B5 - Genes, inheritance and selection

• The genome is the entire genetic material of an organism.
• The genetic material is stored in the nucleus and is arranged into chromosomes.
• Each chromosome is one very long molecule of DNA that's coiled up.
• A gene is a short length of a chromosome.
• Genes determine the production of proteins. The sequence of bases in the gene determines the type of protein which is produced. The production of different proteins controls the development of different characteristics, e.g. dimples, and how an organism functions.
• Genes can exist in different versions. Each version gives a different form of a characteristic,like blue or brown eyes. The different versions of the same gene are called alleles or variants.

• Different species look different. 
• But even organisms of the same species will usually look at least slightly different - e.g. in a room full of people you'll see different colour hair, individually shaped noses, a variety of heights etc. 
• These differences are called the variation within a species. 
• Variation can be genetic - this means it's caused by differences in genotype. Genotype is all of the genes and alleles that an organism has. A organism's genotype affects its phenotype - the characteristics that is displays. 
• An organism's genes are inherited (passed down) from its parents. 
• It's not only genotype that can affect an organism's phenotype though - interactions with its environment (conditions in which it lives) can also influence phenotype. For example, a plant grown on a nice sunny windowsill could grow luscious and green. The same plant grown in darkness would grow tall and spindly and its leaves would turn yellow - these are environment variations. 
• Most variation in phenotype is determined by a mixture of genetic and environmental factors. For example, the maximum height and an animal or plant could grow to is determined by its genes. But whether it actually grows that tall depends on it's environment (e.g. how much food it gets.)

• Continuous variation is when the individuals in a population vary within a range - there are no distinct categories, e.g. humans can be any height within a range, not just tall or short. Other examples include an organism's mass, and the number of leaves on a tree. Characteristics that are influenced by more than one gene or that are influenced by both genetic and environmental factors usually show continuous variation.
• Discontinuous variation is when there are two or more distinct categories - each individual falls into only one of these categories, there are no intermediates. For example, humans can only be blood group A, B, AB, O. Characteristics that are only influenced by one gene and that aren't influenced by the environment are likely to show discontinuous variation.

• Occasionally, a gene may mutate. A mutation is a rare, random change in an organism's DNA that can be inherited.
• Mutations mean that the sequence of DNA bases in the gene is changed, which produces a genetic variant (a different form of the gene).
• As the sequence of DNA bases in a gene codes for the sequence of amino acids that make up a protein, gene mutations sometimes lead to changes in the protein that it codes for.
• Most genetic variants have very little or no effect on the protein the gene codes for. Some will change it to such a small extent that its function is unaffected. This means that most mutations have no effect on an organism's phenotype.
• Some variants have a small influence on the organism's phenotype - they alter the individual's characteristics but only slightly.
• Very occasionally, variants can have such a dramatic effect that they determine phenotype.  

• Small influence on the organism's phenotype:
• Some characteristics, e.g. eye colour, are controlled by more than one gene. A mutation in one of the genes may change the eye colour a bit, but the difference might not be huge.
• Dramatic effect on the phenotype:
• The genetic disorder, cystic fibrosis, can be caused by the deletion of just three bases but it has a huge effect on phenotype. The gene codes for a protein that controls the movement of salt and water into and out of cells. However, the protein produced by the mutated gene doesn't work properly. This leads to excess mucus production in the lungs and digestive system, which can make it difficult to breathe and to digest food.

• Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
• In sexual reproduction, the mother and father produce gametes - in animals these are sperm and egg cells.
• Gametes contain half the number of chromosomes of normal cells - they are haploid. Normal cells (with the full number of chromosomes) are called diploid.
• At fertilisation, a male gamete fuses with a female gamete to produce a fertilised egg. The fertilised egg ends up with the full set of chromosomes (so it is diploid).
• The fertilised egg then undergoes cell division by mitosis and develops into an embryo.
• The embryo inherits characteristics from both parents as it's recieved a mixture of chromosomes (and therefore genes) from its mum and its dad.

Meiosis is a type of cell division. It's different to mitosis because it doesn't produce identical cells. In humans, meiosis only happens in the reproductive organs (ovaries and testes).

