks at sex determination via chromosomes X and Y and some of the genetic traits inherited via these two chromosomes.
In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes, which determine the sex of an organism and various sex linked characteristics.
In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species.
The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome.
In light of this, there are two possibilities that can occur during fertilisation between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilises the egg) they are responsible for determining sex.
Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes.
These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes as indicated below.
The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance.
So when the organism has an XY chromosome compliment (i.e. a male), these sex linked genes are freely expressed in the organisms phenotype, an example being hairy ears developing in old age.
These sex linked genes on the X chromosome display a number of characteristics. The following are just some examples of phenotypes as a result of these genes in expression;
The next page looks at genetic mutations and the consequences as a result of them.
It is natures intention that the exact genetic information from both parents will be seen in the offspring's DNA in the the critical stages of fertilisation. However, it is possible for this genetic information to mutate, which in most cases, can result in fatal or negative consequencies in the outcome of the new ogranism.
One well known example of mutation is non-disjunction. Non-disjunction is when the spindle fibres fail to seperate during meiosis, resulting in gametes with one extra chromosome and other gametes lacking a chromosome.
If this non-disjunction occurs in chromosome 21 of a human egg cell, a condition called Down's syndrome occurs. This is because their cells possess 47 chromosomes as opposed to the normal chromosome compliment in humans of 46.
The fundamental structure of a chromosome is subject to mutation, which will most likely occur during crossing over at meiosis. There are a number of ways in which the chromosome structure can change, as indicated below, which will detrimentally change the genotype and phenotype of the organism. However, if the chromosome mutation effects an essential part of DNA, it is possible that the mutation will abort the offspring before it has the chance of being born.
The following indicates types of chromosome mutation where whole genes are moved:
As the name implies, genes of a chromosome are permanently lost as they become unattached to the centromere and are lost forever
In this mutation, the mutants genes are displayed twice on the same chromosome due to duplication of these genes. This can prove to be an advantageous mutation as no genetic information is lost or altered and new genes are gained
The next page continues looking at these chromosome mutations and mutations that happen within genes that can prove to be more harmful to the organism at hand. The following pages also investigates polyploidy in species.
This page continues from the previous page investigating genetic mutations...
This is where the order of a particular order of genes are reversed as seen below
This is where information from one of two homologous chromosomes breaks and binds to the other. Usually this sort of mutation is lethal
The previous examples of mutation have investigated changes at the chromosome level. The sequence of nucleotides on a DNA sequence are also susceptible to mutation.
All of the genetic mutations looked at through the last 2 pages more or less have a negative impact and are undesired, however, in some cases they can prove advantageous.
Genetic mutations increase genetic diversity and therefore have an important part to play. They are also the reason many people inherit diseases.
The next page looks at polyploidy, a type of mutation that effects chromosome content of an organism, and also investigates the frequency of mutations and factors that play a part in this.
The previous two pages have investigated mutations, and this page continues with more information related to genetic mutations.
Humans are diploid creatures, meaning for every chromosome in our body, there is another one to match it. Read the following
It is possible for a species, particularly plant species, to produce offspring that contains more chromosomes than its parent. This can be a result of non-disjunction, where normally a diploid parent would produce diploid offspring, but in the case of non-disjunction in one of the parents, produces a polyploid.
In the case of triploids, although the creation of particular triploids in species is possible, they cannot reproduce themselves because of the inability to pair homologous chromosomes at meiosis, therefore preventing the formation of gametes.
Polyploidy is responsible for the creation of thousands of species in today's planet, and will continue to do so. It is also responsible for increasing genetic diversity and producing species showing an increase in size, vigour and an increased resistance to disease.
This page and the previous two have investigated the different ways that mutations arise, and the following elaborates on the ways in which mutations are instigated.
Barring all external factors, mutations occur very rarely, and are rarely expressed because many forms of mutation are expressed by a recessive allele.
However there are many mutagenic agents that artificially increase the rate of mutations in an organism. The following are some factors that increase genetic mutations in organisms
As mentioned previously, genetic mutations are a source of new variation in a species because it physically alters the sequence of nucleotides in a given sequence, therefore altering the genome in a unique way.
The next pages investigate genetic diversity in more detail, an how certain alleles (perhaps mutations) are favoured over other alleles in natural selection...
