Chlorosis Chlorosis = when plants aren't producing enough chlorophyll so look pale and yellow --> reduces plant's ability to photosynthesise Most plants showing chlorosis have the right genes for chlorophyll but its influenced by environmental factors : Lack of light --> plants turn off chlorophyll production to not waste resources Mineral deficiencies --> lack of iron or magnesium (iron is cofactor for some enzymes for making chlorophyll and magnesium is used in the chlorophyll molecule) Virus infections --> viruses interfere with the metabolism of cells, infected tissues no longer support chlorophyll synthesis Animal body mass Determined by a mix of genes and environment Environment --> overfeeding or underfeeding, lack of exercise, presence of disease Genetic --> pattern of fat deposition can be altered Creating genetic variation Created by versions of genes inherited from parents Individual mixture of alleles influences the characteristics shown Combination generated by meiosis and random fusion of gametes Genotype = combination of alleles inherited for characteristic Phenotype = observable characteristics of organism Actual characteristics displayed also controlled by environment Dominant alleles are always expressed Recessive alleles only expressed if there's two of them Homozygous = two identical alleles for the characteristic Hetereozygous = two different alleles for the characteristic (only the dominant one is expressed) Continuous and discontinuous variation Continuous = characteristic that can take any value within a range, caused by genetics and environment, controlled by a number of genes --> leaf surface area and animal mass Discontinuous = characteristic that can only appear in distinct categories, caused by genetics, controlled by one or two genes --> blood group, round and wrinkled pea shape

Homozygous genetic cross results in all offspring being heterozygous  Heterozygous genetic cross results in mix of homozygous and heterzygous offspring Codominance = when both alleles are dominant so both are expressed --> blood types A and B (heterozygous is AB) and snapdragon white and red flowers (heterozygous is pink) Multiple genes = when characteristic has more than two alleles (blood type has A, B and O) Sex chromosomes --> females have XX but males have XY Sex linkage = Y chromosome considerably shorter and don't have all the same genes as X so sometimes whatever allele is on the X is expressed if dominant or recessive <-- characteristic is classed as sex linked Colour blindness Haemophilia (slow blood clotting due to absence of blood clotting factor) if male gets the recessive gene on the X they won't have the corresponding dominant one so will get haemophilia. Heterozygous females = carriers

Dihybrid cross is used to show the inheritance of two different genes which may be located on different pairs of homologous chromosomes but control one phenotype --> each gene could have two or more alleles Homozygous parents produce all heterozygous offspring Heterozygous parents produce 9:3:3:1 offspring (usually) Expected ratio different to actual because of random fertilisation and crossing over doesn't always occur

Autosomal linkage Autosomal linkage = linked genes are found on pairs of chromosomes that aren't X or Y Linked genes are inherited as one unit, there is no independent assortment during meiosis (unless the genes are separated by chiasmata) The closer the genes are on the chromosome the less likely they are to be separated in crossing over and the fewer recombinant offspring (different combinations of alleles than either parent) Recombination frequency = measure of the amount of crossing over that has happened in meiosis Recombination frequency = number of recombinant offspring / total number of offspring Recombination frequency of 50% means there is no linkage and the genes are on separate chromosomes Less than 50% means that there is linkage and independent assortment had been hindered Degree of crossing over reduces = recombination frequency reduces <-- determined by how close the genes are on the chromosome Chi-squared test Observed ratios will be different to expected --> due to chance Number of observations made determines how chance will effect the results Important to compare observed and expected to see if the differences are due to chance or if there's another reason Use chi-squared to measure the size of the difference between the expected and observed results and see if they're significant or not If chi-squared result is GREATER than critical value, ACCEPT null hypothesis If chi-sqaured result is FEWER than the critical value, REJECT null hypothesis  Epistasis  Epistasis = interaction of genes at different loci Gene regulation is a form of epistasis with regulatory genes and TFs controlling the activity of structural genes One gene's expression can be "masked" by the lack of or expression of another gene Baldness masks the gene for widow's peak Dominant and recessive epistasis Recessive epistasis = recessive alleles at one locus mask the phenotypic expression of another gene locus Dominant epistasis = dominant allele masks the effects of either allele on the second gene

