Topic 10

Question 1

Describe synapsis

Answer 1

• 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

Question 2

Chiasmata definiton

Answer 2

X shaped points of attachment between non-sis chromatids in HCs

Question 3

Define a linkage group

Answer 3

a group of genes which are located on the same chromosome hence don’t independently assort

Question 4

Thomas Hunt Morgans Drosophilia experiments and what they showed

Answer 4

• 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

Question 5

What are recombinant phenotypes

Answer 5

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

Question 6

Define gene pool

Answer 6

sum total of all alleles of all the genes present in a sexually reproducing population

Question 7

Define evolution

Answer 7

the cumulative change in the heritable characteristics of a population across successive generations (allele freq need to change across the population)

Question 8

5 ways allele frequencies can change in a population

Answer 8

• 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

Question 9

2 ways genetic drift can occur

Answer 9

1. Population bottleneck
   - drastic reduction in population size
   - smaller pop, lower genetic variability -> higher level of genetic drift
   - surviving members repopulate -> changes in allele frequency
2. 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

Question 10

Allele frequency definition

Answer 10

the prevalence of a particular allele in a population in comparison to all the alleles for that gene in the pop

Question 11

3 types of natural selection

Answer 11

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

Question 12

2 types of reproduction barriers

Answer 12

• prezygotic: prevents any offspring from being produced
• postzygotic: offspring not fertile/unviable

Question 13

4 types of prezygotic isolation barriers

Answer 13

• 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

Question 14

Allopatric speciation

Answer 14

when there is a physical barrier separating 2 populations
–> prevents interbreeding. cumulative mutations, genetic drift and natural selection –> populations genetically diverge

Question 15

Sympatric

Answer 15

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

Question 16

Speciation

Answer 16

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.

Question 17

Common cause of sympatric speciation + polyploidy in plants

Answer 17

• 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

Question 17

Common cause of sympatric speciation + polyploidy in plants

Answer 17

• 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.

Question 18

State 2 benefits of polyploidy in plants

Answer 18

• may be larger plants -> more yield (hybrid resistance)
• no seeds as infertile

Question 19

State and describe the 2 paces of speciation

Answer 19

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

Question 20

Explain how speciation occurs, including the different processes of isolation and selection

Answer 20

• 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