In the 19th century, a man called Charles Darwin, a biologist from England, set off on the ship HMS Beagle to investigate species of the island.
After spending time on the islands, he soon developed a theory that would contradict the creation of man and imply that all species derived from common ancestors through a process called natural selection. Natural selection is considered to be the biggest factor resulting in the diversity of species and their genomes. The principles of Darwin's work and his theory are stated below.
This 'weeding out' of less suited organisms and the reward of survival to those better suited led Darwin to deduce that organisms had evolved over time, where the most desirable characteristics of a species are favoured and those organisms who exhibit them survive to pass their genes on.
As a consequence of this, a changing environment would mean different characteristics would be favourable in a changing environment. Darwin believed that organisms had 'evolved' to suit their environments, and occupy an ecological niche where they would be best suited to their environment and therefore have the best chance of survival.
As the above indicates, those alleles of a species that are favoured in the environment will become more frequent in the genomes of the species, due to the organisms higher likeliness of surviving as part of the species at large
The above indicated the theory of natural selection. The next page gives some good examples of natural selection in action.
Darwin's finches are an excellent example of the way in which species' gene pools have adapted in order for long term survival via their offspring. The Darwin's Finches diagram below illustrates the way the finch has adapted to take advantage of feeding in different ecological niche's.
Their beaks have evolved over time to be best suited to their function. For example, the finches who eat grubs have a thin extended beak to poke into holes in the ground and extract the grubs. Finches who eat buds and fruit would be less successful at doing this, while their claw like beaks can grind down their food and thus give them a selective advantage in circumstances where buds are the only real food source for finches.
Polymorphism pertains to the existence of two distinctly different groups of a species that still belong to the same species. Alleles for these organisms over time are governed by the theory of natural selection, and over this time the genetic differences between groups in different environments soon become apparent, as in the case of industrial melanism.
Industrial melanism occurs in a species called the peppered moth, where the occurrence has become of more frequent occurrence since the beginning of the industrial age. The following argument elaborates the basis of principles involved in natural selection as far as industrial melanism is concerned.
Consider this argument of natural selection in the case of sickle cell trait, a genetic defect common in Africa.
This is how science has understood natural selection since the first studies involving Darwin. In the 21st century, humans selectively breed species to create hybrid species possessing the best genes of both parents via a process known as selective breeding.
This is investigated on the next page.
As mentioned in previous pages, scientists from the past harnessing the knowledge of genetics has resulted in many scientific breakthroughs and uses of this knowledge. Most notably, Gregor Mendel's studies into Monohybrid and Dihybrid crossing and Charles Darwin's study of evolution and natural selection has meant that humans have learnt to actively manipulate the phenotype of offspring by selective breeding in animals and plants.
Breeders of animals and plants in today's world are looking to produce organisms that will possess desirable characteristics, such as high crop yields, resistance to disease, high growth rate and many other phenotypical characteristics that will benefit the organism and species in the long term.
This is usually done by crossing two members of the same species which possess dominant alleles for particular genes, such as long life and quick metabolism in one organism crossed with another organism possessing genes for fast growth and high yield. Since both these organisms have dominant genes for these desirable characteristics, when they are crossed they will produce at least some offspring that will show ALL of these desirable characteristics. When such a cross occurs, the offspring is termed a hybrid, produced from two genetically dissimilar parents which usually produces offspring with more desirable qualities. Breeders continuously track which characteristics are possessed by each organism so when the breeding season comes once again, they can selectively breed the organisms to produce more favourable qualities in the offspring.
The offspring will become heterozygous, meaning the allele for each characteristic will possess one dominant and one recessive gene. Most professional breeders have a true breeding cross (ie AAbb with AAbb) so that they will produce a gene bank of these qualities that can be crossed with aaBB to produce heterozygous offspring. This way the dominant features are retained in the first breeding group and can be passed on to offspring in the second instance.
This process of selecting parents is called artificial selection or selective breeding, and poses no threat to nature from man manipulating the the course of nature. It has allowed our species to increase the efficiency of the animals and plants we breed, such as increasing milk yield from cows by continuously breeding selected cows with one another to produce a hybrid.