Population genetics Investigates how allele frequency changes within a population over time Gene pool = total of all genes in a population at one time Allele frequency = relative frequency of a particular allele in a population Frequency isn't linked to whether the allele is dominant or recessive ^ can change over time in response to different conditions Calculating allele frequency P + Q = 1 Where P is frequency of dominant allele and Q is frequency of recessive allele The Hardy-Weinberg principle Mathematical relationship between the frequencies of alleles and genotypes in a theoretical population Theoretical population had 5 features Large population No mutations No natural selection Random mating No immigration or introduction of new alleles P^2 + 2PQ + Q^2 = 1 P^2 is the frequency of homozygous dominant 2PQ is the frequency of heterozygous Q^2 is the frequency of homozygous recessive Use this plus P+Q=1 to work out questions Factors affecting evolution Mutation leads to genetic variation Sexual selection leads to an increase in the frequency of alleles that code for characteristics that improve mating success Immigration and emigration increase the gene flow to a population Genetic drift occurs in small populations --> appearance of a new allele will have a greater effect Natural selection increases the frequency of alleles that allow the organism's survival The impact on small populations Small populations have smaller gene pools so cannot adapt to change as easily and are likely to become extinct Population size limiting factors: Density-dependent factors depend on population size like competition, predation, parasitism and communicable disease Density-independent factors affect populations of all sizes like climate change, natural disasters, seasonal change, human activities Genetic bottlenecks = large reductions in population size for at lease one generation greatly reduces gene pool and genetic variation but a beneficial mutation will have a greater impact Founder effect Extreme example of genetic drift Smaller populations have smaller gene pools  Rare alleles in original population become more common in the isolated population due to the reduced gene pool Evolutionary forces Normal distribution = bell-shaped curve of distribution of different variants Stabilising selection = norm or average is selected got and the extremes are selected against (values in middle of bell are selected) --> results in reduction of frequencies of the alleles for extreme conditions Directional selection = occurs when there's a big change and the normal phenotype is no longer advantageous --> organisms with more extreme phenotypes positively selected --> allele frequency switches to more extreme end of spectrum Disruptive selection = the extremes are selected for and the norm selected against --> very rare but in North America young male lazuli buntings are either dull brown or bright blue this is because the brown aren't seen as threatening by the adults so are left alone but the blue is seen as very threatening by the adults so they're also left alone --> intermediate or normal phenotypes were attacked by the adults

Speciation = formation of a new species through evolution New species will not be able to interbreed with old species to produce fertile offspring Allopatric speciation More common form of speciation for animals Some members of a population are geographically isolated from original group  The environments will be different for each group Selection pressures for either group result in different physical adaptations Smaller group will result in founder effect, leading to genetic drift which further enhances their differences to the original group Eventually the mutations will accumulate in both populations until they're so different that the are no longer able to successfully interbreed --> reproductively isolated and are different species Darwin's finches! ^ adapted in different environments with different food sources so they became suited to these food sources Sympatric speciation Occurs within populations that share the same habitat More common for plants It can occur when two different species interbreed to form fertile offspring The hybrid formed is a new species as it will have a different number of chromosomes to its parents and may no longer be able to interbreed with either parent population  This stops gene flow and reproductively isolates the hybrid Disruptive selection, mating preferences and other behavioural differences can result in individuals or groups becoming reproductively isolated Artificial selection or selective breeding Populations are usually polymorphic (display more than one distinct phenotype) The allele coding for the most common characteristic is the wild type allele Mutants = other forms of that allele resulting from mutations Artificial selection is basically the same as natural selection except the selection pressures are different Instead of changes in the environment, its the desirable traits which provide the selection pressures Individuals with desirable traits are selected and interbred by farmers Offspring of this cross showing the best examples of the desired traits are selected and bred again This process repeated over and over changes the frequencies of alleles within the population and will eventually lead to speciation Problems caused by inbreeding Limiting the gene pool reduces the genetic variation which reduces the chances of evolution and adaptations Many genetic disorders are caused by recessive alleles When organisms are interbred, the frequency of these recessive genes can increase meaning there's more chance of offspring having the genetic disorder Over time, this reduces the ability of the organisms to survive and reproduce Gene banks Seed banks keep seeds from both wild and domesticated varieties of plants Gene banks store biological samples (sperm and eggs) Alleles from gene banks are used to increase genetic diversity in a process called outbreeding Breeding unrelated or distantly related varieties is also outbreeding Reduces the occurance of homozygous recessive and increases the potential for the population to adapt and evolve