Topic 10 - Genetics and Evolution

Question 1

DNA replication

Answer 1

• one of the processes occurring during interphase before meiosis
• involves duplication of chromatids
• so the cell about to undergo meiosis has 2 copies of each chromatid (connected at the centromere) – this is a chromosome

Question 2

how is DNA material exchanged during meiosis?

Answer 2

• synapsis occurs during prophase i
• in which 2 homologous (i.e. chromosomes of same length) chromosomes are brought together to form a bivalent
• each chromosome comes from a different parent so they may contain different alleles
• crossing over occurs (identical breaks in specific loci are swapped in adjacent non-sister chromatids)

Question 3

chiasma(ta)

Answer 3

• the area where a segment from a chromatid joins to a corresponding point on another chromatid
• many can form over all 4 chromatids in synapsis during crossing over
• chromatids tend to repel each other after being attached at chiasmata
• this makes them twist into various shapes

Question 4

bivalent

Answer 4

AKA tetrad

• association of a pair of homologous chromosomes
• the two chromosomes are physically held together by at least one DNA crossover

Question 5

difference between anaphase i in meiosis and anaphase in mitosis

Answer 5

• anaphase: sister chromatids of a single replicated chromosome are pulled apart
• anaphase i: bivalents are separated so that each homologous chromosome is pulled to opposite ends of the cell

Question 6

sister chromatid separation in meiosis

Answer 6

• anaphase ii
• centromeres split so each chromosome separates into 2 chromatids
• they are then pulled to opposite ends of the cell by spindle microtubules

Question 7

what does independent assortment of genes mean?

Answer 7

• one allele doesn’t necessarily follow another when passed to a gamete
• alleles that determine specific characteristics are transmitted independently
  e. g. if the allele for blue eyes is transmitted, it doesn’t necessarily mean the allele for blonde hair will be transmitted as well

Question 8

why does independent assortment of genes occur?

Answer 8

• the orientation of bivalents during meiosis is random
• and as human gametes have 23 chromosomes, there are 2^23 possible different orientations
• crossing over further scrambles alleles across chromosomes

Question 9

autosome

Answer 9

non-sex chromosome

Question 10

sex-linked genes

Answer 10

• genes that have their locus on a sex chromosome

- they are non-autosomal (as they are a sex gene)

Question 11

autosomal traits

Answer 11

traits whose allele locus doesn’t lie on a sex chromosome

Question 12

linkage group

Answer 12

a group of genes inherited together because they’re found on the same chromosome

Question 13

linked genes

Answer 13

• genes with loci on the same chromosome

- during metaphase i, their outcomes will always be AB or ab unless crossing over occurs

Question 14

recombinant genes

Answer 14

aB or Ab

- the alternative selection, if crossing over occurs to linked genes

Question 15

what happens if the ratio found in a punnett grid doesn’t follow the 9:3:3:1 rule?

Answer 15

• we can assume the genes are linked

- as only linked genes don’t follow mendel’s law and therefore the 9:3:3:1 ratio

Question 16

conducting chi^2 tests

Answer 16

chi^2 = Σ((observed - expected)/expected)

degree of freedom = no of conditions - 1

Question 17

gene pool

Answer 17

all genetic info present in the reproducing members of a population at any given time

Question 18

allele frequency

Answer 18

• proportion of a specific variation of a gene in a population
• allele frequency =! no of people who express the trait, because of the nature of recessive and dominant alleles

Question 19

evolution (in terms of allele frequencies and gene pool)

Answer 19

when a gene pool is modified and allele frequencies change, we know some evolution has occurred

Question 20

reproductive isolation of populations

Answer 20

• geographical
• temporal
• behavioural

Question 21

reproductive isolation: geographical

Answer 21

physical barrier separating two prospective mates

e.g. a mountain, forest, etc

Question 22

reproductive isolation: temporal

Answer 22

incompatible time frames

e.g. a population of plants reaching maturity at a different time from another population

Question 23

reproductive isolation: behavioural

Answer 23

when a population’s lifestyle/habits are incompatible with another population
e.g. if a male bird attempts a courtship display to a female bird from a different species, the female may not find him as seductive as other females from the same species

Question 24

directional selection

Answer 24

when a phenotype is favored over another

i. e. choosing between two extremes
e. g. darker moths becoming more prevalent in the population than lighter moths

Question 25

stabilizing selection

Answer 25

when a phenotype is favored over two extremes

i. e. selection towards the mean point
e. g. when a flower producing a medium quantity of nectar is favored over one that produces too little and one that produces too much

Question 26

disruptive selection

Answer 26

when two extreme phenotypes are favored over an intermediate phenotype

i. e. selection against the mean
e. g. spadefoot toads can be either purely carnivorous or omnivorous

Question 27

monoploidy

Answer 27

when a cell contains only 1 set of chromosomes (i.e. haploid)

Question 28

diploidy

Answer 28

when chromosomes in a cell exist in pairs (i.e. diploid)

Question 29

polyploidy

Answer 29

when chromosomes in a cell exist in three or more sets (triploid, tetraploid, pentaploid…)

Question 30

how do polyploid cells occur?

Answer 30

• when cell division doesn’t completely separate copies of chromosomes into distinct nuclei
• so they still end up in the same cell
• much more common in plant cells than animal cells

Question 31

effect of having polyploid cells in plants

Answer 31

• bigger fruits/food storage organs

- more resistant to disease

Question 32

implications of polyploidy

Answer 32

• errors in replication occur more frequently
• if one population of plants is triploid and another tetraploid, the populations’ evolutions will be different
• resulting in a new species arising

Question 33

speciation due to divergence of isolated populations

Answer 33

• gradualism: changes are small, continuous and slow

- punctuated equilibrium: changes are relatively quick, followed by long periods of negligible change

Question 34

difficulty of proving speciation claims

Answer 34

• we only have fossil evidence
• things that help define species are poorly (or not at all) preserved in fossils (e.g. mating calls, pigmentation, behaviour)
• also, it’s impossible to prove that just because an extinct animal looks like a modern-day animal, the two are related or can reproduce together