However, while it is an advantage both to the species and to humans to produce these desirable qualities that may benefit the organisms in question, continuous in-breeding and selective breeding of particular genes runs the risk of losing some of the other genes from the gene pool altogether, which is irreversible. This is called in-breeding depression, where the exclusivity of the advantageous genes mean that some other less desirable genes are phased out. In the long term, it is more advantageous for organisms to remain heterozygous;
Genetic diversity in the long term is reduced, because many organisms end up with similar genomes due breeding with each other constantly. In normal circumstances, this process would be random, and would produce more variable offspring
With the above facts in hand, breeders need to produce more heterozygous offspring to ensure the long term welfare of the species they are breeding and their livelihood. The most important thing here is to preserve the genetic diversity of a species, and preferably keep the gene pool of a species as diverse as possible.
Humans have realised the above dangers, and instead of harnessing and exhausting natures reserves, we have learned to preserve their genetic information for their long term survival and our own well-being. One species becoming extinct can knock the balance of an ecosystem and have a detrimental knock on effect.
With this in mind, humans have gene banks to preserve the genetic information in the case of extinction, and nurture species that are at dangerously low population levels.
Ironically, the human interference that has disrupted so many species can now provide a means of placing genes into organisms, therefore preparing them for the above hypothetical scenarios such as an epidemic of disease. Genetic engineering would provide the means of allowing organisms to suit their environment without the trial and error over time that comes from natural selection.
The next page investigates genetic engineering and the distance it has come.
During the latter stage stages of the 20th century, man harnessed the power of the atom, and not long after, soon realised the power of genes. Genetic engineering is going to become a very mainstream part of our lives sooner or later, because there are so many possibilities advantages (and disadvantages) involved. Here are just some of the advantages :
Of course there are two sides to the coin, here are some possible eventualities and disadvantages.
Genetic engineering may be one of the greatest breakthroughs in recent history alongside the discovery of the atom and space flight, however, with the above eventualities and facts above in hand, governments have produced legislation to control what sort of experiments are done involving genetic engineering. In the UK there are strict laws prohibiting any experiments involving the cloning of humans. However, over the years here are some of the experimental 'breakthroughs' made possible by genetic engineering.
Genetic engineering has been impossible until recent times due to the complex and microscopic nature of DNA and its component nucleotides. Through progressive studies, more and more in this area is being made possible, with the above examples only showing some of the potential that genetic engineering shows.
For us to understand chromosomes and DNA more clearly, they can be mapped for future reference. More simplistic organisms such as fruit fly (Drosophila) have been chromosome mapped due to their simplistic nature meaning they will require less genes to operate. At present, a task named the Human Genome Project is mapping the human genome, and should be completed in the next ten years.
The process of genetic engineering involves splicing an area of a chromosome, a gene, that controls a certain characteristic of the body. The enzyme endonuclease is used to split a DNA sequence and split the gene from the rest of the chromosome. For example, this gene may be programmed to produce an antiviral protein. This gene is removed and can be placed into another organism. For example, it can be placed into a bacteria, where it is sealed into the DNA chain using ligase. When the chromosome is once again sealed, the bacteria is now effectively re-programmed to replicate this new antiviral protein. The bacteria can continue to live a healthy life, though genetic engineering and human intervention has actively manipulated what the bacteria actually is. No doubt there are advantages and disadvantages, and this whole subject area will become more prominent over time.
The next page returns the more natural circumstances of genetic diversity.
We have already discussed some of the reasons why the genetics of a species can change over a long period of time. Charles Darwin's The Origin of Species went into great depth in this matter, and providing substance into the theory of evolution. The key thing to remember about evolution is that it favours more preferable genes in the gene pool, and over time, these preferable characteristics become more exclusive in the gene pool.
This next section rounds up all the factors that can alter the make up of a gene pool
Natural selection will favour genes that are more suited to their environment and become more exclusive in the gene pool over time in such an environment. Different genes will become more exclusive when the environment changes, or the species migrate.
Mutations are random occurrences which change the genome of an organism. They greatly increase genetic diversity, where advantageous mutations are favoured by natural selection and disadvantageous ones are phased out.
Occurs after genetic drift, where two groups of a species become separated and therefore cannot reproduce. The gene pool of these groups differ over time. If these two groups can once again meet up and reproduce, their genetic differences can be merged within the single group and increase genetic diversity.
