Inheritance, Variation and Evolution

Sexual & asexual reproduction:
In sexual reproduction, two sex cells or gametes fuse together. Each gamete contains genetic information from the parent that supplied them. The egg cell and sperm cell or pollen and ovule mix the genetic information to create a new set of genetic information for the offspring. It will be a mixture of the two parents. As the gametes are different each time, this causes variation in the species and, although the offspring may look similar, unless they are identical twins, they will be different with different DNA.
In asexual reproduction there is only one parent, there are no gametes and mingling of genetic information, the offspring is identical in every way to the parent. There is no variation at all and the young can be classed as a clone of the parent. The only types of cell division involved is mitosis.

Mitosis & Meiosis:
In earlier topics, you will have met cellular mitosis. Mitosis (my "T" osis = T for two) makes two identical cells, this is crucial for growth and repair. When we are replacing dead or dying cells and when we grow in age/size, mitosis is what occurs. Each new cell is identical to the original (no variation at all/identical DNA).
In cellular meiosis, there is an extra step in the division which leads to the final sex cells or gametes having only half of the number of chromosomes in there. This process happens in both the male and female gametes so that when they fuse, there will be a complete set of genetic information. No two gametes have the exact same genetic information so (unless identical twins) no two offspring will have the same DNA. This gives rise to variation within a species.

DNA:
DNA is a polymer, as a chemist, this is my favourite part of biology. Polymers are repeating units of monomers and DNA is no exception. Two polymer chains run side by side and are joined by bases, these are A, T, G & C. A and T join and G and C join. The two polymers twist around in a double helix (screw) and it is the sequence of the ATs and GCs that give us our unique genetic signature. DNA is found in chromosomes. A small section of the DNA sequence is called a gene and it is the blueprint for making a specific amino acid that, in turn will make a specific protein. There has been huge amounts of work and research into mapping out the human genome and it is an enormous amount of work but we believe that this code holds the key to curing diseases that we have never been able to cure. There is evidence that certain sequences or genes are the causes of cancers and these can be "turned off".

Sex determination & inherited characteristics:
When gametes fuse, the genetic information that is generated is a 50:50 mixture of the two parents. In humans, there are 23 pairs, 1 pair determines sex and the others are for inherited characteristics. Males have XY chromosomes and females have XX chromosomes. The fusing will either generate XX or XY and this determines the sex of the offspring. By using a punnet square in these diagrams, we can work out the probability of the offspring being a boy or a girl.
In the other examples which give the probability of eye colour and also the probability of offspring having or carrying a genetic disorder.
Notes on these diagrams:
• Dominant alleles are capital letters and always show in the baby if they are there.
• Recessive alleles are lower case and do not show in the baby unless there are two recessives together.
• If parent has the same alleles such as BB or pp, these are homozygous
• If parent has different alleles such as Bb or Pp, these are heterozygous
In cases where there are serious genetic disorders in parents, they can investigate the likelihood of their children having a genetic disorder that may be recessive in themselves. If they are likely to pass this on, they may choose to adopt.

Genetic engineering:
Genetic engineering involves editing a gene or sequence of genes in the genome of an organism. It is done to either add a desired characteristic. There is lots of research into removing sequences that cause certain diseases. To do this, a section of the DNA responsible for that particular gene is cut from one piece of human DNA and inserted into the cell of another organism. Knives, scissors and even scalpels are too clumsy for this, specific enzymes are used in this operation, the gene is then delivered into specific bacteria cell that makes a protein that will make this characteristic.
The best examples of these are making genetically modified crops. To reduce the amount of famine on Earth, we can make crops that survive in environments in which they previously would not have survived, crops that produce a higher yield or are resistant to disease.
We have to be very careful as there are many ethical debates still ringing about "Frankenstein food" and "designer babies". Make sure that you understand arguments for both sides of these.

Variation:
Variation is the differences within the same species in a community. There are two causes for this more commonly referred to as nature and nurture. Individuals are different because of their nature - this is the genes that they inherited through sexual reproduction based on all gametes being slightly different. Nurture is the impact an individual's environment has had on them such as the availability of food affecting their development and growth. There is a third cause and this is simply a combination of both of these combined.

