topic 10 - genetics Flashcards
why does a second meiotic division occur?
sister chromatids separate during meiosis II since meiosis I = diploid –> haploid
what are the stages of meiosis?
meiosis I
meiosis II
both of which follow the same stages as mitosis
what happens during meiosis I?
diploid –> haploid
P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells
what happens during meiosis II?
separates sister chromatids
P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells
what is the outcome of meiosis?
four haploid daughter cells
what is independent assortment?
description of how pairs of alleles separate independently from one another during gamete formation
what is meiosis?
type of cell division that reduces the number of chromosomes in the parent cells by half and produces four gamete cells
the inheritance of one gene/trait is dependent to the inheritance of any other gene/trait - TRUE OR FALSE?
FALSE! It is independent
what is the formula to determine the potential chromosome combinations?
2^n (where n = haploid number)
why is there an equal chance of a gamete containing either the maternal or paternal copy of a given chromosome?
homologous pairs of chromosomes line up in a random orientation along the equator during Metaphase I
can independent assortment occur if two genes are located on the same chromosome? (linked genes)
no
what is synapsis and when does it occur?
the process of homologous chromosomes connecting (forming points of connections between nonsister chromatids) // occurs during prophase I of meiosis
what are chiasmata?
structure that forms between a pair of homologous chromosomes by crossover recombination and physcially links the homologous chromosomes during meiosis
POINTS OF EXCHANGE
what is a bivalent? or a tetrad?
connected homologues (two chromosomes = bivalent, four chromatids = tetrad)
what is crossing over?
nonsister chromatids breaking and recombining with their
homologous partners
OR
exchange of genetic material between non-sister homologous chromatids
what can chiasmata formation between nonsister chromatids result in?
exchange of alleles
until when are nonsister chromatids physically connected by chiasmata?
anaphase
what are recombinant chromosomes?
chromosomes that consist of genetic material from both homologues
what can recombination lead to?
novel allele combinations
what does crossing over do?
increases the genetic diversity of potential offspring
what are unlinked genes?
gene loci are on separate chromosomes
why are genes unlinked?
random orientatino of homologous pairs during metaphase I of meiosis
what is a dihybrid cross?
mating experiment between two organisms that are identically hybrid for two traits
what does a dihybrid cross determine?
the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked
what method is used to work out potential gamete combinations in a dihybrid cross?
FOIL
First / Outside / Inside / Last
ex. AaBb
AB, Ab, aB, ab
What are the steps to complete a dihybrid cross?
- designate characters to represent alleles
- write the genotype and phenotype of the parents
- write down all of the potential gamete combinations
- use a punnett square to work out potential genotypes of offsprings
- write out the phenotype ratios of potential offspring
what is a linkage group?
group of genes whose loci are on the same chromosome and hence don’t independently assort
what is a locus
physical location of a gene or other DNA sequence on a chromosome
like genetic street address :)
do linked genes follow the law of independent assortment?
NO! they function as a single inheritable unit
who demonstrated the law of independent assortment?
mendel // peas!!!
how are linked genes represented?
in pairs, vertically (AB / ab)
can linked genes be separated? if so, how
yes, through crossing over
do recombinant phenotypes occur more frequently or less frequently?
LOWER FREQUENCY as they are only evident when crossing over increases
Frequency increases the further apart two genes are
who determined the concept of gene linkage and how
Thomas Morgan, fruit fly experiment
what is the chi-squared test?
statistical measure used to determine whether the difference between an observed and expected frequency distribution is statistically significant
what does it mean if the observed frequencies do not conform to the expected frequencies for an unlinked dihybrid cross?
genes are linked, inheritance is not random and are potentially affected by natural selection
how is a chi-squared test conducted? (STEPS)
- identify hypothesis
- table of frequencies
- chi squared formula
- degrees of freedom
- identify the p value
what is the chi squared formula?
x^2 = sum of (O-E)^2 / E
where
O = observed frequency
E = expected frequency
how are degrees of freedom calculated?
df = (m-1)(n-1)
where m = # of rows
n = # of columns
what are monogenic traits?
characteristics controlled by a single gene locus (finite pattern of expression)
what are polygenic traits
characteristics controlled by more than two gene loci (bell shaped distribution)
polygenic traits can be influenced by the environment: True or False
True
what are two examples of polygenic traits and how are they effected by the environment?