• Before the cell starts to divide, it duplicates its DNA (so there's enough for each new cell). One arm of each X-shaped chromosome is an exact copy of the other arm.
• In the first division in meiosis the chromosomes line up in pairs in the centre of the cell. One chromosome in each pair came from the organism's mother and once came from its father.
• The pairs are then pulled apart, each new cell only has one copy of each chromosome. Some of the father's chromsomes and some of the mother's chromosomes go into each new cell.
• Each new cell will have a mixture of the mother's and father's chromosomes. Mixing up the genes like this is really important - it creates genetic variation.

• In the second division the chromosomes line up again in the centre of the cell. It's a lot like mitosis. The arms of the chromosomes are pulled apart.
• You get four haploid gametes - each only has a single set of chromosomes. The gametes are all genetically different.

• During sexual reproduction, the male gamete fuses with the female gamete during fertilisation and the resulting cell has two copies of every chromsome.
• There are 23 pairs of chromosomes in every human body cell. The 23rd pair are labelled XY. These are sex chromosomes - they decide whether you turn out male or female.
• Males have an X and Y chromosome: XY. The Y chromsome cause male characteristics.
• Females have two X chromosomes: **. The lack of a Y chromosome causes female characteristics.
• Like other characteristics, sex is determined by a gene.
• The Y chromosome carries a gene which makes an embryo develop into a male as it grows. Females, who always have two X chromosomes, don't have this gene and so they develop in a different way.

• You can draw a genetic diagram to show the probability of a child being a boy or a girl.
• Genetic diagrams are often used to show the inheritance of individual alleles, but here one is being used to show how whole sex chromosomes are inherited.
• The parents' gametes are written along the top and on the left side of the diagram. The male parent has an X and a Y chromosome and the female parent has two X chromosomes.
• Then the combinations of the alleles from both parents are written in the relevant box in the table, to give all the possible combinations in the offspring.
• There are two ** genotypes and two XY genotypes, so there's a 50% chance of having either a boy or a girl. This means there is a 50:50 ratio of boys to girls.

Asexual reproduction is another form of reproduction - it's different to sexual reproduction in several ways:

• In asexual reproduction there's only one parent so the offspring are genetically identical to that parent.
• Asexual reproduction happens by mitosis - an ordinary cell makes a new cell by dividing in two.
• The new cell has exactly the same genetic information (i.e. genes) as the parent cell - it's called a clone.

Bacteria, some plants and some animals reproduce asexually.

• You have two copies of each gene (i.e. two alleles) - one from each parent.
• If the alleles are different, you have instructions for two different versions of a characteristic (e.g. freckles or no freckles) but you only show one version of the two (e.g.freckles). The version of the characterstic that appears is caused by the dominant allele. The other allele is said to be recessive. The characterstic is caused by the recessive allele only appeears if both alleles are recessive.
• In genetic diagrams, letters are used to represent genes. Dominant alleles are always shown with a capital letter (e.g. 'C') and recessive alleles with a small letter (e.g. 'c').
• If you're homozugous for a trait you have two alleles the same for that particular gene e.g. CC or cc. If you're haterozugous for a trait you have two different alleles for that particular gene, e.g. Cc.
• An organism's genotype is the genes and alleles it has and its phenotype is the characteristics that it displays.

• Some characteristics are controlled by a single gene, e.g. blood group - this is called single gene inheritance. Genetic diagrams help to predict the phenotype of the offspring when you know the genotype of the parents.
• However, it's not always quite this simple - most characteristics are actually controlled by multiple genes, e.g. height. (You don't need to be able to draw the genetic diagrams for these though.)

• Looking at the similarities and differences between organisms allows us to classify them into groups.
• Scientists have been doing this for thousands of years but the way in which organisms are classified has changed over time.
• There are two different classification systems you need to know about, artificial and natural.

• Early classification systems only used observable features (things you can see) to place organisms into groups, e.g. whether they lay eggs or if they can fly. This system of putting organisms into groups is known as an artificial classification system. 
• Artificial classification systems are still used to make keys so that scientists can easily identify and group organisms but they're no longer seen as the best way to classify organisms. 

• As people began to understand more about evolution, evolutionary relationships became much more important when classifying organisms. 
• Natural classification systems use information about organsims' common ancestors and about their common structural features to sort organisms. For example, even though bats and humans have many differences, the bone structure of a bat wing is similar to that of a human hand, so in a natural classification system, bats and humans ae grouped together. 
• In natural classification systems, living things are divided into five kingdoms (e.g. the plant kingdom, the animal kingdom).
• The kingdoms are then subdivided into smaller and smaller groups - phylum, class, order, family, genus, species. 
• The hierarchy ends with species - the groups that contain only one type of organism (e.g. humans, dogs, E. coli). A species is defines as a group of similar organisms that are able to reproduce to give fertile offspring. 