Selective breeding:
For centuries, farmers and breeders of animals have selectively bred plants and animals to pass on their desired characteristics. Dairy cows and beef cattle are very different but have common ancestry. If a cow has a huge milk yield, you will want as many of her calves as possible in the gene pool and if a bull has a huge muscle mass and beef yield, he will be bred preferentially with beef cows to pass on those genes too. It is easy to see with dogs that have been bred for hunting, chasing rabbits, going down holes and those who are just amazingly cute.
Plants have been through the same process too. over 1000 years ago, carrots were purple and other colours. When orange ones appeared, they ere more appealing so were bred and cultivated as a preferred crop. Work is still going on now to breed specific colours of roses. It is not just colour in vegetables, certain kinds of crops are resistant to extreme temperatures, flooding or drought and certain diseases. These characteristics are then passed on to improve the overall yield.
We met genetic engineering before, this selective process does not have the same level of ethical issue as there is no manipulation with DNA and it has been done for centuries.
Alas, this still can have problems. Back to the subject of dogs, some pedigree dogs rely on too small a gene pool and this causes defects caused by inbreeding. It may make them prone to diseases and/or have conditions that limit their life span.

Evolution & the fossil record:
Evolution is the most iconic part of biology, it even beats photosynthesis to earn that trophy. Darwin's theory shows us that through variation there will be occasional mutations that give individuals a characteristic that is not in that species. If this characteristic is not good, then this individual is unlikely to live to breeding age and so will not pass on their genes to any offspring. If, however, their mutation makes them strong, better hunters, better at camouflage or more resistant to disease, then they will live longer and pass on their genes to even more young. This mutation is now in the gene pool and those who have it will, over a long period of time, pass it on to offspring. Eventually, every member of this species will have this new trait. Those with this will have had a longer life as they are more adapted to their environment. This is where the term survival of the fittest comes from. As a whole species evolves, we call it natural selection. This is when the environment get's tougher, only those most well adapted will survive and pass on their genes to the next generation, the "weaker" ones die out.
Mutations are changes in the order of DNA bases (GC and AT) that results in a code change. Interbreeding can only produce fertile offspring if the parents are of the same species. Examples include horse & donkey parents can produce a mule or a hinny but these are themselves infertile.
This process of evolution has been going on for a very long time and Darwin shows that we are all evolved from the first primitive life that appeared 3 billion years ago.
Darwin's theory was not widely accepted at first, particularly by the church as it directly opposes the Bible's version of creation. Wider acceptance has grown since more and more extensive evidence appeared in the fossil record. The discovery of the Burgess shales, a formation of approximately 508 million years old, showed strange creatures that had some similarities to modern species. Later finds showed links between these very old ones and present day. A clear trail of evolution can be shown that is indisputable. There are, however, gaps as only a few organisms become fossils. Most soft-bodied organisms are not preserved at all and weathering, erosion and other geological activity destroys existing fossils.
The more alarming side of evolution is with bacteria. Bacteria reproduce so much faster than other organisms so they evolve faster. We discussed evolving a resistance to a disease, bacteria can evolve a resistance to antibiotics. Antibiotic resistance is a crisis that we will soon face and scientists are working on alternative formulae all of the time. MRSA is an example and if it gets inside our bloodstream, treatment often involved amputation to prevent the spread as there are no antibiotics that can kill off the infection.

Extinction:
Extinction can be caused by a sudden and catastrophic event like the asteroid that made so many species of dinosaur extinct or it can be caused by less sudden events. New diseases, new predators, predators evolving or behaving differently to improve their success. There are some that take more time like environmental changes or weather. Geological events in Earth's history have caused many extinctions, much more than a single volcanic eruption, the eruption of flood basalts have released poisonous gases into the atmosphere. In 1783, the eruption of Lakagigar in Iceland killed 75% of livestock and a quarter of the population of Iceland. This eruption was approximately 714000 times smaller than others that we have discovered in our geological history (based on volume of lava to escape).
Then we get to the sad truth about species that are extinct or on the brink of extinction because of our actions.