Genetic variation and change

what is meiosis

what is meiosis

the process of cell division that occurs in the sex organs (ovaries, testes), and produces gametes (egg, sperm)

what are the stages of meiosis

• starting diploid cell (2n)

• DNA replicates (4n) tetraploid cell

• crossing over occurs - 1st step causing variation

• homologous pairs line up at cell equator randomly (independent assortment) - 2nd step causing variation

• spindle fibres separate homologous pairs and pull to ends of cell

• first division, 2 diploid cells (meiosis I)

• spindle fibres attach randomly to a chromatid (segregation) - 3rd step causing variation

• chromatids are pulled to cell pole

• cell membrane pinches and each forms two haploid cells, each has one chromatid, each has. a unique combination of genes (meiosis II)

what is crossing over

after the chromosomes have replicated they are now called homologous chromosomes, and each chromosome is called a chromatid, the inner chromatids exchange their tips, exchanging DNA and therefore genes, this results in each chromatid being unique

what is independent assortment

the chromosome pairs now line up in the centre of the cell (equator), it is random which paris will be on each side to be pulled apart by spindle fibres, this results in different combinations of alleles being combined

what is segregation

during meiosis II, spindle fibres will randomly grab one chromatid from each pair, to be pulled to opposing cell poles, this means when these cells divide each haploid cell will have a unique set of genes

what is dihybrid inheritance

dihybrid inheritance is where we look at the inheritance of two traits together, the traits can be on different chromosomes (unlinked), or on the same chromosome (linked)

what is a test cross

a testcross is when we breed an organism with an unknown genotype with a homozygous recessive, we look at the offspring produced from the cross, and if any after multiple crosses have the recessive phenotype, the unknown genotype must be heterozygous, if after multiple breedings all offspring have the dominant phenotype, we can assume the unknown genotype is homozygous (purebred)

what is a monohybrid cross

the inheritance of one gene

what is pure breeding

individuals that are homozygous will always produce the same offspring when crossed together

what is co-dominance

an interaction between two alleles when neither allele can mask the other, but both alleles are fully expressed in the phenotype

what is incomplete dominance

an interaction between two alleles when neither allele can mask the other, but both alleles are partially expressed to produce an intermediate phenotype

what is complete dominance

an interaction between two alleles when the dominant allele masks the presence of the recessive allele

what are multiple alleles

an interaction between alleles when a gene has more than two possible alleles

what are lethal alleles

a condition when an allele is able to produce a phenotype that kills the organism, if the allele is recessive, only the homozygous condition is lethal

species

a group of organisms capable of breeding to produce fertile offspring

population

a group of individuals of the same species in the same location at the same time

gene pool

total alleles within a population

allele frequency

the number of times a particular allele appears in a gene pool

gene flow

the movement of alleles in (immigration) or out (emigration) of a gene pool

evolution

the change in allele frequency of a trait within a population over generations

natural selection

the process by which alleles better suited for an environment make an organism more likely to reproduce thus passing on those alleles more frequently, increasing their number

artificial selection

the process by which organisms are selectively bred by humans based on desired traits

sexual selection

the process by which females select breeding male based on appearance, not fitness

directional selection

environmental change favours one extreme of phenotype variation, average phenotype shifts in one direction

stabilising selection

extreme phenotypes selected against and decrease/ disappear, average phenotypes are favoured and increase

disruptive selection

both extreme phenotypes are favoured and increase in number, average phenotypes selected against and decrease in number

what is a mutation

mutations are changes in the base sequence of a gene which makes a new protein/allele

what does a new allele mean

a new allele in a gene pool means there is increased variation which natural selection can act upon like causing multiple alleles for a trait

what is a heritable mutation

a mutation that occurs in the gametic cells and can be passed on

what happens due to heritable mutations

if the new mutated allele gives that individual an advantage it will be more likely to reproduce, and pass on the mutated alleles which can now be considered part of the gene pool

silent mutations

no effect on reproductive success

beneficial mutations

increased reproductive success

harmful mutations

decreased reproductive success

comparing variation caused by meiosis vs mutation

• both mutation and meiosis are major sources of variation within a population. They create variation in very different ways. Meiosis shuffles genes/alleles during gamete production creating unique combinations of genes/alleles to make the offspring different from parents and siblings. No new genes/alleles are created

• mutations change the base sequence of a gene. This, by definition creates a new gene/allele. New alleles increase variation in a population. if the mutation is gametic, and provides a survival/reproductive advantage it will be established in the gene pool

what is genetic drift

the change in allele frequency due to chance events, such as an environmental disaster, new unstoppable predator/disease

what is the bottleneck effect

a large population is reduced to a small number. The surviving individuals alleles are not representative of original population. Some alleles will be permanently lost, and some less common/ less desirable alleles become permanent (fixed)

what is the founder effect

a small portion of a larger population becomes reproductively isolated. The founding population has less variation, fewer individuals, so some alleles will be lost, some will become fixed

what happens post genetic drift

because of the loss of alleles/ change of allele frequency populations post genetic drift are extremely susceptible to environmental change. Even if numbers rebound, variation will remain low. It takes a long time for new alleles to be introduced via beneficial mutation. Humans may increase variation by increasing gene flow (introducing new individuals into the gene pool), although this is not always possible. Evolution (change in allele frequency), occurs a lot more rapidly in small populations as changes have a larger proportional effect

why is variation important

variation is important as natural selection requires variation to cause differential reproductive success. If there was no variation, no individual could have alleles favoured over others and thus natural selection could not occur

what causes variation

• mutation (source of new alleles)

• meiosis (shuffles existing alleles to make unique gametes)

• sexual reproduction (randomness of which sperm and egg fertilise)

what changes allele frequency (variation) in a population (evolution)

• natural selection - decreases variation

• gene flow - immigration (increases variation) emigration (decreases variation)

• sexual selection - decreases variation

• artificial selection - decreases variation

• genetic drift - decreases variation

• mutation - increases variation