Topic 10 - Genetics and Evolution Flashcards
DNA replication
- 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
how is DNA material exchanged during meiosis?
- 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)
chiasma(ta)
- 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
bivalent
AKA tetrad
- association of a pair of homologous chromosomes
- the two chromosomes are physically held together by at least one DNA crossover
difference between anaphase i in meiosis and anaphase in mitosis
- 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
sister chromatid separation in meiosis
- anaphase ii
- centromeres split so each chromosome separates into 2 chromatids
- they are then pulled to opposite ends of the cell by spindle microtubules
what does independent assortment of genes mean?
- 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
why does independent assortment of genes occur?
- 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
autosome
non-sex chromosome
sex-linked genes
- genes that have their locus on a sex chromosome
- they are non-autosomal (as they are a sex gene)
autosomal traits
traits whose allele locus doesn’t lie on a sex chromosome
linkage group
a group of genes inherited together because they’re found on the same chromosome
linked genes
- genes with loci on the same chromosome
- during metaphase i, their outcomes will always be AB or ab unless crossing over occurs
recombinant genes
aB or Ab
- the alternative selection, if crossing over occurs to linked genes
what happens if the ratio found in a punnett grid doesn’t follow the 9:3:3:1 rule?
- 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
conducting chi^2 tests
chi^2 = Σ((observed - expected)/expected)
degree of freedom = no of conditions - 1
gene pool
all genetic info present in the reproducing members of a population at any given time
allele frequency
- 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
evolution (in terms of allele frequencies and gene pool)
when a gene pool is modified and allele frequencies change, we know some evolution has occurred
reproductive isolation of populations
- geographical
- temporal
- behavioural
reproductive isolation: geographical
physical barrier separating two prospective mates
e.g. a mountain, forest, etc
reproductive isolation: temporal
incompatible time frames
e.g. a population of plants reaching maturity at a different time from another population
reproductive isolation: behavioural
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
directional selection
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
stabilizing selection
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
disruptive selection
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
monoploidy
when a cell contains only 1 set of chromosomes (i.e. haploid)
diploidy
when chromosomes in a cell exist in pairs (i.e. diploid)
polyploidy
when chromosomes in a cell exist in three or more sets (triploid, tetraploid, pentaploid…)
how do polyploid cells occur?
- 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
effect of having polyploid cells in plants
- bigger fruits/food storage organs
- more resistant to disease
implications of polyploidy
- 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
speciation due to divergence of isolated populations
- gradualism: changes are small, continuous and slow
- punctuated equilibrium: changes are relatively quick, followed by long periods of negligible change
difficulty of proving speciation claims
- 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