human height: diet
human skin colour: sun exposure
what is a gene pool
the sum total of alleles for all genes (and alleles) present in a sexually reproducing population
what does a large gene pool indicate? what does a small gene pool indicate?
large: high amount of genetic diversity
small: low amount of genetic diversity
which is preferred? a high amount of genetic diversity or a low amount? why?
high amount as it increases the chance of biological fitness and survival
what is evolution?
cumulative change in the heritable characteristics of a population across successive generations
what are the mechanisms of change?
mutation (random change in the genetic composition) gene flow (movement of alleles into/out of a population) sexual reproduction (assortative mating) genetic drift (result of chance or random event changing composition of gene pool) natural selection (differentially selective environmental pressures)
what is the allele frequency?
relative proportion of a particular allele within a population
what is genetic drift?
change in the composition of a gene pool as a result of chance or random events
genetic drift is more significant in larger populations: true or false?
FALSE, it is more significant in smaller populations where chance events have a bigger impact on the gene pool
what the two mechanisms by which a large population is reduced to a small population?
population bottlenecks, founder effect
what is a population bottleneck?
event reduces population size by an order of magnitude around or greater than 50%
can be environmental (ex. fires, floods) or human induced (ex. overhunting)
what is the founder effect?
small group breaks away from a larger population to colonize a new territory
original population remains intact!!
the new population has less genetic variability (more susceptible to genetic drift)
what is natural selectioN?
change in the composiiton of a gene pool in response to a differentially selective environenmental pressure
what are the three types of natural selection?
stabilizing selection
directional selection
disruptive selection
what is stabilizing selection (with example)
when an intermediate phenotype is favored at the expense of extremes (conditions are stable)
ex. human birth weights (too large = birth complications, too small = high infant mortality)
narrow width of distribtion, gets rid of extreme variations
what is directional selection (with examples)
when one phenotypic extreme is selected at the cost of the other (when conditions change)
ex. antibiotic resistance (w/ antibiotic, resistance up, no antibiotic = susceptibility up)
shifts distribution left/right, favours ONE extreme
what is disruptive selection (with examples)
when both extremes are favored at the expense of the intermediate (conditions fluctuate)
ex. moth pigmentation (camouflage, depends on conditions)
favours both extremes
bimodal distribution
when does reproduction isolation occur?
when barriers prevent two populations from interbreeding
what are the two main categories of reproductive isolation barriers?
prezygotic isolation (before fertilization)
postzygotic isolation (after fertilization, offspring are not viable or infertile)
what are the three main examples of prezygotic isolation barriers?
temporal
behavioural
geographic
what is temporal isolation?
populations have distinct / separate reproductive cycles
what is behavioural isolation?
populations exhibit or respond to different specific courtships
what is geographic isolation
populations occupy different habitats / niches in an area
what is speciation?
evolutionary process that results in the formation of a new species from a pre-existing species
when does speciation occur?
when reproductive isolating mechanisms prevent two breeding organisms from producing fertile, viable offspring
what are the two basic mechanisms via which speciation occurs?
allopatric (geographical)
sympatric (reproductive)
what is allopatric speciation?
geographic!!
physically separated populations are exposed to different environmental conditions and begin to diverge
what is sympatric speciation?
divergence of species in the same geographic location (due to reproductive isolation)
what is sympatric speciation usually caused by?
meiotic failure during gamete formations
what is polyploidy?
heritable condition of posessing more than two complete sets of chromosomes
what is polyploidy usually caused by?
failure to undergo cytokinesis
what is polyploidy most common in?
plant species that can self pollinate or reproduce asexually
why do farmers want polyploid crops?
allows for the production of seedless fruits
usually larger and disease resistant
what are the two models of evolution via speciation?
phyletic gradualism
punctuated equilibrium
what is phyletic gradualism?
speciation occurs uniformly (constant pace over a period of time, continuous/gradual accumulation of mutations)
supported by the evolution of the modern horse hoof
what is punctuated equilibrium?
speciation occurs in rapid bursts with periods of stability (characteristics are maintained if conditions are stable, environmental change promotes rapid divergence)
gaps in the fossil record support this
what is the Hardy-Weinberg principle? (definition + equations)
means by which the frequency of two alleles can be predicted within a population
p + q = 1
(p+q+^2 = 1
what are the population conditions assumed when using the Hardy-Weinberg equation?
population is large with random mating
no mutation/gene flow
no natural selection