• As technology improves, scientists are able to learn more and more about organisms and how they're related to each other. Many years ago, the invention of the microscope helped scientists to classify organisms as they could examine the structure of organisms in more detail. Nowadays, as well as improvements to microscopes, other new technologies are resulting in new discoveries being made and the relationships between organisms being clarified. For example, new evolutionary relationships are continully being discovered through molecular phylogenetics. 
• DNA sequencing is used in molecular phylogenetics to see how closely related organisms are. 
• DNA sequencing is a technique that compares the sequence of DNA bases for different species. The more similar the DNA sequence between species, the more closely related they are. E.g. the base sequence for human and chimpanzee DNA is about 94% the same. 

• Populations of species usually show a lot of genetic variation - this means that there's a big mix of gene variants (alleles) present in the population. 
• Variants arise when DNA randomly mutates.
• The resources living things need to survive are limited. Individuals must compete for these resources to survive - only some of the individuals will survive. 
• Some genetic variants give rise to characteristics that are better suited to a particular environment (e.g. being able to run away from predators faster). This means that these organisms have an advantageous phenotype. These individuals will have a better chance of survival and so have an increased chance of breeding and passing on their genes. 
• This means that a greater proportion of individuals in the next generation will inherit the advantageous variants and so they'll have the phenotype that help survival.

• Over many generations, the characteristic that increases survivial becomes more common in the population. The "best" characteristics are naturally selected and the species becomes more and more adapted to its environment.

An example is:

• All rabbits had short ears until a mutated gene meant that one rabbit was born with big ears that meant it could hear better and was always the first to dive for cover at the sound of a predator. When this rabbit reproduced, it meant eventually the rabbit population all had big ears which helped them with survival due to better hearing.

• Natural selection leads to the evolution of species. Evolution is the change in inherited characteristics of a population over tme, through the process of natural selection.
• The speed at which a species evolves depends partly on how quickly it reproduces - some species reproduce very quickly (e.g. bacteria can be ready to start dividing in just 20 minutes), whereas others reproduce much more slowly (e.g. usually humans only start producing after around 20-30 years).
• Being quick to reproduce means that inherited characteristics are passed on to future generations much more quickly, so the time taken for the population to adapt to its environment is reduced.
• Evolution can mean that a species' phenotype changes so much that a completely new species is formed (i.e. the old and new version of the species wouldn't be able to breed together to produce fertile offspring).

• This can happen when a physical barrier seperates two populations of a species - conditions on each side of the barrier will be slightly different so the phenotypes that are beneficial will be different for each population. Natural selection acts on each population to increase the proportion of the advantageous phenotype in that population, until they are so different that they can no longer breed together.

• Scientists believe that all complex organisms on Earth have evolved from simple organisms that existed about 3500 million years ago. Of course, they wouldn't think this without good evidence to back it up. Fossil records and antibiotic resistance in bacteria both provide evidence for evolution.

• A fossil is any trace of an animal or plant that lived long ago.They are most commonly found in rocks.
• They can tell us a lot about what the organisms looked like and how-long ago they existed. Generally, the deeper the rock, the older the fossil.
• By arranging fossils in chronological (date) order, gradual changes in organisms can be observed. This provides evidence for evolution, because it shows how species have changed and developed over many years. For example, if you look at the fossilised bones of a horse, you can put together a family tree to suggest how the modern horse might have evolved.

• Like all organisms, bacteria sometimes develop random mutations in their DNA, which introduces new variants into the opulation. These can lead to changes in the bacteria's phenotype - for example, a bacterium could become less affected by a particular antibiotic (a substance designed to kill bacteria or prevent them from reproducing).
• For the bacterium, this ability to resist antibiotics is a big advantage. The bacterium is better able to survive, even in a host who's being treated with antibiotics, and so it lives for longer and reproduces many more times.
• This leads to the resistant variant being passed on to offspring and becoming more and more common over time - it's just natural selection.
• The emergence of antibiotic resistant bacteria provides evidence for evolution (as there is a change in the inherited characteristics of a population over time, through the process of natural selection). Bacteria reproduce so quickly, scientists are able to monitor the evolution as it's occuring.