Topic 10 Flashcards
Describe synapsis
- When homologous chromo are connected in P1 via synapsis
- HCs are joined together by synaptonemal complex
- autosomes ALWAYS undergo synapsis (heteorsomes usually stay unpaired)
- tetrad structure is held together at the chiasma
Chiasmata definiton
X shaped points of attachment between non-sis chromatids in HCs
Define a linkage group
a group of genes which are located on the same chromosome hence don’t independently assort
Thomas Hunt Morgans Drosophilia experiments and what they showed
- Crossed wild type red-eyed with mutant white-eyed
- female white eyed only produced white eyed males
- male red-eyed only produced red-eyed females
- the white eye trait was determined to be sex-linked
- other traits also didn’t conform to Mendelian ratios –> the genes don’t independently assort
- idea of gene linkage –> crossing over phenotypes occur in a much lower freq.
- crossing over is a product of the distance between the 2 genes –> diff genes had different ratios of crossing over
What are recombinant phenotypes
phenotype combinations not found in either of the parents –> due to crossing over in P1
- frequency of recombinant phenotypes is lower than normal phenotypes
- identified by performing a test cross of the org with homozygous recessive
Define gene pool
sum total of all alleles of all the genes present in a sexually reproducing population
Define evolution
the cumulative change in the heritable characteristics of a population across successive generations (allele freq need to change across the population)
5 ways allele frequencies can change in a population
- natural selection -> selecting certain alleles due to environmental selective pressures
- sexual reproduction –> assortative mating promotes the increase in certain freq.
- gene flow -> migration/immigration
- genetic drift -> large scale
2 ways genetic drift can occur
- Population bottleneck
- drastic reduction in population size
- smaller pop, lower genetic variability -> higher level of genetic drift
- surviving members repopulate -> changes in allele frequency - Founder effect
- small group from OG population go colonise a new area
- not the same amount of genetic diversity -> subject to more genetic drift
- as the pop increases in size -> the pop not representative of the original population
- the OG pop is mainly intact, unlike in bottleneck
Allele frequency definition
the prevalence of a particular allele in a population in comparison to all the alleles for that gene in the pop
3 types of natural selection
Stabilising:
- doesn’t favour the extremes
- the extreme phenotypes are reduced, central phenotypes increase in proportion
- normal distribution curve
- environmental conditions are stable
Disruptive:
- favours 2 extremes
- normal distribution curve is low in the centre and high at the extremes
- seasonal variation
Directional:
- one phenotype favoured
- response to gradual/sustained changed in the environment
- followed by stabilising selection once optimal phenotype established
2 types of reproduction barriers
- prezygotic: prevents any offspring from being produced
- postzygotic: offspring not fertile/unviable
4 types of prezygotic isolation barriers
- geographical: physical barrier isolates the 2 populations -> adapt to their own environmental niches -> they cannot reproduce
- temporal: mating season timings do not align
- physical: physically difficult for the 2 populations to mate
- behavioural: different mating calls –> prevent them from interbreeding
Allopatric speciation
when there is a physical barrier separating 2 populations
–> prevents interbreeding. cumulative mutations, genetic drift and natural selection –> populations genetically diverge
Sympatric
when there is no physical barrier, but the 2 populations are reproductively isolated.
- may be due to chromosomal abnormalities/meiotic errors
- if error not present in the OG pop, the pop with error can only produce fertile offspring with the pop with error
Speciation
When new species form from a pre-existing species. Populations within a species become reproductively isolated and cannot interbreed to produce fertile, viable offspring anymore.
Common cause of sympatric speciation + polyploidy in plants
- usually due to errors in meiosis such as failure during cytokinesis
- resulting in 2n gamete
- form 3n cell when fused with an n gamete
- Polyploid organisms can interbreed (if same chromo. number) -> can’t produce viable offspring with their parent population
why polyploidy more common in plants:
- self fertilisation common
- asexual reproduction common
example: allium species has multiple
Common cause of sympatric speciation + polyploidy in plants
- usually due to errors in meiosis such as failure during cytokinesis
- resulting in 2n gamete
- form 3n cell when fused with an n gamete
- Polyploid organisms can interbreed (if same chromo. number) -> can’t produce viable offspring with their parent population
why polyploidy more common in plants:
- self fertilisation common
- asexual reproduction common
example: allium genus has multiple different species, with different multiples of the chromosome number
- garlic, chives, onions, etc.
State 2 benefits of polyploidy in plants
- may be larger plants -> more yield (hybrid resistance)
- no seeds as infertile
State and describe the 2 paces of speciation
Punctuated equilibrium: long periods of no change in genetic composition, follow by an abrupt and drastic change. supported by general lack of transition state fossils for many species -> could also be due to difficulty in obtaining fossils.
Phyletic gradualism: slow, small changes over a long time period. the process is smooth and continuous -> big changes from many little changes. evidence: the many intermediates to form the present day horse in fossil record
Explain how speciation occurs, including the different processes of isolation and selection
- species is an interbreeding population that can interbreed to produce fertile, viable offspring
- variation exists in a species;
- certain members have traits better adapted to environmental conditions;
- higher chance of survival to reproductive age to pass on alleles;
- speciation is the formation of new species;
- occurs because populations have become reproductively isolated
- temporal isolation - difference in timing of courtship patterns/mating
- behavioural - difference in courting/mating behaviours;
- geographical/allopatric - physical barrier preventing population from interbreeding
- polyploidy can cause reproductive isolation;
- stabilising selecting - extreme phenotypes not favoured, normal distribution curve;
- disruptive selection - extreme phenotypes favoured more, bimodal spread;
- directional selection - one extreme phenotype favoured over the other
