ch10: every time i see ABO i involuntarily shudder Flashcards
outline the formation of chiasmata during crossing over (5)
chiasmata is formed in prophase I of meiosis
pairing of homologous chromosomes
chromatids break and rejoin after non-sister chromatids exchange alleles
X-shaped structure formed
chiasmata form at position where crossing over occurs
chiasmata holds homologous chromosomes together
chiasmata become visible when homologous chromosomes unpair
draw a labelled diagram of the formation of a chiasma by crossing over (3)
google docs crossing over between homologous chromosomes centromere single strand break DNA cut between homologous chromosomes sister chromatids labelled
explain how genetic variation is promoted by crossing over (3)
mutual exchange of DNA between non-sister chromatids
split up combinations of linked alleles → recombination → produce new combinations of alleles
independent assortment of genes on a chromosome
explain how recombinants are formed during meiosis (3)
recombination occurs in prophase 1 of meiosis
homologous chromosomes pair up
chiasmata form between non-sister chromatids
chromosomes exchange segments → crossing over takes place
explain how meiosis results in an effectively infinite genetic variety of gametes (8)
one homologous chromosome is from the mother and one from the father
prophase I: homologous chromosomes pair up crossing over recombination of linked genes’ alleles many possible points of crossing over occurs at random positions due to the two chromatids crossing over are not identical
metaphase I: random orientation of homologous chromosomes
metaphase I + anaphase I
chromosomes move to opposite poles
independent assortment of chromosomes
meiosis II: separates alleles further
2n combinations: 4 genetically different nuclei gametes from each meiosis
outline the relationship between Mendel’s law of independent assortment and meiosis (6)
independent assortment of unlinked genes
genes are inherited independently
unlinked genes are on different chromosomes
presence of one allele does not influence presence of other allele
metaphase I: orientation of homologous chromosomes is random
orientation of one bivalent does not affect orientations of others
evidence seen in dihybrid crosses
all allele combinations (AB, Ab, aB, ab from AaBb) in gametes
phenotypic ratio of 9:3:3:1 in double heterozygous cross shows equal probability of all gametes
how observed ratio can arise (3)
linked genes are on the same chromosome
Mendel’s genes were on different chromosomes
linked genes don’t have independent assortment
can only separated by crossing over
non-recombinants are…
recombinants are…
test cross expect 2 phenotypes but get all 4 with smaller % of recombinant
outline how the chi-square test can be used in analysing monohybrid crosses (6)
determine whether difference between expected (E) & observed (O) results is due to chance → tests the significance of data
data must consist of discrete variables between observed and expected result
the null hypothesis is that there are no significant differences between them
chi-squared = ∑(O-E)^2/E
statistical table used to determine probability
establish degree of freedom
critical value of 5%
define the term gene linkage and outline an example of a cross between two linked genes (8)
gene linkage
pair of genes on same chromosome
genes tend to be inherited together
do not follow expected ratio in cross between heterozygous parents
more offspring with parental characteristics
fruit flies L = long wings l = short wings R = red eyes r = white eyes
cross: google docs
why are plants with 16 and 24 chromosomes are reproductively isolated (4)
diploid: plants with 16 chromosomes
tetraploid: plants with 32 chromosomes
triploid hybrid: plants with 24 chromosomes
meiosis fails in triploids because three chromosomes cannot pair up → triploids are infertile
discuss whether the plants with 16 and 32 chromosomes are separate species (4)
google docs
outline how reproductive isolation can occur in an animal population (4)
can be sympatric or allopatric
polyploidy: chromosomes cannot pair up
geographical: a population being separated by a barrier to contact (e.g. river/mountain)
behavioural: difference in courtship behaviours (a great and underrated trope tbh)
temporal: members of difference populations reproducing at different times
explain how new species can emerge by directional selection/disruptive selection/polyploidy (8)
directional selection:
natural selection favours one end of the range of variation
progressive change in a population in that direction
isolated population subjected to directional selection while rest of the species are not
species changes sufficiently over time → cannot interbreed → different species
disruptive selection:
extreme types selected for → adapted to different niches
reproductive barriers become established between extreme types
polyploidy:
having 3(+) sets of chromosomes
sometimes occurs due to an error in mitosis/meiosis
many new species formed as tetraploids
triploids are infertile: tetraploids and diploids do not produce fertile offspring together
discuss gradualism and punctuated equilibrium on the pace of evolution (4)
google docs
using skin colour as an example, describe polygenic inheritance (5)
more than one gene control same characteristic
combination of alleles determines the phenotype
no. of genes ↑ → possible no. of phenotypes
allows for continuous variation of skin colour
phenotypes do not follow simple Mendelian ratios of dominance and recessiveness
incomplete dominance
skin colour due to the amount of melanin in the skin
the more recessive alleles there are the lighter the skin colour
the environment also affects gene expression of skin colour
sunlight/UV light stimulate melanin production
if need 2nd example: grain colour in wheat + allele promote pigment production or not