Can cause in-breeding depression by continuous inbreeding, non-random mating is also known as selective breeding, where the breakthroughs of Mendelian genetics have allowed us to predetermine what genes are present in offspring. As advantageous genes are desired by the breeder, some of the less 'popular' genes are lost due to this random mating, therefore decreasing genetic diversity.
It is important for a species to have a large gene pool, because in the event of danger, some alleles will allow the species to survive and reproduce to produce a larger and more variant gene pool. For example, an extremely contagious disease may threaten 99% of a species, though the remaining 1% may possess an allele that provides them with resistance to the disease. If this allele was not present in the population, then chances are the entire population would be wiped out
Sometimes, species can be split into groups, usually as a result of a geographical factor preventing the two groups from contacting one another. This means that the two groups are unable to reproduce with one another.
As the two groups now live in different environments, natural selection will favour slightly different genes in each of the groups that will favour them in their particular environment. Over time, the difference in gene pool between the two groups can be quite dramatic.
If these two groups once again become re-united, gene migration occurs (see above). If they remain seperated, their genetic differences become greater which can result in the formation of a new species.
When the genetic differences become so great, it can come to the stage where the two groups can no longer reproduce with one another. This results in the formation of a new species (because organisms who are capable of reproducing but cant reproduce with a member of the same species are deemed another species).
Long ago, the land of Earth was all on one continent. Over time these continents separated, with members of the same species drifting away on each continent. Over time, speciation has occurred. This is evident by looking at the marsupials of Australia, who have been isolated from other mammals in their ancestral line, and therefore have many differences to that of mammals on other continents.
Adaptive radiation is the slow change of genotype and phenotype of a species from its common ancestor, meaning that species with a common ancestor become more diversified over time. The next page investigates adaptive radiation.
When Charles Darwin was in the Galapagos islands, one of the first things he noticed is the variety of finches that existed on each of the islands. All in all, there were many different species of finch which differed in beak shape and overall size. This is adaptive radiation and natural selection at work.
These finches, better known as 'Darwin's Finches' illustrated adaptive radiation. This is where species all deriving from a common ancestor have over time successfully adapted to their environment via natural selection.
Previously, the finches occupied the South American mainland, but somehow managed to occupy the Galapagos islands, over 600 miles away. They occupied an ecological niche with little competition.
As the population began to flourish in these advantageous conditions, intraspecific competition became a factor, and resources on the islands were squeezed and could not sustain the population of the finches for long.
Due to the mechanisms of natural selection, and changes in the gene pool, the finches became more adapted to the environment, illustrated by the diagram below.
As competition grew, the finches managed to find new ecological niches, that would present less competition and allow them, and their genome to be continued.
As indicated by the diagram above, the finches adapted to take advantage of the various food sources available on the island, which were being used by other species. Over the long term, the original finch species may have disappeared, but by diversifying, would stand a better chance of survival.
All in all, the finches had adapted to their environment via natural selection, which in turn, has allowed the species to survive in the longer term, the prime directive of any species.
The marsupial mammals occupy Australia, and dither from placental mammals because they bear their young inside a pouch. Long ago, the land mass of Earth consisted of one single continent, Pangaea, where all animals existed. When this continent fragmented into smaller continents, the geographical barrier meant the mammals of the time could no longer reproduce with one another. From here on in, the gene pool of these groups of mammals would become increasingly different until they could no longer reproduce with one another. This occurrence applies to the marsupial family of mammals that occupy Australia.
All the marsupials in present day Australia would have evolved from one common ancestor. However, over time and via natural selection, the many marsupial species (i.e. kangaroo and koala) have occupied their own ecological niche and adapted accordingly. Kangaroo's have long powerful legs to cover the wide area of land that they occupy while the koala's smaller structure and more centralised centre of gravity allow them to climb trees and obtain the eucalyptus that they feed on.
Humans, the most evolved species on the planet, have also underwent many changes over time from our ancestors. Here are some examples
It is worth noting that if two groups of humans were totally isolated over time, it is entirely possible that a different species could spawn from own, because the two groups would be adapting to their own environment over time. This would mean changes in the gene pool in each of the groups, to the point where the two groups could not reproduce with another and produce fertile offspring due to these genetic differences. At this point, with any species, a new one has begun.